An investigation into heat recovery from the surface of a cyclone dust collector attached to a downdraft biomass gasifier

Applied Thermal Engineering 98 (2016) 1158–1164 Contents lists available at ScienceDirect Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / a p t h e r m e n g Research Paper An investigation into heat recovery from the surface of a cyclone dust collector attached to a downdraft biomass gasifier Nwabunwanne Nwokolo a,b,*, Sampson Mamphweli a, Golden Makaka b a b Institute of Technology, University of Fort Hare, P/Bag X1314, Alice, 5700, South Africa Physics Department, University of Fort Hare, P/Bag X1314, Alice, 5700, South Africa H I G H L I G H T S • • • At a temperature of 450 °C–500 °C, hot syngas is regarded as a good heat carrier A significant quantity of energy (665893.07 kcal) is lost via the surface of the cyclone The surface temperature 150 °C–220 °C was within the low waste heat recovery temperature A R T I C L E I N F O Article history: Received 12 September 2015 Accepted January 2016 Available online 15 January 2016 Keywords: Waste heat Downdraft gasifier Producer gas Cyclone dust collector Temperature A B S T R A C T The gas leaving the reactor of a downdraft biomass gasifier contains large quantities of heat energy; this is due to the fact that the gas passes through a hot bed of charcoal before leaving the reactor This heat is normally wasted in the gas scrubber/cooler that cools it from between 400 °C–500 °C to ambient temperature (around 25 °C) The waste heat stream under consideration is the raw syngas that emanates from a gasification process in a downdraft gasifier situated at Melani Village, Eastern Cape This loss of heat is undesirable as it impacts on the thermal efficiency of the system This study investigates the feasibility of heat recovery from the surface of the cyclone dust collector prior to entering the gas scrubber It was shown that there was a downward decrease in temperature along the length of the cyclone It is found that the total quantity of heat contained in the gas was 665893.07 kcal, which could indicate the viability of recovering heat from the cyclone © 2016 Elsevier Ltd All rights reserved Introduction Biomass gasification is a thermochemical process that involves the production of gaseous fuel from carbonaceous feedstock A wide range of carbonaceous fuels have been used for the purpose of gasification, such as pine wood, eucalyptus wood, rice husk, wheat straw, corn cob, sugarcane bagasse, corn stalk, poplar, hazelnut shell, switchgrass, olive husk, coconut shell and many others [1,2] Biomass conversion process provides a more versatile application for the gaseous fuel, thus increasing the efficiency of energy utilization of biomass Gasification when compared to combustion achieves a better and more efficient energy production [3] Gasification is made possible by the use of a controlled amount of air, oxygen, steam or mixtures of two or more of these The choice of the gasifying agent used determines the heating value of the gaseous product For instance, air gasification results in a low to medium heating value of gas (4–7 MJ/Nm3) while oxygen or steam gasification result in medium heating value of 10–14 MJ/Nm3 [4] * Corresponding author Tel.: +27833433195 E-mail address: nnwokolo@ufh.ac.za (N Nwokolo) http://dx.doi.org/10.1016/j.applthermaleng.2016.01.014 1359-4311/© 2016 Elsevier Ltd All rights reserved Variation in the ratio of the gasifying agent to the carbonaceous feedstock, impacts on the quality of the final gaseous product However, other factors particularly reactor temperature also contribute to the quality and quantity of gaseous fuel produced The quality of syngas produced via gasification is crucial as the presence of impurities and undesirable products such as particulate char, tar, nitrogen oxides, and sulfur dioxides can interfere with the downstream application of the syngas These undesirable products are traceable to carbonaceous feedstock composition and incomplete gasification, which stems from variation in operating and design parameters Some end user applications of the gaseous fuel require a more intense gas cleaning and conditioning technique Some of these uses range from heat and power application such as integrated gasification combined cycle (IGCC) to production of synthesis fuels such as methanol and ethanol [5] Development and commercialization of biomass gasification unit is still hindered by inconsistency in quality of syngas produced The presence of impurities such as particulate matter and tar can cause operational problems such as fouling, clogging, obstruction of pipes and filters, reduction in heat exchange efficiency [6] Particulate matter mostly constitutes of inorganic compound, which include alkali metals (potassium