Biomass integrated gasification–SOFC systems: Technology overview

Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Biomass integrated gasification–SOFC systems: Technology overview Zia Ud Din a,b, Z.A Zainal a,n a b School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia School of Mechanical and Manufacturing Engineering, National University of Sciences and Technology (NUST), H-12 Campus, Islamabad, Pakistan art ic l e i nf o a b s t r a c t Article history: Received 24 February 2015 Received in revised form 31 May 2015 Accepted 13 September 2015 The combination of biomass gasification with fuel cells, especially high temperature Solid Oxide Fuel Cells (SOFCs) promises sustainable and highly efficient (decentralized and modular) energy conversion systems This review encompasses the components of biomass integrated gasification–SOFC technology including biomass characteristics, the thermochemical conversion in gasifiers and the factors affecting the gasification process, the cleaning technologies for raw producer gas and its conditioning and finally the integration of gasifier with SOFCs The influence of impurities present in biomass producer gas such as particulates, tar, H2S, HCl and alkali compounds based on recent experimental studies and their tolerance limits towards SOFCs are presented Even though analysis based on the probable tolerance limits of impurities towards SOFCs and a comprehensive overview of the cleaning technologies for producer gas impurities indicate that producer gas cleaning at various temperatures using current technologies to meet SOFC requirements is possible, more experimental studies are still needed to acquire the detailed information on the tolerance limits of impurities for SOFCs The recent theoretical modeling and experimental studies of biomass integrated gasification–SOFC systems are also presented & 2015 Elsevier Ltd All rights reserved Keywords: Biomass gasification SOFC Gasifier–SOFC system Gas cleaning Contents n Introduction 1.1 Technology overview Biomass characteristics 2.1 Proximate and ultimate analysis 2.2 Effects of biomass properties on gasification process Biomass gasification technology 3.1 Types of gasifiers 3.2 Effects of operating parameters on gasification process Producer gas cleaning 4.1 Cold and hot gas filtration 4.2 Thermal and catalytic tar cracking 4.3 Other contaminants cleaning Fuel cells 5.1 The choice of fuel cell type for biomass integrated gasification–SOFC systems 5.2 Solid oxide fuel cells 5.3 Biomass derived fuels for SOFC Effects of producer gas impurities on SOFC performance 6.1 Particulate matter 6.2 Tar 6.3 Sulfur compounds 6.4 Halides - HCl 6.5 Alkali compounds 6.6 Nitrogenous species – NH3 Corresponding author Tel.: ỵ 60 5937788; fax: ỵ60 5941025 E-mail address: mezainal@eng.usm.my (Z.A Zainal) http://dx.doi.org/10.1016/j.rser.2015.09.013 1364-0321/& 2015 Elsevier Ltd All rights reserved 1357 1357 1357 1358 1358 1358 1359 1360 1360 1361 1361 1361 1364 1364 1365 1366 1367 1367 1367 1368 1369 1369 1369 Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 1357 Gas-cleaning schemes for biomass integrated gasification-SOFC systems 7.1 Producer gas conditioning Studies of biomass integrated gasification–SOFC systems Conclusions Acknowledgment References Introduction Fossil fuels are covering the global energy demand today and will continue to produce approximately 80% of the world’s primary energy by 2040 [1,2] Also, majority of the fossil fuel resources are concentrated in few parts of the world that have already and could continue to give rise to serious conflicts between various countries At the same time, world emissions of carbon dioxide has increased by 44% above 1993 levels by 2011 [3] and it is expected that the earth temperature will increase 1.7– 4.9 °C from 1990 to 2100 [4] Such an energy scenario may lead to disastrous outcomes if alternative fuels and alternate energy systems are not developed and utilized Among the alternative fuels, biomass and fuel cells have recently received significant attention Biomass is considered a renewable energy source if it is based on sustainable utilization Its energy potential is promising due to the reason that biomass is more evenly dispersed source over the earth and thus available nearly worldwide If utilized at the same rate as new biomass material grows, energy production from biomass does not contribute to the atmospheric CO2 emissions At the same time, the residues and wastes as biomass feedstock has negligible greenhouse gas emissions being the part of the short carbon cycle It is estimated that bioenergy (biomass and waste) would contribute between a quarter and a third of global primary energy supply in 2050 [5] Fuel cell is an electrochemical device that converts the chemical energy of the fuel directly into electrical energy (without combustion) cleanly and efficiently and produces water as its main by-product While low temperature fuel cells operate mainly on hydrogen as a fuel, for high temperature fuel cells; hydrogen, methane, carbon monoxide and their mixtures are found to be good fuel The high temperature SOFCs produces high quality heat also as a by-product which can be used for cogeneration or could be used in combined cycle applications Traditionally the fossil fuels are mainly converted into electricity and the world net electricity generation is likely to increase by 93% in 2040 [1] due to the fast developing countries and growing population Thus, there is a demand to increase the efficiency of electricity production At the same time, heat is the largest energy service demand worldwide [2] The gasification of biomass in a gasifier is more efficient technique as compared to biomass combustion (steam cycles), particularly on a smaller scale In order to achieve highest electrical efficiencies, the use of SOFCs with biomass gasification is a promising approach hence combining the merits of renewable energy sources with fuel cell energy systems Such systems could achieve higher electrical efficiencies as compared to their competing systems such as gasification-internal combustion engines Efficiencies as high as 50–60% could be reached if the high quality exhaust heat from SOFC is used in gas turbines downstream [6] Remaining heat fraction from SOFC could be utilized for heating services The first step in this technology is converting the biomass into a combustible mixture of gases (referred as producer gas in this text) through the process of gasification inside a gasifier The producer gas composition varies with the type of biomass feedstock used, employed gasifier type, the gasification agent and gasifier operating parameters The producer gas consists of hydrogen, carbon monoxide, carbon dioxide, methane, nitrogen, water vapor and impurities (mainly of six types i.e particulates, tar, sulfur-, chlorine-, nitrogen- and alkali- compounds) The impurities will also vary with the type of biomass used and the gasification technique employed The impurities are harmful and are required to be sufficiently removed in the second step so that the (cleaned) producer gas could be used for the selected fuel cell type in the last step of this technology The producer gas cleaning requirements depend upon the type of the fuel cell used, catalyst materials of the cell and its operating conditions The subsequent sections of this paper provide the most important aspects and components of this technology Biomass characteristics are briefly discussed in Sections Section describes the main aspects of the gasification technology including the producer gas impurities and their effects on the equipment The available cleaning technologies are comprehensively summarized in Section before discussing the effects of producer gas impurities on SOFCs in Section Section focuses on the fuel cell types and the reasons why SOFCs are the best choice for integration with biomass gasification Section concludes the gas processing aspects The most recent theoretical and experimental studies of integration of biomass gasification with SOFCs are elaborated in Section and the paper concludes in Section Biomass characteristics The variety of biomass can be classified mainly as (i) Primary (directly from plants or animals) and (ii) Waste (waste or different biomass derived products) The crops grown specifically for the purpose of energy production are called Energy crops and are suitable to be used in gasification [7] On the other hand, agricultural plant residues are produced in huge quantities every year and are underutilized worldwide, rise husk being one of the common example Animal manure is also included in agricultural waste but it is technically not viable as the only fuel for gasification because the producer gas from it is of generally very low Supply chain management and pre-treatment of biomass Gasification Biomass agent Fuel Cell System Gasification - Feedstock - Gasifier type - Gasification agent - Operating parameters 1.1 Technology overview Outline of the core components of biomass integrated gasification–SOFC systems is given in Fig 1369 1370 1370 1372 1373 1373 Raw PG - FC type (SOFC) Gas cleaning Clean - FC design PG - FC materials - Operating conditions Electricity Heat PG = Producer gas Fig Technical outline of a biomass integrated gasification–SOFC system 1358 Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 heating value but could be used as a supplementary fuel Cow dung has been used together with sawdust in a downdraft gasifier [8] The primary source for small scale industrial energy is forestry waste in the rural areas of the developing countries Industrial residues and Municipal Solid Waste (MSW) could be potential biomass feedstock [9] However, MSW pose major problems like ash deposits in large amounts [10] leading to active studies for the economical methods for the ash management [11] 2.1 Proximate and ultimate analysis Proximate analysis describes the biomass composition in terms of gross components such as Volatile Matter (VM), moisture content, Fixed Carbon (FC) and ASH The proximate analysis (Table 1) is used to establish the first measure of the suitability of biomass material for oxidation/ gasification.VM and FC contents of biomass fuels are found to be higher than that of solid fuels such as bituminous coal indicating the easiness of the fuel ignition The ash fraction is mostly less in woody biomass as compared to husk and straw materials (see Table 1) The HHV of Biomass fuels normally ranges from 17 to 21 MJ/kg [12] Herbaceous materials have lower calorific values as compared to the woody fuels The Ultimate analysis presents the elemental composition of the fuel It is required to determine the theoretical air/fuel ratio in various gasification processes and in evaluating the potential emissions The ultimate analysis of different biomass materials and coal is given in Table for comparison a gasifying agent, converting volatiles and char to producer gas as well as taking part in water–gas shift reaction which increases the hydrogen content in the producer gas [22–25] However, biomass with more than 40 wt% moisture content decreases the process conversion efficiency [26] due to the irretrievable loss of heat of vaporization and heating of steam to gasification temperature In case of air blown gasification for producer gas production, biomass should be dried to o30 wt% moisture content, preferably to $ 15 wt% [27] The moisture content also affects the handling, storage and transportation of biomass fuels [28] On the other hand drying of biomass involves cost Different methods of biomass drying have been researched [29–35] As a heat source, the hot air, recycled heat from flue gas [33] or steam may be utilized in direct drying or in indirect drying i.e heat transfer through hot surface to biomass In case of gasifier–SOFC systems, hot effluent exiting SOFCs may be circulated to the drier for a possible cost effective solution The amount and composition of ash in biomass significantly affects the gasification process The higher the ash content, the greater are the chances of impeding the chemical reactions and clogging the equipment Biomass such as miscanthus and straw contains high sulfur contents and would be less suitable for SOFC applications downstream because of its poisonous effect on the Ni/ YSZ anodes (see details in Section 6.3) unless it is removed from the producer gas The heating value of biomass materials is lowered by high values of ash composition and moisture content 2.2 Effects of biomass properties on gasification process Biomass gasification technology Moisture content has a significant effect on overall energy balance of biomass gasification process and it is directly proportional to the gas yield and composition Some moisture content in biomass fuel is desirable because steam generated from this moisture acts as Gasification involves thermal conversion of biomass into a combustible gas mixture through incomplete combustion This gas mixture consists of hydrogen, carbon monoxide, methane and carbon dioxide along with nitrogen, water vapour and impurities Table Ultimate and proximate analysis of some biomass materials compared with bituminous coal Biomass type Larch wood Camphor wood Wood sawdust Rice husk Rice straw Wheat straw Switch grass Cotton stem Bituminous coal Ultimate analysis (% w/w, dry basis) Proximate analysis (% w/w) C H O N S Ash VM FC M 44.15 43.43 46.2 45.8 38.61 46.1 47 42.8 80.9 6.38 4.84 5.1 6.0 4.28 5.6 5.3 5.3 6.1 49.32 38.53 35.4 47.9 37.16 41.7 41.4 38.5 9.6 0.12 0.32 1.5 0.3 1.08 0.5 0.5 1.0 1.55 – 0.1 0.06 – 0.65 0.08 0.1 0.2 1.88 0.12 0.49 1.3 0.8 12.64 6.1 4.6 4.3 76.86 72,047 70.4 73.8 65.23 75.8 58.4 72.3 35 14.86 14.75 17.9 13.1 16.55 18.1 17.1 15.5 45 8.16 12.29 10.4 12.3 5.58 (dry basis) 20 7.9 11 LHV (MJ/kg) Ref 19.45 17.48 18.81 13.36 14.40 17.2 18.7 15.2 34 [13] [14] [15] [16] [17] [18] [19] [20] [21] Table Overview of biomass gasification sub-processes Sub-process/ Temp (°C) Processes involved/ Reaction equations – Reaction name Drying o 200 Pyrolysis 200–600 Reduction 600–1000 Drying of biomass Release of VM (gas, vaporized tar, char) C ỵ CO2 2CO (H 0298 ẳ ỵ 172 kJ/mol) – Boudouard Products in producer gas at sub-process CO, H2, H2O, CO2, CH4, C2H2, N2, tar CO, H2, H2O, CO2, CH4, C2H2, N2, tars, particles, H2S, NH3 C ỵ H2O CO ỵ H2 (H 0298 ẳ ỵ 131 kJ/mol) Watergas C ỵ 2H2 CH4 (H 0298 ẳ 75 kJ/mol) Methanation CO ỵ H2O CO2 ỵ H2 (H0298 ẳ 41 kJ/mol) Watergas shift CH4 ỵ H2O CO ỵ 3H2 (H0298 ẳ þ206 kJ/mol) – Steam-methane reforming Oxidation 1000–1500 C þ 0.5O2 CO (ΔH 0298 ¼ À 111 kJ/mol) – Char partial combustion C ỵ O2 CO2 (H 0298 ẳ 399 kJ/mol) Char total combustion H2 ỵ 0.5O2 H2O (ΔH0298 ¼ À 242 kJ/mol) – H2 partial combustion CO, H2, H2O, CO2, CH4, C2H2, N2,tars, particles, H2S,NH3 Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 Gasification inside a gasifier involves four sub-processes; drying, pyrolysis, oxidation and reduction [36] summarized in Table These sub-processes may be identified sequentially in fixed bed gasifiers The gasification is autothermal if the required energy for gasification is provided by partial combustion of biomass within the same gasifier In allothermal gasification, the necessary heat for gasification is supplied from outside the gasifier mostly via the heated bed material Heat for gasification may be supplied 1359 from the depleted fuel and air streams of SOFC in allothermal gasification The composition of the producer gas and the impurities in it depend on the type of feedstock used, gasifier type, the gasification agent and gasifier operating parameters The six different types of impurities in producer gas include particulates, tars, sulfur-, chlorine-, nitrogen- and alkali- compounds and may exist in different phases of appearance as shown in Fig and described in Table 3.1 Types of gasifiers Producer gas impurities Solid Particulates Alkali compounds Liquid Tar Halides The gasifiers are mainly divided into two classifications [38], fixed bed and fluidized bed Fixed bed gasifiers are further divided into updraft [39–41], downdraft [42–46] and crossdraft gasifiers In a (countercurrent) updraft gasifier, the biomass descends downward while the gasification agent (normally air) ascends upward from bottom and producer gas comes out at the top of the gasifier As the tar produced in pyrolysis zone could only pass through the drying zone along with the producer gas, thermal tar Gaseous Sulfur species Nitrogenous species Fig Overview of impurities in biomass gasification gas [37] Table Overview of impurities in producer gas (adapted from [48,49], particular references indicated inside the table) Impurity Source/ Description of impurity Effects of impurity Gas quality requirements [49] Particulate matter – Solid agglomerations of unreacted carbon and ash (K, Na, Ca, SiO2, Fe, Mg) [50] – Also, condensed tar, ammonia and sulphur species – Elutriated bed material/ catalysts from FB gasifiers – Size range from o μm to 100 μm – Formed during pyrolysis and successive reactions with char – Complex mixture of diverse organic (aromatic) compounds – All hydrocarbons with molecular weights benzene are called Tar [51] – In vapour phase inside gasifier – The characterization of primary, secondary and tertiary tar is found in detail in literature [52–55] – – – – Fouling on reactor walls and bed materials Clogging of gas cleaning filters Blocking engine nozzles and ICE systems Corrosion and erosion of downstream equipment and turbine blades ICE: o 50 (PM10) Turbine: o 20 (PM0.1) (unit: mg/Nm3) (PM10 and PM0.1 means particles smaller than 10 and 0.1 μm resp.) – Tar contributes one of the biggest problems in gasification systems – Depending on their molecular structure condenses from 300 °C to end use zone and become sticky – Plugging and fouling of pipes, tubes, equipment – Clogging of gas cleaning filters – Undergo dehydration reaction around 430 °C to form solid coke which causes abrasion of turbine blades – Deactivate catalysts/ sorbents for reforming/cleaning – Contaminate water of wet clean-up processes – Poison catalysts for upgrading producer gas [57] and catalysts for gas cleanup – Burning syngas produces SO2 – a regulated pollutant – Sulphur compounds corrode metal surfaces ICE: o 100 Turbine: o (all vapour) (unit: mg/Nm3) Tar Sulfur compounds: H2S mainly and COS Halides: HCl Alkali compounds mainly K, Na Nitrogenous species: mainly NH3, HCN – Biomass inherent 0.1% wt Sulfur lower than Coal (1% wt) – H2S in producer gas varies 20 ppm – 200 ppm [56] – Stringent cleanup not required for most of the applications except solid oxide fuel cells (see Section 7.3) – Chlorine in biomass vaporizes in gasifier and react with water vapour to form HCl vapour ( boiling point 57 °C) – HCl in syngas varies 99 ppm [58] to 200 ppm [59] – HCl (vapors with other contaminants) form NH4Cl, NaCl – Biomass contains alkali and alkali earth metals – Alkali based catalysts and transition metal promoters also contribute alkali metal contaminants – Vaporize above 600 °C leaving reactor as aerosols and/or vapors [61] and can readily condense downstream – Originate from the nitrogen content and protein containing materials in biomass – Predominantly NH3 is the primary contaminant [63–67] – Typically in gaseous phase (boiling point o 30 °C) ICE: Turbine: o 1(unit: ppm) ICE: – Hot corrosion of turbine blades [59,60] – Condensed NH4Cl and NaCl causes fouling Turbine: o 0.5(unit: ppm) and they deposit in cooler downstream piping and equipment – condensed form causes fouling and corrosion in downstream applications [62] – Deposition in gas inlet nozzle in IC Engines and air inlet of the fluidized bed gasifiers – Agglomeration of the bed materials – Promotes slagging of ash in equipment – Poison some catalysts and damage ceramic filters – Not sensitive to engines and turbines – To be controlled to keep NOx emissions below the limits according to emission regulations – Poison some catalysts ICE: Turbine: o 50(unit: ppb) ICE: Turbine: -(unit: ppb) 1360 Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 Table Comparison of current main designs of biomass gasifiers (adapted from [47,87–90], particular references indicated inside the table) Downdraft Updraft Crossdraft Bubbling fluidized bed Circulating fluidized bed Dual Fluidized bed Gasification agent Fuel size (mm) Air 20–100 Air 5–100 Air 5–20 Air/ H2O/ O2 $ Air/ H2O/ O2 $ H2O $ Allowable fuel moisture (%) Syngas temperature (°C) Reaction temperature (°C) Syngas LHV (MJ/Nm3) Tar in producer gas (g/Nm3) Particles in producer gas (g/Nm3) Ash in syngas Reactor size MWth Residence time (of biomass particles) Hot gas efficiency (%) Technology Gasifiers manufactured (%) World [91], Europe [92] 12( o 25) 43(o 60) 700 200–400 $ 1090 – 4.5–5.5 5.5–6 0.01–5 30–150 0.02–8 0.1–3 Low High o1 0.1–20 Particles remain in bed and 10–20 1250 – $ 0.01–1 – High $ 0.01 gasified 85–90 90–95 – Proven, simple with low investment cost 75 2.5 remaining cracking could not occur resulting in high tar loads (see Table 4) But the particle load will be low (see Table 4) because the producer gas is filtered as it passes through the biomass feed bed In a (concurrent) downdraft gasifier, biomass enters at the top while air is introduced right above the throat (oxidation zone) through a number of nozzles from the reactor sides [47] The producer gas is taken out from the bottom of the gasifier As the tar produced are thermally cracked through the oxidation zone at around 1300 °C, relatively clean gas from tar as compared to the producer gas from updraft gasifiers is produced even if the particulates in the gas are high (see Table 4) In a fluidized bed gasifier, a fluidized bed of fine inert material (sand and/or catalyst) is used as a heat transfer medium Biomass is fed directly into the sand and kept in fluidized state by means of gasification agent/fluidization medium at the appropriate velocities called minimum fluidized velocity [68] Sub-processes of gasification (Table 2) cannot be separately identified like in a fixed bed gasifier as the biomass and bed material are thoroughly mixed Fluidized Bed (FB) gasifiers are further categorized into Bubbling Fluidized Bed (BFB) [69], Circulating Fluidized Bed (CFB) [70,71] and Dual Fluidized Bed (DFB) gasifiers [72–74] Tar and particle content from fluidized bed gasifiers are higher as compared to downdraft gasifier because the oxidation temperature in fluidized bed reactor is generally lower than that of the downdraft gasifier However, compared to the updraft gasifier, the tar content is lower in the producer gas from fluidized bed gasifiers as there is enough free board for tar to