and sodium); alkaline earth metals (calcium); silica (SiO2) N Nwokolo et al./Applied Thermal Engineering 98 (2016) 1158–1164 In addition elements such as arsenic, selenium, antimony, zinc, and lead are also present in particulate matter, but in trace amounts [7–9] According to internal combustion engine, manufacturer’s particulate matter content in the syngas should be lower than 50 mg/Nm3 so as to guarantee long-life failure-free operation [10] Secondly, tar is a complex mixture of condensable hydrocarbons, comprising single-ring to 5-ring aromatic compounds and other oxygen-containing organic molecules The application of the syngas determines the tar tolerance limit; for instance, in compressors, the allowable limit is 500 mg/Nm3 with 100 mg/Nm3 for internal combustion systems, and mg/Nm3 for direct-fired industrial gas turbines [11] The severe operational problems induced by the presence of impurities in syngas necessitate the need for syngas cleaning before its end use application Synthesis gas stream produced from gasification can be cleaned through a number of methods or techniques Some of these methods can only remove one contaminant while some others can remove more than one contaminant in a single process such as wet scrubber The cleaning techniques can be classified based on the process temperature range: Hot gas cleanup (HGC) and cold gas cleanup (CGC) Hot gas cleanup refers to the cleaning that occurring within a temperature range of 400 °C to 1300 °C On the other hand, cold gas cleanup are cleanings that occur near ambient condition Cold gas cleaning makes use of water sprays such as wet scrubbers Although cold gas cleaning technologies are effective, they still suffer from energy inefficiencies and also generate waste water [5] The aim of this study is to investigate and ascertain the possibility of recovering heat energy at the cyclone prior to the gas entering the wet scrubber where it loses the inherent heat energy through cooling The temperature of the syngas is cooled at the wet scrubber from about 400–500 °C to ambient temperature before reaching the gas engine As a result, a significant amount of energy in the form of heat, which could be harnessed for other heating purposes, is lost Harnessing the heat energy at the cyclone before the wet scrubber will improve the thermal efficiency as well as the overall efficiency of the gasification system The overall aim is to integrate a heat exchanger around the cyclone so as to recover the waste heat from the gas and convert it to usable heat But before the heat exchanger integration, there is a need for baseline study so as to determine the possible amount of energy that could be recovered from the surface of the cyclone The use of waste heat recovered from a biomass gasifier was examined by attaching a thermoelectric generator system (TEG) to the surface of a catalytic reactor used for cleaning the syngas In addition, the electrical properties of the thermoelectric generator along side with the efficiency of the gasification system were studied The measured surface temperature (473 °C–633 °C) of the catalytic reactor was high enough to serve as a heat source to the hot junction of the TEG The power output and power density of the TEG was found to be approximately 2.9 W–6.1 W and 91.5 W/m – 193.1 W/m2, respectively More also, a cold gas efficiency (CGE) of 76.26% was obtained [12] Pavlas et al [10] evaluated the utilization of waste heat using a heat pump from a biomass gasification unit integrated with an existing boiler The integration of a heat pump was so as to utilize the low grade heat more effectively and efficiently The study concluded that a significant energy savings can be achieved through the use of heat pump A combined heat and power system using gas from gasification of biomass was analyzed to determine the effect of using a different fuel than was originally designed on the thermodynamic characteristics of the system The efficiency of heat and electricity generated was found to depend on the type of system An overall efficiency of 67% taking into account the gas generator efficiency was obtained [13] Duan et al [14] developed a comprehensive model using ASPEN Plus for the energy assessment of an integrated coal gasification 1159 combined with a blast furnace slag waste heat recovery system Blast furnace is a by-product of an iron making process that is discharged at a high temperature of about 1500 °C–1600 °C The optimal temperature required to simulate the gasification reaction as well as recover the blast furnace heat was found to be 800 °C Guangul et al [15] compared the temperature profiles for gasification of oil palm frond using high temperature air and unheated air The temperature profile along the height of the gasifier as well as the