be converted [38] Fluidized bed gasifiers are suitable for larger installations and biomass throughputs are about ten times higher than in fixed bed gasifiers In addition to the fixed and fluidized bed gasifiers, there are advanced entrained flow gasifiers and plasma gasifiers Characteristics of various commonly used gasifiers are compared in Table As far as the selection of the gasifier for biomass integrated gasification–SOFC systems is concerned, downdraft gasifier might be a good selection for typical SOFC power generation modules already in demonstrative operation in the ranges such as 25 kW, 100 kW or 250 kW This selection is based on the reason that downdraft gasifier has moderate cost and most importantly produces relatively low level of impurities (see Table 4) For the SOFC based power generation systems around MW range, (bubbling) fluidized bed gasifier might be a good option but an effective gas clean-up system is required because such gasifiers produce higher level of impurities (particulates and tar, see Table 4) and more energy and cost would be required to clean the higher amounts of impurities Hence, SOFC integrated with Entrained flow bed O2 $ 0.15 (only fines) o 55 o 55 11–25 o 15 800–1000 – 800–1000 1260 800–1000 – 800–1000 $ 2000 3.7–8.4 4.5–13 14.2–18.1 8.8–9.3 3.7–62 4–20 0.2–2 0.01–4 20–100 8–100 8–100 – High High High Low 1–50 20–200 30–600 Considerable time in Few seconds – circu- Considerable time in Few seconds bed late in loop bed 89 89 90–95 80 Proven with coal, high investment cost Complex Complex 20 (fluidized bed gasifiers including BFB, CFB, DFB) remaining gasification can offer highly efficient power generation systems, especially in modular solutions 3.2 Effects of operating parameters on gasification process In addition to the type of feedstock and gasifier used, the gasification agent and gasifier operating parameters also influence the composition of producer gas (see Table 5) Air, the most common and economical gasifying agent yields nitrogen diluted (50 vol%) [75] and low heating value gas of 4–6 MJ/Nm3 (HHV) Steam gasification in fluidized bed gasifier is able to produce middle heating value (10–16 MJ/Nm3) gas [76–79] while dual fluidized bed gasification produces the gas as high as 12–20 MJ/ Nm3 (HHV) [74] Oxygen is the most expensive gasifying agent CO2 alone [80] or the mixture of two or more of the above gasification agents may also be used for gasification of biomass [81] Equivalence Ratio (ER) is a key parameter because it influences the calorific value of the producer gas significantly The suitable value of ER is found to be between 0.2 and 0.4 [82,83] to get better calorific value and controlled tar level Gasification at lower temperatures results in low conversion of char and high amounts of tar in producer gas H2 and CO contents of producer gas can be increased by raising the gasification temperature, however the materials for construction used in gasifiers with ash melting, set the temperature tolerable limits to about 1000 °C [84] As far as gasifying agent to biomass ratio is concerned, the producer gas with maximum H2 content with the least tar concentration was produced at the steam/biomass ratio of 1.4 in a dual fluidized bed gasifier [85,86] For entrained bed gasifiers the recommended O2/ biomass ratio is 0.4 [14] Producer gas cleaning The producer gas from the gasification process unavoidably contains unwanted substances as outlined in Fig and Table and must be removed to the acceptable levels (Table 3) depending on the individual application and to meet pollution control regulations Acceptable levels of impurities which are expected to be even stringent for solid oxide fuel cells are discussed separately in detail in Section based on recent experimental studies An approach for removing the impurities inside the gasifier is called “primary” or “in-situ” clean-up Primary clean-up with the selection of proper design [98–100], optimum operating parameters [101–103] and use of appropriate additives and catalysts Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 1361 Table Gas composition from different biomass gasification processes under different conditions Biomass Feedstock Gasifier type Gasifying agent Gasification Temp (°C) ER Gas composition (vol%) H2 CO CO2 CH4 8.47 10.53 13.50 13 11.90 8–12 30–50 22.78 24.94 23.8 23 25.70 15–22 22–25 11.81 12.80 13.51 11 9.90 5–8 25–30 – – – – – – H2S (35–39), COS ( o 2) 1.5–1.8 – NH3 (3100), Cl2 (260) 11.8 – 10.4 – 3.75 – 15 – 8–11 – Oil palm fronds Oil palm fronds Wood chips Hazelnut shells Wood pellets Wood waste Cedar wood DD DD DD DD DD DD UD Air Air(350 °C) Air Air Air Air O2 750–980 650–980 $ 1000 $ 1000 $ 1000 900–1050 650–950 0.27 0.22 0.35 0.35 0.28–0.30 0.20–0.35 0–0.3 Juniper wood Rice straw UD FB Air Air 800–1000 700–850 2.7 2.5–3.5 21–25 9–12 0.07–0.25 6–10 10–18 14–19 Wood pellets Almond shells Olive kernel Olive kernel Pine woodchips DFB DFB BFB DD DFB H2O H2O Air H2O H2O 812 ? 750 1050 700–900 – – 0.2 – 0.3 25.1 33.1 23.98 40 26–42 might reduce the costly subsequent cleaning of the producer gas in secondary processes [104] Cleaning of the raw producer gas downstream, out of the gasifier using multitude of technologies are termed as “secondary processes” some of which remove only one impurity at a time while others can remove several impurities in a single step, wet scrubber being the best example Other categorization based on the process temperature range of cleaning technologies is cold gas cleaning [101,105,106] and hot gas cleaning [107] Hot cleaning methods increase gasification efficiency by 3–4% [108] as energy is wasted when producer gas is cooled Most of the pilot scale gasification used combination gas cleaning methods to reach the required specifications for a particular application 4.1 Cold and hot gas filtration Particulates and Tar are not only monitored inside the gasifier but also cleaned in secondary processes either at ambient temperatures (physical or mechanical filtration) or at high temperatures (mainly mechanical filtration) Producer gas filtration carried out at 260 °C and above is considered as hot gas filtration in accordance with the VDI guideline 3677-3 [109] Producer gas cooling is an essential step for downstream applications where gas of ambient temperature is required i.e engines Cooling could be done before cleaning or after cleaning, however, it could be done simultaneously Cold gas cleaning is further divided into either dry gas cleaning [103,110,111] or wet gas cleaning [112] Tar filtration (physical removal) is done via cold gas cleaning when in condensed form over the particulate matter [113,114] The cold and hot gas filtration equipment includes various types of scrubbers, dry and wet cyclones, electrostatic precipitators (dry/wet) and various kinds of barrier filters which include fabric filters, rigid filters (ceramic/metallic) and granular (sand) filters An overview of cold and hot gas filtration technologies along with their advantages and features are given in Table 4.2 Thermal and catalytic tar cracking Tar treatment is mainly carried out to decompose heavy aromatic tar species to lighter and less problematic non-condensable gases by means of catalytic cracking or thermal cracking Thermal cracking is decomposition of large organic molecules to lighter gases by heating for a certain residence time at temperature range 33.1 25.1 14.26 25 25–37 14.8 19.3 19.42 28 16–19 Syngas HHV (MJ/Nm3) Tar (g/ Nm3) Ref 4.66 5.31 5.77 5.0 5.80 4.5-6.25 6.5–12.1 – – – – – – 33 to o – 15.2– 0.4 7.8 10.6 – 25.26 – [46] [46] [43] [43] [93] [44] [40] Contaminants (ppmv) 2.02 2.03 2.6 2.60 1–3 8–10 3.5–3.9 3.62–5.14 – – 6.54 (LHV) 13.62 (LHV) 17 [41] [69] [94] [95] [96] [97] [73] from 1000 to 1300 °C [128], with higher temperatures needing shorter residence time [129] High temperatures can be generated (i) using high temperature gasifiers [98,130] (ii) by heating the gas stream via heat exchangers (iii) by introducing air or O2 downstream of gasifier, and (iv) using energy efficient radio frequency [131,132] Thermal cracking increases soot production when applied downstream of the gasifier [129] which hampers the SOFC performance Detailed discussion on thermal tar cracking is available elsewhere in literature [128,133] Tar undergoes reforming or cracking reaction on some catalysts to form gaseous products at certain temperatures The catalysts can be used in-situ either in the gasifier (bed) or can be fed with the biomass feed Alternatively, catalysts are used in the secondary tar reformers/ beds downstream outside the gasifier Catalytic tar cracking reduce the decomposing activation energies hence can occur at low temperature range (500–900 °C) as compared to thermal cracking This method is more advantageous in terms of reducing the complications and costs related to high temperature processes A comprehensive overview (on the basis of Abu El-Rub's classification [134]) of catalysts for tar cracking with their features, limitations and improvements is given in Table It is to be noted that thermal and catalytic tar cracking are more beneficial as compared to cold gas tar filtration (Section 4.1) in terms of energy efficiency by possibly eliminating the gas cooling and subsequent reheating step for hot gas downstream applications like SOFC 4.3 Other contaminants cleaning Sulfur species, halides, alkali compounds and ammonia are soluble in water and they can be removed using wet scrubbing which is more favorable for the cold downstream applications For high temperature applications like SOFC, the removal of these contaminants at high temperatures is favorable Alkali vapors are condensed below 550–700 °C forming particles (o5 μm) adding to particulate matter or combine with tars which are subsequently removed with particle removal systems [135] At higher temperatures than their vaporization temperatures, alkali compounds are removed by hot adsorption onto solid sorbents termed as alkali ‘getter’ (Table 8) For HCl removal, alkali metal compounds are injected into the gasifier which results in forming salts that can be filtered by particulate filtration systems Good solubility of NH3 in water makes wet scrubbing a reliable technology for NH3 removal but thermal catalytic decomposition is employed for hot gas 1362 Table Overview of cold and hot gas filtration technologies for particulate matter Operating principal/ Types Category Temp (°C) Features Advantages Wet scrubbers Liquid (water/oil) removes particulate matter: – venturi scrubbers – impactor scrubbers – spray scrubbers – cyclonic spray scrubbers – wet dynamic scrubbers – electrostatic scrubbers Inertial separation: Particles separated using centrifugal force generated by spinning gas stream in a circular direction Cold, wet, physical o65– 100 Hot, dry, mechanical 1000– 1300 o 500 Efficiency: - Spray scrubbers: 90% ( μm), 40% ( o μm) – Wet dynamic and cyclonic spray scrubber: 95% (5 μm) and o 75% ( o μm) – Venturi: 50% (o μm) – Electrostatic scrubbers: 99% ( o μm) [115] – Removes large quantity of large particles, integral part of fluidized bed gasifiers – Remove 90% of particles with maximum μm dia – Initial gas cleanup device Cyclone with a cylindrical rotating metal gauze in it – Achieve load o 25 mg/Nm3 – Pressure loss; 30–200 mbar – Achieve load 10–20 mg/Nm3 (venturi scrubbers) – Simultaneously cools the gas – Simultaneously removes tar and other – Heat loss contaminants (NH3, HCl, H2S, SO2, alkali – Large size scrubbers – Large water supply for large compounds together with particles) systems – Costly waste water management – Pressure loss; o10 mbar – Simple, robust, cheap – Not effective with sub– Can be connected in series micron particles – Also remove condensed tar and alkali – de-dusted gas still may compounds – Very high flow capacities contain 30 mg/Nm3 – Low energy requirement – Enhanced efficiency from conventional – Clogging by Tar at low temcyclones (95%) peratures [118] – Very low pressure loss – Expensive – High energy required – Low flow capacities – Excessive filter cake causes – Filter cake increases efficiency pressure drop – Tar reduction catalysts can be added into – Tar foul/ plug filters filter elements – Simple, cheap materials used (polyester/ – Materials limit operation wool) temperature and flow capacities – 99.9% removal of o 100 μm – Thermal stresses and reac– Moderate flow capacities tion to alkali vapour reduce filter life – Might get sintered and – Material strength ceramics corroded – 99.9% removal of o μm – Moderate flow capacities – Particles and harmful gaseous compo- – Complexity of solid handling – High energy required nents’ combined removal – Resistant to stresses, corrosion – High flow capacities Cyclones Rotating particle separators Electrostatic precipitators – Inertial separation – filtration Particles charged then collected by opposite polarity collector plates Hot, dry, mechanical Warm/ Hot, dry, mechanical Barrier filters (General) Gas penetrate the porous filter materials but inhibit particles Hot/ warm, dry, physical 90–800 Barrier filters types (Specific) Fabric filters (use temperature resistant woven fibers) Warm , dry physical 90–260 Rigid ceramic filters (use SiC or monolithic and composite ceramics) Hot, dry, physical 400– 800 200– 500 – Remove o μm (dry) particles – filter cake removed periodically by reverse pulsing – Effective for 500 μm – Expensive Nextel’s M material withstand 700 °C [123] – In candle [124] or tube form – Achieve load 5–10 mg/Nm3[63] Rigid metallic filters ( use Iron aluminide/ Hot, dry, physical Iron-Chrome-Aluminum) $ 500 – Achieve load o 10 mg/Nm3 Granular (sand) bed filters:- Utilizes silica Cold/ hot, dry, and sand- Fixed bed or moving bed physical 550– 600 – Efficiency 99% (4 μm), 93% (0.3 μm) [126] – Moving bed eliminates periodic filter cake removal Disadvantages Ref [116] [48,117] [119,120] [48,121] [122] [122] [122,125] [125] [127] Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 Filtration technology Table Overview of catalysts for tar cracking with their main features (adapted from [48, 134, 136], particular references indicated inside the table) Classification A Mineral based Catalysts: – Physically treated only – Abundant – Cheap Catalyst Calcined rocks – Most popular, cheap, disposable – 95% tar conversion – Used in beds to guard expensive metal catalysts from deactivation – Effectiveness: Secondary In-situ Calcites, Magnesites – Activity: Dolomite calcite magnesite Olivine: (Mg,Fe)2SiO4 (Fe, Mg content – Activity: Dolomite Olivine (in-situ) enhances its activity) – Attrition resistance: Olivine Dolomite – Increases C conversion in-situ Silica-Alumina catalyst – Activity: Ni on Al2O3 dolomite activated alumina Clays SiC(inert) – easy disposal Reduced metallic Iron (more active – Activity: Dolomite Fe2O3 Fe3O4 than their oxide form) – abundant – Catalyzes water–gas shift reaction Ni-based – Activity: 8–10 times Dolomite [128] – Water-gas shift capabilities – Complete tar elimination ( $ 900 °C) – Decomposes Ammonia (99% , 900 °C) – Used normally in fixed beds (secondary) – Ni-aluminate cheaper than Pt, Ru and Rh – Ni and Ni/ Al2O3 coating is used on monolithic reactors (Pt, Zr, Rh, Ru, Fe)-and Tungsten based – Activity: Rh Pd Pt Ni ¼ Ru [128] – Rh/CeO2/SiO2 exhibit considerably greater activity at 650-700 °C [138–140], higher coking and H2S tolerance as compared to Ni [141] – Alkali metal carbonates (Direct cat- – Activity: lower than dolomite generally, alysts; K2CO3, Na2CO3, CsCO3 etc.) (rarely exceeds 80% in secondary beds) – Catalysts can come from ash within gasifier – K2CO3/Na2CO3 as direct catalyst added to FB gasifier by mixing with dry biomass Al2O3 (activated by heating to remove – Activity: high (Al2O3 ¼ dolomite) hydroxyl groups from hydrous alumina – High mechanical & thermal stability – Alumina can be combined with metal oxides; NiO, V2O5, CoO, CuO, MoO3, Fe2O3 etc 4Aluminosilicate Zeolites (SiO4) and – Experienced with FCC units ( in petroleum 5(AlO4) industry) – Activity: Dolomite Zeolites – Cheaper, Sulfur tolerant and more stable than regular alumina based catalysts – Cheapest among synthetic catalysts Synthetic, non-metallic (Semicoke, – Natural production inside gasifier charcoal, activated carbon, char from poplar wood-produced by pyrolysis) – Activity: Ni Pinewood/biomass char biomass ash FCC dolomite olivine silica sand [142] – Dolomites (CaCO3.MgCO3) – limestone (CaCO3) – Magnesium Carbonate (MgCO3) Silicate Mineral Clay minerals Iron rich minerals Fe2O3, Fe3O4, HFeO2, FeTiO3 B Synthetic Transition metal based catalysts catalysts: – Chemically treated – mainly Ni based – Expensive Alkali metals (group 1A) – K and Na exist naturally in biomass (switchgrass), its ash acts as catalyst Activated Alumina Fluid Catalytic Cracking catalysts (zeolite based) – Crystalline aluminosilicate minerals Char Dolomites (calcined by losing bound CO2 when heated) at 900 °C Features/Advantages Limitations/ Disadvantages Improvements – Deactivated by carbon deposition and – Activity increases by increasing recarbonation if CO2 partial pressures Ca/Mg ratio & active metal content (Fe) in the reactor are too high – Low attrition rate in secondary – High attrition rates in-situ beds than in-situ – Deactivated by carbon deposition – Adding Ni into natural olivine increased activity, reduced deactivation (in-situ) [137] – Degrades (pore structure of catalyst) – Activity increases by increasing pore dia ( 0.7 nm), internal at 800 °C surface area – Promote coking – Rapidly deactivated (in absence of H2) – Combined iron oxide with silica by carbon deposition – Adding supports (Al2O3) and pro– Poisoned by H2S – Contaminated by coking ( o 900 °C) moters (La and Co) for in-bed use – Not robust for Fluidized Bed due to – Adding supports (Al2O3, SiO2, deactivation and attrition MgAl2O4) and promoters (Mg, Ca, – Sintering causes loss of surface area K) for secondary use – Expensive catalysts – Long term stability not proven and activity – High Price can be mitigated by combining cheaper supporting and promoting materials – Deactivated by particle agglomera- – Wet impregnation of biomass with K2CO3 decreases agglomeration tion, coking, K vaporized at 900 °C – Increase in ash byproduct and hence deactivation – Difficult/ expensive catalyst recovery – Rapid deactivation from coking as – Adding MgO as promoter and Ni compared to dolomite as active sites reduce coking and H2S poisoning – Deactivated by coking – Poisoned by steam, nitrogen and alkaline metal compounds – Quickly elutriated from the bed of fluidized bed gasifier – Deactivated by coking – Consumed by gasification reactions inside gasifier Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 Material – Activity & lifetime: Zeolites ỵ Ni pure zeolites Coking resistance: Zeolite supported Ni pure Ni catalysts – Successive gasification regenerate the catalyst – Char-dolomite mixtures are used 1363 1364 Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 Table Overview of various sorbents used for cleaning of contaminants other than particulate matter and tar Syngas Containment Sorbents / catalysts – Most common, 99% H2S removal to ppmv at 350-450 °C Above 450 °C ZnO is reduced which is prevented by combining with other metal oxides i.e zinc ferrite (can be used upto 600 °C) and zinc titanates (can be used even above 600 °C) – Doping ZnO sorbents with CuO – enhances sulfur adsorption and regeneration ability (have longer lifetime) Metal oxides (Titanium, Fe, Mg, – 1) Disposable: Ca based sorbents, injected into the gasifier Al2O3 etc.) – 2) Regenerable: Used in separated fixed bed reactors downstream Removal capabilities are (between 77-650 °C) [147] Copper based – It is also reduced like ZnO at elevated temperatures Ceria based – Reduces H2S concentration from 300 ppmv to ppmv at 700 °C Dolomite and lime particles – Removal of Sulfur via adsorption ( 85%) on dolomite and lime particles injected into gas stream Removal from gas with particle filtration – Alkali and alkali earth metal car- – HCl adsorption through chemisorption, HCl removal at 550 °C bonates: (Na2CO3, K2CO3, KCl, [148], commonly employed in fixed beds – Nahcolite (NaHCO3): inexpensive, suitable for fixed and FB gasiNaCl etc.) – Alumina/ Silica minerals fiers, reduces HCl o ppm at 600-700 °C – Ca(OH)2 , Mg(OH)2 and NaHCO3 (all three) supported on Al2O3 reduces HCl to o ppm at 550 °C Alumina/ Silica minerals – Chemisorption, removal of alkali at 840 °C Clays or kaolinite – Chemisorption, kaolinite used at 1000 °C both in-situ (injecting sorbents) and secondary (bed of sorbents) Activated alumina/ bauxite – Physisorption, used at 1000 °C both in-situ and secondary, η ¼ 98– 99% in 0.2 s, sorbent regenerated by boiling water and reused – Dolomite, Ni- and Fe- based – Thermal catalytic decomposition employed for hot gas cleanup of catalysts NH3 instead of its removal from syngas – Ni-, Mo-, Ru-based (expensive) catalysts exhibits $ 70% conversions at 700–960 °C – Dolomites and Fe- based (cheaper) catalysts exhibits ammonia conversion rates of 85% to 99% at 900 °C ZnO Sulfur compounds (mainly H2S, SO2) – Sulfur is removed as H2S rather than as a combustion byproduct (SO2) Sorbents can be regenerated Halides (mainly HCl) Alkali compounds Nitrogenous species (mainly NH3) Features cleanup of NH3 instead of its removal from producer gas Sulfur species from coal gasification syngas have long been removed using alkaline wet scrubbers On the other hand, ZnO sorbents have been used for the elimination of sulfur species in fluidized bed gasifiers at higher temperatures The overview of different sorbents/ catalysts used for removal of sulfur-, chlorine-, nitrogenand alkali- compounds is briefly given in Table Fuel cells A fuel cell converts the chemical energy of a fuel into electrical energy, water vapour and excess heat Fuel cells give much higher efficiencies being one-step energy conversion (chemical to electrical) systems as compared to conventional multi-step (chemical to thermal to mechanical to electrical) thermo-mechanical processes A fuel cell comprises of three components (i) anode; a catalytic fuel electrode, (ii) cathode; a catalytic oxidant electrode and (iii) an electrolyte which prevent the electrodes to come into electronic contact In SOFCs, fuel gas (H2/CO) is fed continuously and gets dispersed over the surface of porous anode which conducts the electrons released by the oxidized fuel components An oxidant (O2 or air) is fed continuously and get distributed over the surface of porous cathode which conducts the electrons coming from anode via the external circuit These electrons react with O2 at the cathode–electrolyte interface to