producer gas temperature was reported The peak of the temperature was observed at the combustion and reduction zone as expected Furthermore, a dedicated ASPEN Plus model was used by Francois et al [16], to predict the mass and energy balance (including pollutant emissions) of a combined heat and power (CHP) biomass gasification plant from biomass dryer to gasifier, gas cleaning and IC engine A total of 10.3 MW of electricity and 13.3 MW of heat were produced from the biomass CHP plant while utilizing about 34.4 MW of anhydrous wood [16] Damartzis et al [17] assessed a small CHP biomass gasification system consisting of a fluidized bed reactor, a gas cleaning system and internal combustion engine for power generation Most studies on CHP application are based on model development and predictions, but limited study are available on the experimental application of CHP Gasification mechanism The major chemical reactions that occur during gasification are summarized as shown in Table The heat that supports the reaction is either provided by partial oxidation of the gasified materials or is externally supplied These reactions are made possible because of the high operating temperature of reactors used for gasification Regardless of the type of reactor used, gasification process involves four basic steps, namely drying, pyrolysis, oxidation and reduction Each of this process corresponds to the different zones found in a reactor Reactors also referred as gasifiers are majorly classified into three, fixed bed, fluidized bed and entrained flow gasifiers Fluidized bed and entrained flow are mostly used for large scale or industrial application while fixed bed (conventional type) is used for small scale applications [4] This conventional type of gasifier consists of a bed of solid fuel that moves down slowly during the gasification process In fixed bed gasifier, feedstocks are fed into the gasifier system through the top and the oxidizing agent either goes in the same direction or opposite direction with the feedstock They are characterized by long residence time, low ash carry over, high carbon conversion and low gas velocity [19] Fixed bed is further classified into downdraft, updraft and cross draft gasifier Each differs in the flow direction of feedstock and gasifying agent 2.1 Description of gasifier system The Johansson biomass gasifier under study is of a downdraft type, it offers the advantage of producing a tar free gas, which Table Basic gasification reactions [18] Reactions Heat of reaction Type of reaction C + CO2 ↔ 2CO C + H2O ↔ CO + H2 C + 2H2O ↔ CO2 + 2H2 C + 2H2 ↔ CH4 2CO + H2O ↔ CO2 + H2 CH4 + H2O ↔ CO + 3H2 CH4 + CO2 ↔ 2CO + 2H2 172.5 kJ/mol 131.3 kJ/mol 90.2 kJ/mol −74.9 kJ/mol −41.2 kJ/mol −206.2 kJ/mol 247.4 kJ/mol Boudouard Water gas primary Water gas secondary Methanation Water gas shift Steam reforming Dry reforming 1160 N Nwokolo et al./Applied Thermal Engineering 98 (2016) 1158–1164 Table Average gas composition of Johansson biomass gasifier system Barrel Gases CO H2 CO2 CH4 N2 Composition (%) 22.3–24.3 22.3–22.5 10.7–9.8 1.90–2.10 42.9–41.5 makes it suitable for engine application This system comprises of many components that include the reactor where the solid fuel is fed into and subsequently gasified The other components are collectively known as the purification unit where the syngas is cleaned of impurities such as carbon particles and as well cooled down to meet the gas engine quality requirement Finally the gas is then used to drive the generator, which generates the electricity A typical composition of the gases produced in this system is shown in Table The system component is depicted in Fig The cooling down of the syngas occurs at the scrubber, where water is sprayed over a scrubbing medium consisting of a low resistance, but porous large surface area This scrubbing media usually consist of a coarse or even graded charcoal The water used in the scrubber is recycled through an ambient pond over a long period of time [20,21] A significant amount of energy in the form of heat is lost at the scrubber during the cooling of the syngas to room temperature Usually the gas is cooled down to improve the volumetric efficiency of the engine, but at the same time it impacts on the overall thermal efficiency of the system Therefore, this study seeks to investigate the quantity of heat that could be harnessed from the body of the cyclone based on surface temperature measurement 2.2 Cyclone separator The cyclone is the first purification unit for the syngas after the gas exits the reactor and before entering the scrubber The main purpose of the cyclone is to remove the fine carbon particles that exit the reactor with the gas Generally, cyclone is less prone to explosion; hence, it offers a better advantage when compared