form oxygen ions which migrate to the anode through the electrolyte where they react with fuel at the anode–electrolyte interface to produce H2O/CO2 whilst releasing electrons An electrolyte is either a negative ion conductor or positive ion conductor (depending upon the fuel cell type) by blocking the electrons creating a current through an external circuit which could be used before reaching the cathode There are different possible combinations of fuel and oxidant depending upon the type of the fuel cell (see Table 9) Fuel cells are classified Ref [143–146] [60,148,149] [60] [61,150] [149] primarily by the type of the employed electrolyte Each type differs in their operating temperatures, power outputs, electrical efficiencies and typical applications A comprehensive comparison of main fuel cell types i.e polymer electrolyte membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC) is given in Table As compared to the conventional energy generation technologies, fuel cells offer advantages like higher efficiencies, environmental friendly, quite operations, fuel flexibility, modularity and quick installation, energy security and good opportunities for cogeneration systems especially with high temperature fuel cells i.e SOFCs Fuel cells are viewed by many as the energy source of the 21st century [151] 5.1 The choice of fuel cell type for biomass integrated gasification– SOFC systems While low temperature fuel cells such as PEMFC, AFC and PAFC require pre-processing of fuel as they work essentially with hydrogen only, high temperature MCFC and SOFC are capable of using hydrocarbons as fuel Light hydrocarbons are internally reformed to H2 and CO inside MCFC and SOFC itself due to their high operating temperature At the same time, CO is electrochemically oxidized in MCFC and SOFC which is otherwise considered poison for PEMFC, AFC and PAFC Comparing MCFC and SOFC, the latter has relatively more advantages SOFC is all-solid construction and the problems associated with the molten electrolyte (corrosion and electrolyte loss) of the MCFC are thus eliminated Solid construction allows several stack configurations while MCFC cannot be oriented in different directions Additionally, the cathode of MCFC requires CO2 that adds significantly to the system complexity Furthermore, biomass gasification is generally carried out around 900 °C – the typical operating temperature of SOFC, that makes the integration of gasification and SOFC, Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 1365 Table Comparison of various fuel cell technologies (adapted from [151–154] in principle, simple and viable High quality heat produced by SOFC provided excellent opportunities for cogeneration and combined cycle systems Hence, SOFC are the most promising candidate for utilization with biomass gasifier, however there is a need for cleaning of the producer gas (see Section 7) prior to feeding into the SOFC 5.2 Solid oxide fuel cells The structure and properties of materials in SOFC are of great importance in achieving high performance and long term durability Throughout the years, different anode [155–158], cathode [157,158] and electrolyte [158] materials have been investigated Cathode of SOFC is typically made of Sr- doped lanthanum manganite (LSM) which has high electronic conductivity and catalytic activity for oxidant reduction The electrolyte of SOFC is yttria stabilised zirconia (YSZ), a solid- completely gas impermeable and possesses high ionic conductivity and no electronic conductivity The state-of-the-art anode material is a composite of Ni cermet and YSZ Nickel being a good electronic conducting material, is also a good catalyst and enhances the fuel oxidation while YSZ is only an ion conductor Another type, Ni/GDC (Gadolinium Doped Ceria) anode also performs well with hydrocarbons as fuel [156] As compared to YSZ, GDC provides a larger surface area for reactions at the electrode–electrolyte boundary as it is a mixed conductor in reducing environment The use of SOFCs with different fuels i.e biogas (Section 5.3), producer gas from coal gasification and biomass producer gas (Sections and 8) has been actively researched recently Most of the research has been done on the commonly used Ni-YSZ anodes Hydrogen as a fuel exhibits to times higher oxidation rate as compared to CO (at 1000 °C) due to the larger diffusion resistance of CO than H2 on the Ni-YSZ anode surface [159] While CO (a major component in producer gas) is poor fuel as compared to H2, it takes part in water gas shift reaction and produces additional H2 Anodes of different materials have been tested for better SOFC performance and resistance to carbon deactivation from the fuels containing hydrocarbons For example, Cu/CeO2/YSZ anodes indicated higher electrochemical oxidation of H2 and CO as compared to Ni/YSZ anodes and Cobalt incorporation to Cu-CeO2 resulted in even higher power densities with CO than those of H2[160] The producer gas from biomass gasification has been tested in SOFCs containing Ni/YSZ and Ni/ GDC anodes (see Sections and 8) Sulphur tolerant anode materials might reduce the producer gas cleaning requirements (Section 6.3) hence reducing the cost of biomass-SOFC systems The performance of SOFC on producer gas depends mainly on the type of the anode used, as the fuel is fed to the anode side Secondly, the tolerance level and effects of impurities present in producer gas may vary considerably based on the anode materials Therefore, the selection of the SOFC materials especially the anodes and suitable gas cleaning systems are very critical in the development of efficient yet cost effective biomass integrated gasification–SOFC systems However, selection of the suitable anodes at this point is rather premature to decide because tolerance towards several impurities for various anode materials is still not well known and requires further research A single SOFC generates V of DC current, so to reach a higher potential they are connected in series in stacks using interconnecting plates between the cells Interconnecting plates also called bipolar plates connect the anode of the one cell to the 1366 Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 cathode of the next cell (see Fig 3) Several cell designs exist, but the two most common designs are the planar and tubular designs (Fig 3) SOFCs can be classified into various types according to their operational temperature, type of support, cell and stack design and flow configuration as described in the Table 10 5.3 Biomass derived fuels for SOFC SOFCs offer a great advantage of being fuel flexible Biomass can be converted to the various products which could be used as a fuel for SOFCs Ethanol is biochemically produced from biomass fermentation and considered a very promising fuel for SOFC due to its high hydrogen content and high heating value [161] Sulzer HEXIS (former) kW stack has been successfully operated with fermentation gas for over 5000 h with steam reforming employed for conversion of ethanol to hydrogen [162,163] Biogas is another example which is biochemically produced through anaerobic digestion of biomass Biogas is mainly composed of methane in the range of 55–60% [164] Biogas from a sewage treatment plant has been used to generate electricity and heat using SOFCs [165] Amongst thermochemical conversion methods, fast pyrolysis converts biomass into bio-oil which is a liquid mixture of (carbonyl, phenolic and carboxyl) oxygenated compounds For bio-oilSOFC integrated systems, the bio-oil should be reformed to hydrogen, catalytic steam reforming considered being the best choice [166] Biomass gasification and SOFC systems have been studied the most as compared to the SOFCs integrated with other biomass conversion methods Anyhow, all of these products and Fig SOFC stack types (a) planar (co-flow) (b) planar (counter-flow) (c) planar (cross-flow) (d) planar radial by Sulzer HEXIS [168] (e) tubular [169] Table 10 Classification of solid oxide fuel cells Classification Types Description Temperature – High temperature: HT-SOFC – Intermediate temperature: IT-SOFC – Low temperature: LT-SOFC – Advantages of HT-SOFC (800–1000 °C) over IT-SOFC (650–800 °C) and LTSOFC (500–650 °C): (1) Decreased resistivity of components, hence decreased ohmic polarization (2) Increased electrode kinetics results in faster reactions, hence decreased activation polarization (3) Higher exhaust heat availability – Disadvantages of HT-SOFC over IT- and LT-SOFC: Type of support – Self-supported (by anode, cathode or electrolyte) – Externally supported (by interconnect or substrate support (1) Require longer start-up and shut-down time (2) Structural integrity becomes weaker (3) Corrosion rate increases – Supporting component should be thick enough to give strength to the cell – For HT-SOFC, electrolyte support is preferred because electrolyte ionic resistivity decreases at higher temperature For IT- or LT-SOFC anode or cathode support preferred – Sometimes interconnect or a porous substrate gives support/ strength to the cell – Advantages of planar SOFC over tubular SOFC: Cell and stack design – Planar (flat- and radial- SOFC) – Tubular (by formerly Siemens-Westinghouse now Siemens) Anode, cathode and electrolyte layers deposited on (1) Simple geometric configuration (2) Low fabrication cost (3) More compact cylindrical tube as large voids required in tubular design (3) Simpler series of electrical connection between cells (4) Shorter current path hence lower ohmic losses – Disadvantages of planar SOFC over tubular SOFC: Flow configuration – Co-flow, - Cross-flow and – Counter –flow (1) Gas tight sealing required (2) Cells may expand and contract without constraints in tubular design – The co-flow configuration exhibits uniform temperature distribution and smallest thermal gradients at same operating conditions as compared to other two types Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 their conversion technologies are compatible to be integrated with SOFCs resulting in highly efficient energy conversion systems which could make them one of the key energy technologies in future Once the biomass based SOFC systems (Bio-SOFC) are fully realized, these can be used as major distributed generators providing combined heat and power on site as well as contributing to levelling of solar and wind unstable power supplies (see Fig 4) [167] Effects of producer gas impurities on SOFC performance SOFC anode performance on gas compositions representing producer gas from different gasifiers show that if producer gas is sufficiently cleaned, it will not exhibit challenging problems for SOFC [170,171] The amount of impurities in the producer gas could be mitigated to a certain level by the selection of appropriate biomass type, gasification agent and gasification technique but could not be eliminated completely Hence, additional downstream gas cleaning is necessary to meet the producer gas cleaning requirements for SOFC The influence of producer gas impurities on SOFC operation are presented below in following sections 1367 observed except in one of the tests when producer gas containing particles (o5–10 μm) reached the SOFC anode when particle filtration was not sufficient (without metal candle filter) This test was stopped early Particulates were found on the anode surface during post investigations Hofmann et al suggested that prolonged exposure to particulates could clog the porous structure, impede diffusion paths and reduce catalytic area of anode causing irreversible degradation of SOFC This scenario might lead to the delamination of anode layer due to mechanically induced tensions [173] However, several experiments showed successful short term SOFC operation on cleaned producer gas using ceramic or sintered metal filters [173–176] Particles in producer gas are carbon rich compounds at room temperature and might oxidize at the anode at operating temperature of SOFC but this need to be confirmed Therefore, further research is required to know the detailed influence of particulates on successful operation of SOFC particularly on long term basis Moreover, if particle removal is employed below 400–450 °C, tar removal has to be carried out before particle removal because tar condenses at 400–450 °C On the other hand, alkali compounds in producer gas condense at 700–550 °C, if the particle filtration is carried out above this temperature; alkali getters (see Table 8) will be required unless the fuel contains very low alkali content 6.1 Particulate matter 6.2 Tar Precise allowable limits of the particulates in the producer gas to SOFCs and their influence on cell performance are either not documented in detail or not found in literature However, it is presumed that, particles are essentially to be reduced as much as possible (a few ppmw probably) for successful SOFC operation on long term basis Particles in the producer gas are in sub-micron to micron sizes (see Table 3) The SOFC anode pores match with the particulate sizes and could be blocked by them Study performed on different anode pore structures showed that SOFC performance (on H2 fuel) is significantly reduced by decreasing the effective gas diffusion paths, number of electrical conduction paths and the electro-chemically catalytic sites for anodic reactions [172] Hofmann et al [173] tested planar electrolyte-supported SOFC with Ni/GDC anode on real producer gas from biomass gasification The gas was hot cleaned from HCl, H2S and particulates (cyclone and sinter metal filter) No considerable contamination of anodes was Tar may not be poisonous to SOFC as they may be reformed and oxidized hence acting like fuel to SOFC But the presence of tar may also induce carbon deposition deactivating the catalysts (Ni) and degrade the cell especially in dry conditions [177,178] The impact of tar, such as carbon deposition on SOFC significantly varies with the anode material and operating conditions of SOFC i.e steam/carbon (S/C) ratio, temperature, and current density Singh et al [179] presented a thermodynamic analysis (of 32 species) to study carbon deposition in SOFC anodes due to the tar present in producer gas Gas composition representing that from an air blown gasifier was used with varying steam content The mixture of toluene, naphthalene, phenol and pyrene represented the complex nature of tar The increase in S/C ratio resulted in decrease of carbon deposition on the anode It also showed initial decrease and then increase with the increase of temperature but Fig Micro-grid system using Bio-SOFC as a major distributed power source as depicted by Shiratori et al [167] 1368 Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 was found minimum in the upper operating range (850–950 °C) of SOFC The increase of current density caused carbon deposition to be decreased until it became zero beyond the threshold current density [179] This theoretical study was found in agreement with the experimental results of Mermelstein et al that increase in current density reduces the carbon deposition [180] Mermelstein et al also showed that carbon deposition was reduced significantly with increase of S/C ratio ( 41) in both (Ni/YSZ and Ni/GDC) anodes fed with 15 g/Nm3 of benzene as model tar in a H2/N2 mix fuel at 765 °C for 30 [181] Lorente et al confirmed that carbon deposition reduces with the increase of S/C ratio [182] Anyhow, Ni/ GDC anodes are found to be more resistant to carbon formation as compared to Ni/YSZ anodes because ceria is capable of resisting carbon deposition [180,183] Aravind et al reported no significant impact on planar Ni/GDC anodes in single gas (humidified H2) atmosphere up to 110 ppmv naphthalene (model tar) for 90 operation at the temperature range of 750–850 °C Rather, it was indicated that tar might have reformed on the anode [184] Liu et al recently studied electrolyte supported SOFC (with Ni/GDC anode) fed with producer gas composition and tar (toluene) concentration identical to that from an air blown gasifier under wet and dry conditions It is reported that SOFC anode is likely to tolerate carbon deposition with the feed gas under wet conditions It was reported the same with the feed gas under dry conditions but only under mild current load, however carbon is likely to be induced on the anode at open circuit under dry conditions Adding CO2 to the feed gas showed the suppression of carbon deposition too The SOFC performance was enhanced with the increase in tar concentration in the feeding gas under the suitable SOFC operating conditions, indicating that the tar reforms at Ni/GDC anode [185] In recent studies, Lorente et al compared the effect of a model tar (toluene) and real tar (from a coal gasifier) to carbon deposition behavior over Ni-GDC and Ni-YSZ anode materials (powders) Materials were exposed to tar (15 g/Nm3) in a H2/N2 mix fuel at 765 °C for h It was found that a model tar (toluene) overestimated the anode degradation as compared to real gasification tar [182,183] Some of the published work showed promising SOFC operation on real producer gas as fuel One kW stack from HEXIS was operated by Nagel et al for 30 h with producer gas from an updraft gasifier (tar $ g/Nm3) employing catalytic partial oxidation before SOFC It was reported that carbon deposition was satisfactorily eluded however ash deposits found to be the main obstacle [176] Hoffman et al reported successful operation of a SOFC with Ni/GDC anode for 150 h on producer gas from an innovative gasifier (Viking [186]) The producer gas was (almost) tar free from Viking gasifier but was cold cleaned from other impurities [175] A similar cell was operated with no significant degradation or carbon deposition during h operation with tar laden (410 g/Nm3) gas from CFB gasifier at Delft [174] Biollaz et al have not reported problems for tubular SOFC fed with biomass derived gas from updraft gasifier diluted with synthetic gas (1:20) with tars (5 mg/ Nm3) for 1200 h [187] It can be concluded from the current published results that the probable tolerance level of tar in the producer gas for SOFC with Ni/YSZ anode is around a few ppmv However Ni/GDC anodes might be fed with large amounts of tar which might get reformed at these anodes under suitable SOFC operating conditions Further research is required to confirm the effect of different types of tar on different types of anodes and also to find the detailed information about the suitable SOFC operating conditions for successful long term SOFC operation 6.3 Sulfur compounds At anode conditions, all sulphur compounds are converted into H2S which is chemisorbed on the nickel surface, passivating the active sites, thus affecting the fuel oxidation reactions The influence of H2S is widely studied on SOFC performance and considered poisonous even at a few ppmv levels The H2S poisonous effect is considered reversible at moderate levels, while higher concentrations will cause irreversible damage to the SOFC anodes The effect of sulfur and their allowable limits for SOFCs varies with fuel composition, SOFC anode material and operating conditions i.e current density and temperature Matsuzaki and Yasuda found that H2S poisoning increases with decreasing temperature The increase in the polarization resistance and overvoltage was noted for Ni/YSZ anode (fed with H2) when H2S in the feed gas was increased by 0.05 ppm at 750 °C, by 0.5 ppm at 900 °C and by ppm at 1000 °C The poisoning effect was reversible when H2S was removed [188] Lohsoontorn et al also found that H2S poisoning effect increases with decreasing temperature from 600 °C to 555 °C on Ni/GDC anode [189] Rasmussen and Hagen demonstrated that a single cell (with Ni/YSZ anode) had a reversible effect in H2S concentration between ppm and 100 ppm Voltage dropped by 10% from adding ppm H2S in H2 containing fuel at 850 °C and 1000 mA/cm2 Voltage changes of the cell suggested that poisoning effect is mainly due to the chemisorption of sulfur on Ni and the saturation coverage reached at 40 ppm H2S No significant change of anode microstructure was detected after the tests [190] Norheim et al tested a cell containing Ni/YSZ anode with the mixture of H2/CO2 containing H2S and found that cell performance reduced with the increase of sulfur content up to 80 ppm but no further decrease was observed from 80 to 100 ppm The operating voltage at a current density of 200 mA/cm2 dropped from 0.81 V at ppm H2S to 0.79 V at 100 ppm H2S [191] In another test the cell performance fully recovered after the 240 ppm H2S exposure to cell was removed indicating no irreversible changes again [191] Aravind et al showed that the electrochemical oxidation of hydrogen at Ni/GDC anodes is not affected significantly up to ppmv of H2S added in the feed gas (humidified H2) at 750 °C and 850 °C for 1.5 h operation [184] While it has been previously demonstrated that even ppmv of H2S presence in the feed gas containing methane reduce the performance of the cell significantly for methane oxidation [171] Schubert et al studied a single cell SOFC stack (with Ni/GDC anode) fed with simulated producer gas (27% H2, 11% CO, 6.2% H2O, 2.8% CO2, 2% CH4, and 51% N2) and H2/steam mixture (45/5/50 Vol% H2/H2O/ N2 respectively) at operation temperature of 850 °C The voltage drop of 20 mV was observed immediately after the H2S addition of ppm followed by stable cell voltage for 24 h Further addition of H2S up to 30 ppm caused additional voltage drop of only 1–2 mV followed by the stable voltage for several hours [192] These results [192] suggest that SOFC with Ni/GDC anodes can be used for producer gas applications for extended time durations as compared to Ni/YSZ anodes On the other hand, Błesznowski et al recently showed that Ni-YSZ anodes can tolerate up to only ppm H2S The 200 h tests indicated negligible impact of up to ppm H2S in the fuel (47.5% H2/ 47.5 N2/ 3% H2O) at 800 °C Significant voltage declined at ppm H2S [193, 233] In one of the studies, Nagel et al investigated the influence of thiophene (S-containing tar component) added in natural gas as a fuel on kW SOFC stack at 950 °C Sound stack operation was reported Thiophene added up to 400 ppmv caused 6% stack degradation [194] Several studies regarding the influence of H2S on catalytic activity of Ni (anodes) revealed that internal reforming reactions are more sensitive to H2S poisoning as compared to electrochemical reactions [171,195] Also, H2S poisoning effect is more prone to methane reforming and water gas shift reactions as compared to CO oxidation [196] Producer gas from steam gasifiers Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 usually have high methane content and might be more sensitive towards H2S