to Inlet Duct Cone Fig Schematic and pictorial view of the cyclone [22] fabric filters in high temperature application The schematic flow diagram of a cyclone is shown in Fig [22] As the raw gas exits the gasifier it enters the cyclone in a tangential manner The tangential entry results in a spiral flow of gas beginning at the cylindrical part of the cyclone to the conical part At the conical section, the clean gas reverses and exits in a straight stream through the vortex finder, whereas the particulates collide with the outer wall and fall to the bottom (collection chamber) About 80% of these particulates are removed when operating at full power and this is equivalent to g/Nm3 Table shows the main dimensions of the cyclone The removal of particulates present in the syngas at the cyclone is enhanced by centrifugal force The cyclone performance is usually rated in terms of particle cut diameter or cut size and is represented mathematically as follows: ⎡ 9μ W ⎤ dp50 = ⎢ ⎥ ⎣ 2π NVi ρp ⎦ Fig Schematic diagram of Johansson biomass downdraft gasifier (1) N Nwokolo et al./Applied Thermal Engineering 98 (2016) 1158–1164 Table Main dimensions of the cyclone Item Dimensions (cm) Cyclone cylinder height Cyclone cone height Cyclone outside diameter Cyclone inlet duct length Vortex finder length 22 101 111 50 45 Where μ = Gas viscosity (kg/ms) W = Width of inlet duct (m) N = Number of turns inside the cyclone Vi = Gas inlet velocity (m/s) ρp = Particle density (kg m3 ) 1161 One thermocouple was also inserted into the inlet duct of the cyclone through a drilled hole The hot bed temperature of the gasifier was also monitored by a thermocouple that was inserted at the lower zone of the gasifier This monitored the temperature of the gas leaving the reduction zone of the gasifier before making its way to the cyclone All the thermocouples were connected to the channels of a CR1000 data logger The data logger was powered with a 12 V external power supply The initial temperature at the inlet duct and surface of the cyclone were noted prior to igniting the gasifier The total quantity of heat that could be recovered from the syngas was as well determined using equation Q = V × ρ × Cp × ΔT (2) Where This formula is predicted both for general cyclone and high efficiency cyclone, and it represents the particle size that can be separated at 50% efficiency Q is the heat content in kcal V is the flow rate of the substance in m3/hr ρis density of the flue gas in kg/m3 Cp is the specific heat of the substance in kcal/kg °C ΔT is the temperature difference in °C Temperature measurements Temperature results and discussion For the temperature measurement both contact (thermocouples) and non contact (infrared camera) temperature measuring technique was used The setup for the temperature measurement is shown in Fig 3, it comprises of thermocouples, CR1000 data logger, external power supply and some gas sensors, but the gas sensors were not used for the purpose of this study Type k thermocouples were used because of its wide operating temperature range (−270 °C to 1260 °C) It has a measuring accuracy of ±2.2°C The two thermocouples fitted on the body of the cyclone were 50 cm apart Four thermocouples were used in all; the third thermocouple was inserted at the reduction zone of the gasifier and the fourth to the inlet duct of the cyclone The entire measurement was conducted outside at the location of the biomass gasification system Effect of ambient temperature was not considered in the surface temperature measurement The surface temperature of the cyclone was as well measured with FLIR thermaCAM (infrared camera) with a temperature range of −20 °C to 250 °C and an accuracy of ±2°C FLIR thermaCAM (infrared camera) is a non contact instrument that can visualize the temperature distribution of a surface The gasifier was loaded with chunks of pine wood sourced from a nearby sawmill The pine wood chips varied in sizes owing to the fact that they were off cuts Before the ignition of the gasifier two thermocouples were fitted at two different heights on the surface of the cyclone This was done so as to determine if there are temperature variations between the bottom and top part of the cyclone In assessing the potential of recovering heat from any system, one of the parameters of significance is temperature The magnitude of the temperature difference between the heat source and heat sink determines the quality of heat to be recovered Recovering heat from the cyclone section of the Johansson biomass gasification system will improve the system from a standalone power system to a combined heat and power system Combined heat and power systems based on gasification are valuable to sawmills and wood processing industry In this study the heat source is the hot syngas