poisoning as compared to the producer gas from airblown gasifiers In designing the gas cleaning systems for biomass-SOFC systems, it is assumed that sulfur levels in producer gas should be brought down to ppmv for SOFCs (especially with Ni/YSZ anodes) 6.4 Halides - HCl There is scarce literature found on the influence of HCl on SOFC operation in detail It is postulated that HCl and Cl2 may attack the Ni of the anode causing cell degradation [197,198] but there still remains unanswered questions about the nature of HCl poisoning of the SOFC and requires further research Anyhow, HCl definitely cause corrosion of the system components Buchinger et al showed that micro-tubular SOFC (with Ni/YSZ anode ỵ 5% CeO2 inside) fed with H2 gas with HCl up to 47.4 ppm at 850 °C did not cause considerable effect on cell operation When combining HCl with H2S in feed gas, increasing the HCl concentration had no prominent effect but increasing H2S concentration showed slight decrease in cell power Anyhow, stable cell performance was demonstrated for several tens of hours when fed with 10 ppm HCl and ppm H2S [199,200] Trembly et al tested an electrolyte supported SOFC (with Ni/GDC anode interlayer ỵ Ni/YSZ current-collection layer) using simulated coal syngas at 800 °C Introduction of 20 ppm and 160 ppm HCl in the fuel showed degradation of 17% and 26% in 100 h respectively The cells have shown to sustain stable operation after 50 h of operation with 20 ppm and after 100 h with 160 ppm HCl [197] Haga et al found the degradation rate of cell voltage poisoned by ppm Cl2 to be 3% / 1000h [198] Aravind et al showed that HCl up to ppmv in humidified H2 (as fuel) is well tolerated in Ni/GDC anode at 850 °C and cell did not show any significant degradation in 90 [184] In one of the recent studies, Xu et al [201] found 1.6% performance loss during 300 h test of anode (Ni/YSZ) supported SOFC exposed to simulated coal syngas with 100 ppm HCl under constant current load at 800 °C The degradation was relatively less than reported by Trembly et al [197] and Haga et al [198] who tested electrolyte supported cells Xu et al [201] suggested that anode supported cell might have more tolerance to HCl than electrolyte supported cell Recently, Błesznowski et al showed that anode supported SOFC with Ni/YSZ anode can tolerate upto 10 ppm HCl without significant performance degradation [193] These results discussed above from literature indicate that cleaning of HCl to a few ppm in biomass producer gas may be required before it could be fed to SOFC 6.5 Alkali compounds Alkali content of biomass feedstock (mainly K and Na) can vaporize at gasification temperatures (see Table 3) and may exist in the producer gas from the gasifier in the range of 1–10 ppm 1369 measured just after the cyclone without any cooling [202] The Cl present in the biomass correlate with alkalis forming NaCl, KCl, ZnCl etc which have been barely studied with regard to their influence on SOFCs [203] or neither Nurk et al nor we could find it in literature In one of the rare studies, Nurk et al demonstrated that the SOFC performance decreased with the presence of ppm KCl in the feeding fuel while 3500 ppm KCl in the feed caused anode pore blocking resulting delamination of anode due to alkali salt deposits [203] Also, alkali compounds are known to cause corrosion in downstream equipment and poison fuel reforming catalysts [117] This indicates that stringent alkali cleaning would be required (upto levels of a few ppm) before feeding producer gas into SOFC However, this has to be confirmed with experiments 6.6 Nitrogenous species – NH3 Ammonia is identified as a fuel for SOFC as it dissociates into H2 and N2 at the SOFC anode SOFC fed with NH3 as a fuel has demonstrated comparable performance with SOFC fed with hydrogen as a fuel [204] Dekker et al showed that decomposition of ammonia depends upon the catalyst availability and the SOFC temperature The total decomposition of ammonia takes place at 850 °C at the anode [205] Hence removing Ammonia from producer gas as SOFC feed gas is considered to be not required Gas-cleaning schemes for biomass integrated gasificationSOFC systems Information gathered from literature regarding the impact of impurities on SOFCs and an analysis of the available cleaning options (see Tables 6–8) indicates that the producer gas from biomass gasification could be cleaned at different temperatures using available technologies to meet SOFC cleaning requirements (see Table 11) However, it has been noted that Ni/GDC anodes are comparatively more tolerant to the impurities as compared to Ni/ YSZ anodes While gas cleaning at lower temperatures (near ambient temperature) is more mature, high temperature gas cleaning will help to yield higher efficiencies in biomass integrated gasification–SOFC systems Aravind and de Jong developed high temperature gas cleaning scheme (Fig 5-left) with 600 °C as the minimum temperature within the cleaning system [107] An intermediate temperature gas cleaning unit (Fig 5-right) has been tested at Technical University of Graz [206] and the same design was improved for higher flow rates at Technical University of Delft [207] A relatively low temperature gas cleaning scheme has also been tested to clean gas from two stage downdraft gasifier at Federal University of Itajuba, Brazil The scheme used was; Gasifier Sand filter at 150 °C - Venturi scrubber at 150 °C - spray scrubber at 50 °C - H2S cleaning with activated carbon at 150 °C - Ceramic filter at 350 °C Experiments showed that this low Table 11 Summary of probable tolerance limits of producer gas contaminants for SOFC with cleaning options Contaminants Tolerance limit Low temperature cleaning options High Temperature cleaning options Particulate Few ppmw Sand filters, wet scrubbers, wet electrostatic precipitators, ceramic filters Wet scrubbers, wet electrostatic precipitators Cyclones in combination with - rigid barrier (metal/ceramic) filters/ granular bed filters/ bag filters Tar cracking with catalysts (see Table 10), tar thermal cracking Wet scrubbers, activated carbon Wet scrubbers Removal with particulate matter via filtration (see Table 6) Sorbents (see Table 8) Sorbents (see Table 8) Removal as solid particles ( o 600 °C), alkali getter ( 4700 °C) Tar Tens to few hundred ppmv H2S ppmv HCl Few ppmv Alkali compounds Few ppmv 1370 Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 From gasifier 850 °C From gasifier 350 °C Ceramic filter at 850 °C Ceramic filter at 350 °C Tar cracking with dolomite at 900 °C HCl cleaning with NaOH/Al2O3 at 350 °C HCl cleaning with Na2CO3 at 600 °C H2S cleaning with ZnO at 350 °C H2S cleaning with Zinc Titanate at 600 °C H2S cleaning with CuO at 450 °C Alkali cleaning with activated Alumina at 850 °C Tar cracking with dolomite at 800 °C Ceramic filter at 850 °C Tar cracking with Ni at 800 °C To SOFC at 850 °C To SOFC at 350 °C Fig Flow scheme for the high temperature (left) and intermediate temperature (right) gas cleaning [207] temperature gas cleaning system is able to remove 95%, 96%, 91% and 68% of tars, particulates, H2S and HCl respectively [207] 7.1 Producer gas conditioning Producer gas conditioning encompasses the system-integrated conversion of hydrocarbons and oxygenated organic compounds of the producer gas into hydrogen rich gases which is more suitable fuel for the fuel cells Steam reforming is one of the most common gas conditioning method of hydrocarbons which takes place over 800 °C with suitable catalyst, Ni being the most common The oxygenated organic compounds in producer gas found to be reformed even at lower temperatures [166,208] Operating temperature of SOFC and Ni being used as a catalyst in them offer the possibility of either direct internal steam reforming inside the SOFC at the anode or external steam reforming in a separate reactor by means of heat exchangers utilizing heat of flue gas from SOFC The later method allows using low cost standard steam reforming reactors but heat transfer limitations hampers the system efficiency The former method is simpler and more efficient as the electrochemical oxidation of hydrogen at anode generates the steam and heat required for internal steam reforming An Alternative to steam reforming is partial oxidation (PO) of hydrocarbons using oxygen at 850–1700 °C [209] PO process has been improved by using catalysts which reduces the operation temperature to less than 800 °C and called Catalytic Partial Oxidation (CPO) [210] Autothermal reforming process [211,212] is the combination of the steam reforming and CPO hence reducing the required steam in steam reforming process because such system uses both molecular oxygen as well as steam as oxygen source CPO and autothermal reforming have almost completely reformed the tar of the pyrolysis gas into H2 and CO [212] Studies of biomass integrated gasification–SOFC systems When looking at the studies performed regarding gasifier– SOFC systems, experimental investigations as well as thermodynamic/ simulation analysis has been performed in parallel Theoretical models and simulations are very useful tool to predict the complex details of the systems involved in achieving high efficiencies On the other hand, understanding the technical feasibility of such systems essentially require experimental studies This review is mainly based on experimental studies (mostly covered in Section 6) The detailed review of simulation and modeling studies are not the part of this paper, anyhow the results of some of the recent theoretical studies are summarized here Table 12 Comparison of gasifier-SOFC systems with gasifier-engine systems Bold: Modeling results Gasification system Electric power unit Power output (kWel) ηel (%) ηth (%) ηCHP (%) Ref Updraft (Harboore plant) FICFB (Guessing plant) DD DD DD (Viking) DD (Viking) CFB CFB ICE 1400 28 55 83 [225] ICE 2000 25 56 81 [226] 12.5 21.3 36.4 50.3 41.4 68.4 – – 30 30 – – 50 84.4 80 80 – – [227] [228] [217] ICE ICE SOFC SOFC-MGT SOFC SOFC-MGT 200 10 181.5 251 37.1 61.3 [222] (also Table 13) to show that such systems offer the possibility of higher efficiencies Table 12 compares gasifier–SOFC systems with their counterpart i.e gasifier-ICE systems Alderucci et al in 1994 was probably the first who performed a thermodunamic analysis of FB gasifier integrated with SOFC He predicted SOFC electrical efficiencies of 47% and 51% for steam and CO2 respectively as gasifying agents when gasifier operating temperature was 700 °C [213] Omosum et al studied the integration of SOFC with biomass gasification for CHP using steady-state model in the gPROMS modeling tool He compared SOFC combination with DD gasifier with cold gas cleaning (efficiencies of 20.8% electrical and 33.9% overall) and a FB gasifier with hot gas cleaning (efficiencies of 22.6% electrical and 59.6% overall) The capital cost was marginally higher for hot cleaning option (2900 d/kW) as compared to cold cleaning process (2600 d/kW), the higher efficiency and available extra heat which could be sold may justify the additional cost [214] Sucipta et al performed electrical efficiency analysis of biomass based (tubular) SOFC-MGT system fueled by producer gas using air, oxygen and steam as gasification agents and reported electrical efficiencies to be 46.4%, 48.9% and 50.8% respectively [215] Fryda et al simulated the combination of air blown FB gasifier with tubular SOFC and/or mGT in a CHP system (o1 MWe) using AspenPlusTM software and achieved the exergetic electrical efficiency of 35.6% for SOFC-MGT system [216] Bang-Moeller et al worked using simulation tool Dynamic Network Analysis (DNA) based on a 500 kWth fixed bed air steam blown gasification plant Results of this study are summarized in Table 13 [217,218] Aravind et al reported thermodynamic simulations with a 100 kWe power plant based on a fixed bed air gasifier and a hybrid SOFC-MGT system reporting an electrical efficiency of 54% [6] Toonssen et al studied the influence of gasification technique and gas cleaning Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 1371 Table 13 Summary of recent theoretical (simulation and modeling) studies on biomass integrated gasification-SOFC systems Power unit Gasification and cleaning equipment specifications SOFC and auxiliary equipment specifications Results Ref Gasif type Biomass type Oxidant Gasif temp (°C) Gas cleaning Tmin(°C) Tar refr SOFC type (Anode) J (A/ cm2) {Press.