stream and the aim is to recover the heat from the surface of the cyclone prior to the gas entering the scrubber Fig presents the inlet gas temperature profile and cyclone surface temperature profile Prior to starting of the gasifier system the temperature of the gas entering the cyclone (Tin) and cyclone surface temperatures (TSL and TSU) were 18.79 °C, 22.59 °C and 21.31 °C, respectively After the ignition of the gasifier the temperature of the gas entering the cyclone was the first to show an increase while the two surface temperatures followed after minutes A maximum temperature of 608.8 °C was obtained from the syngas stream as it exits the 500 TSL 450 TSU Tin 400 Thermocouple Wires 350 Temperature (C) Infrared Camera 300 250 200 150 External Power supply 100 50 Data Logger Fig Temperature measurement setup [23] 0 20 40 60 Time (min) 80 100 120 Fig Gas inlet and cyclone surface temperature profile within the first 120 mins N Nwokolo et al./Applied Thermal Engineering 98 (2016) 1158–1164 reduction zone of the gasifier Guangul et al [15] obtained a similar temperature profile for gas outlet temperature, the temperature increased from about 50 °C to a maximum of 600 °C In addition, the obtained gas temperature of 608.8 °C did not differ much from the temperature range (623 °C–700 °C) obtained by Balas et al [3] This was the temperature range within which Balas et al [3] obtained the individual gas components: H2, CO2, CH4, N2, CO As the gas stream approached the cyclone inlet duct, a decrease in temperature was observed This is as a result of the utilization of some of the sensible heat of the syngas in heating the air entering the gasifier through an internal heat exchanger Hence, there is no waste of heat in this regard There were fluctuations in the temperature profiles, particularly the temperature of the gas entering the cyclone within the first 120 mins of operation Similarly the gas outlet temperature profile obtained by Guangul et al [15] showed some fluctuation as it was increasing This is also in agreement with the different zone temperature profile reported by Mamphweli and Meyer [23] Afterwards, some stability was recorded as shown in Fig The percentage difference between the temperature of the gas entering the cyclone and the surface temperature of the cyclone showed that above 65% of heat in the gas is transferred to the wall of the cyclone This implies that about 65% of the energy entering the cyclone is currently lost and this is waste heat available for conversion to useful energy From Fig 5, the gas temperature was observed within a temperature range of 450 °C–500 °C while the two surface temperatures ranged from about 150 °C–220 °C Comparing this result with that reported by Ma et al [12] in which the temperature of the gasifier outlet is about 350 °C– 500 °C and surface temperature of the catalytic reactor is approximately 200 °C–360 °C The two gas temperatures compared very closely The difference is that the heat recovery in Ma et al [12] study occurred at the catalytic reactor while in the current study, the heat recovery is intended to take place at the cyclone However, there are some similarities in terms of the position of the heat recovery unit, in both cases heat is recovered from the gas before entering the wet scrubber For maximum heat recovery to occur, the position of the heat recovery unit is important as well as the choice of the heat recovery equipment In addition, the surface temperature at the upper part of the cyclone was found to be higher than the surface temperature at the lower part of the cyclone This indicates that there was a downward decrease of temperature along the surface of the cyclone as shown in Fig 220 200 180 160 Temperature (C) 1162 140 120 100 80 60 40 10 20 30 40 50 60 Length (cm) 70 80 90 100 Fig Temperature gradient along the length of the cyclone The temperature gradient along the length of the cyclone shown in Fig was obtained using a FLIR thermal camera As observed from Fig 6, the highest obtained temperature was around 200 °C, which compares closely to the surface temperature result (Fig 5) obtained using thermocouple Fig shows that a larger part of the decrease in temperature occurred at the lowest part of the cyclone, which is closer to the collection chamber of the particulates This major decrease is represented between 59 cm and 97 cm, which corresponds to the lowest part of the cyclone The decrease in temperature could be attributed to more deposit of particulates at the lower part of cyclone, thus, inhibiting the ease of heat transfer Secondly, because the gas enters from the top of the cyclone consequently, the upper part gets heated up first The actual thermal image (Thermogram) is presented in Fig 235.3°C Upper/cylindrical part 600 TSL 550 TSU Tin %Diff