} (bar) Uf (%) SOFC exit temp (°C) Turbine inlet temp (°C) Pnet (kWel) ηel (%) ηCHP (%) – Air – Steam 600 900 800 50 430 70 No No Yes – – 0.25 – – 70 950 950 900 – – – 122 120 140 21 23 36 34 60 – [214] SOFC DD FB FB SOFC-ST SOFC – FB Wood Olive kernel Cathode air 1300 Air 807 544 450 Yes Yes 80 75 995 900 1179 – 10700 170.3 42 20 – 62.3 [230] [216] – 85 85 – 900 1000 900 900 1000 225.7 349.9 88.47 26.1 40.6 53.9 70.7 58.1 72.3 [6] 85 85 – 85 800 800 – 800 – – 900 $ 700 1000 181.5 140.1 251 64.4 36.4 28.1 50.3 80.6 79.7 76.1 79.7 [231] [217] 79.9 80 56 62 47 56 950 800 814 816 827 827 1120 950 – – 1420 1000 30,000 428 37.1 37.1 61.3 50.2 50 44 41.4 41.3 68.4 56.0 – – – – – – [219] [221] [222] SOFC MGT SOFC-MGT SOFC-MGT DD – Air 800 600 Yes SOFC-ST SOFC MGT SOFC-MGT FB Straws DD/ Wood chips Viking Steam Air/ steam 950 800 700 50 No No SOFC-GT SOFC-MGT SOFC CFB FB CFB – Wood chips Woods Air Steam/ O2 S/B 0.41 S/B 0.41 S/B 0.40 S/B 0.40 950 820 850– 900 700 Yes No STR ATR STR ATR SOFC-MGT 500 Planar Tubular (Ni/YSZ) – Tubular (Ni/YSZ) – 0.377 { $ 1} {4} 0.43 {4} Planar (Ni/ 0.25 GDC) – 0.37 0.3 Topsoe {3.7} Fuel Cell 0.3 A/S and Riso National Lab – 0.25 – – 0.175 Planar 0.161 Anode supported 0.375 0.508 [229] Gasif.¼ Gasifier; Tmin ¼ minimum temperature; Refr ¼reformer; J ¼ current density; Uf ¼ fuel utilization factor; ST¼ steam turbine; GT ¼ gas turbine; MGT ¼ micro-gas turbine; S/B ¼ steam to biomass ratio; STR¼ Steam reforming; ATR ¼Autothermal reforming Fig (a) Gas cleaning unit and (b) SOFC test rig connected to gasifier (taken as it is from [173]) 1372 Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 technology (hot/cold) on the performance of integrated hybrid SOFC-Gas turbine systems of 100 kWe and 30 MWe scale (results from the solution with maximum efficiency are given in Table 13) [219] Recently, Di Carlo et al simulated a SOFC-MGT system using CHEMCAD software, based on BFB gasifier and a mix of steam and O2 enriched air as gasifying agent Varying both the utilization factor and % of O2 purity in air stream (25% to 95%) showed the overall efficiency ranging from 36% to 44% which is much higher as compared to the efficiency from producer gas based internal combustion engines [220,221] Morandin et al performed thermoeconomic analysis of nine different system configurations as combinations of two different (CFB and Viking DD) gasifiers with SOFC and MGT hybrid system and/or steam turbine (combined system) The hybrid configuration (with CFB) showed the highest efficiency of 68% and a specific cost of 8000 $/kWel[222] Masoud Rokni analyzed recently a complete balance-of-plant of an integrated SOFC-Stirling engine (120 kW) for CHP with domestic hot water used as a heat sink for the Stirling engine Thermodynamic analysis showed that plant efficiency of 42.4% (LHV) is achieved [223,224] There are few examples till date in which SOFCs have been coupled with real biomass gasifiers Producer gas from the FICFB (Fast Internally Circulating Fluidized Bed) steam gasifier in Guessing, Austria [225] was cleaned from particulates at 500-700 °C using metal filter candle and from sulfur at 350–450 °C using ZnO based pellets before feeding it to SOFC [127,232] As a part of the EU Framework “UNIQUE” project [233], SOFC cells were tested on-site with producer gas from 10 kWth ICFB gasification facility at ENEA research centre in Trisaia, Italy and at FICFB gasifier in Guessing, Austria The results of SOFCs showed that the cleaned producer gas from contaminants to the desired limits for SOFC (with appropriate concentrations of methane in producer gas and humidification) from both gasifiers yields comparable or better performance than the reference fuel i.e H2/N2 mixture The cell voltage stability near 0.66 V was observed during 26 h test at Guessing [234] As a part of the EU Framework “BioCellus” project [206], different gasifiers in Europe were integrated with SOFCs for testing purposes These include an autothermal downdraft Viking gasifier at Danish Technical University (DTU) [175], an autothermal fixed bed downdraft gasifier at Graz University of Technology (TUG) [173], an allothermal bubbling fluidized bed reformer (BioHPR) at Technical University of Munich (TUM) [235] and 100 kWth CFB gasifier at Technical University of Delft (TUD) [174] Almost all test results (from to 160 h duration) from the four sites suggested that the planar electrolyte supported SOFC with Ni-GDC anode could cope with tar laden producer gas ranging from $ to 10 g/Nm3 without degradation due to carbon formation Anyhow, the particulates, H2S and HCl were removed from the producer gas before feeding it to SOFC as shown in the Fig The anode microstructures were found intact after the tests [206] University of North Dakota studied a thermally integrated SOFC-biomass gasifier power system in which high temperature effluent from SOFC was recycled to the downdraft air gasifier Experiments on a bench-scale gasifier using wood chips demonstrated tar output from 9.9 to 234 mg/Nm3 A passive sponge iron bed filtered H2S below ppm to use this producer gas in Ni-based SOFC technology SOFC button cells as well as a 100 W stack tested for 10 h showed steady operation with 22% and 15% performance drop respectively, relative to hydrogen [236] Oudhius et al used real producer gas as SOFC feed and showed proof of concept by coupling a downscaled (5 cells) Sulzer HEXIS (former) SOFC stack to a two stage gasifier (pyromaat [237]) at Energy Research Centre (ECN), Netherlands The producer gas was cleaned in a scrubber, ceramic filter, ZnO filter and active coal filter Stack was well able to operate for less than 48 h using willow and rofire as feedstock (pelletized paper recycling rejects) exhibiting electrical efficiency of 41% and 36% respectively Soot formation affected the stack performance negatively [237] Investigations at ECN were later upgraded from to 30 cells stack (350 W) and tests duration from 48 to 250 h Facility was upgraded by adding ceramic candle filters (300 °C) and commercial adsorbents reactors for S- and Cl- compounds removal at 200 °C The stack performance on producer gas from clean beech wood, rofire pellets and industrial waste in the upgraded facility was found comparable to the performance with synthetic gas without accelerated degradation [238] Nagel et al operated kW HEXIS stack with wood producer gas from an updraft gasifier at Paul Scherrer Institute, Switzerland The gas after catalytic partial oxidation was cleaned from particulate, H2S and HCl It was reported that carbon deposition was satisfactorily eluded however ash deposits were the main reason for the performance loss of 6% in 30 h of operation [176] Hoffman et al [175] in BioCellus project operated a single planar SOFC with Ni-GDC anode on producer gas for 150 h successfully at a temperature of 850 °C, current density of 260 mA/cm2, fuel utilization of 30% and S/C ratio of 0.5 Only 10 mV (minor) increase in overpotential was noted The producer gas was (almost) free of tar, from the Viking two stage gasifier and was passed through a bag house filter for soot/ particulate cleaning and activated carbon adsorber unit for S removal [175] Similar SOFC was tested with hot cleaned gas from particulates, H2S and HCl but with a high tar load ( 410 g/Nm3) from atmospheric CFB gasifier at TU Delft and no carbon deposition or performance loss was observed for short term (7 h) operation [174] This can be observed from the literature that efforts made to integrate biomass gasifiers with SOFCs have shown promising results Biomass integrated gasification–SOFC systems have been evaluated theoretically and the steps towards their technical realization have been already made through experiments Conclusions Biomass is considered renewable and environmental friendly which is widely available and an adaptable energy source The dispersed nature of biomass resources and their heterogeneity in composition make it necessary to have inventories of biomass of different origin in order to facilitate the optimal mixture compositions and feeding of gasifiers Moreover, local use of biomass to convert it to electricity and heat, primarily in decentralized modular solutions would avoid the extensive cost of biomass transportation The producer gas is generated in a gasifier via thermo-chemical gasification of biomass which can serve as a fuel for SOFCs if it is sufficiently cleaned from impurities The composition of the gas from gasifier and the composition of impurities in it, varies with the type of biomass used, employed gasifier type, the gasification agent used and gasifier operating parameters An analysis based on the influence of impurities present in biomass producer gas such as particulates, tar, H2S, HCl and alkali compounds based on recent experimental studies and their tolerance limits towards SOFCs and a comprehensive overview of the cleaning technologies for producer gas impurities indicate that producer gas cleaning using available technologies at various temperatures to meet SOFC requirements is possible Recent results in literature have shown that Ni/GDC anodes are more resistant to impurities as compared to Ni/YSZ anodes The fact that SOFC have rather high tolerance towards producer gas impurities as previously thought is very encouraging and the development of biomass integrated gasification–SOFC plants as highly efficient and sustainable systems is gaining more attention In future, more tolerant new anode materials towards impurities might reduce the producer gas cleaning requirements hence reducing the cost of cleaning of the producer gas Z Ud Din, Z.A Zainal / Renewable and Sustainable Energy Reviews 53 (2016) 1356–1376 Biomass (wood) gasification typically reaches around 25–30% electrical and 50% thermal efficiencies in engines or turbines Numerous theoretical studies have shown that gasifier–SOFC systems could possibly reach electrical efficiencies as high as 42– 48%, even for the systems of few hundred kW power Electrical efficiencies might be significantly enhanced to around 50–60%, if SOFCs are connected to turbines downstream Part of the remaining heat fraction from SOFC could be available for cogeneration applications At 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