Volume 5 biomass and biofuel production 5 05 – biomass co firing

Volume 5 biomass and biofuel production 5 05 – biomass co firing Volume 5 biomass and biofuel production 5 05 – biomass co firing Volume 5 biomass and biofuel production 5 05 – biomass co firing Volume 5 biomass and biofuel production 5 05 – biomass co firing Volume 5 biomass and biofuel production 5 05 – biomass co firing Volume 5 biomass and biofuel production 5 05 – biomass co firing

5.05 Biomass Co-Firing A Nuamah, The University of Nottingham, Nottingham, UK; RWE npower, Swindon, UK A Malmgren, BioC Ltd, Cirencester, UK G Riley, RWE npower, Swindon, UK E Lester, The University of Nottingham, Nottingham, UK © 2012 Elsevier Ltd All rights reserved 5.05.1 5.05.1.1 5.05.1.2 5.05.2 5.05.2.1 5.05.2.2 5.05.2.2.1 5.05.2.2.2 5.05.2.3 5.05.2.3.1 5.05.2.3.2 5.05.2.3.3 5.05.2.3.4 5.05.2.3.5 5.05.2.3.6 5.05.2.3.7 5.05.2.4 5.05.2.5 5.05.2.5.1 5.05.2.5.2 5.05.2.5.3 5.05.2.5.4 5.05.2.6 5.05.3 5.05.3.1 5.05.3.2 5.05.3.3 5.05.3.4 5.05.3.5 5.05.3.5.1 5.05.3.5.2 5.05.3.6 5.05.3.6.1 5.05.3.6.2 5.05.3.6.3 5.05.4 5.05.4.1 5.05.4.2 5.05.4.3 5.05.5 5.05.5.1 5.05.5.1.1 5.05.5.2 5.05.5.2.1 5.05.5.2.2 5.05.5.2.3 5.05.5.3 5.05.5.3.1 5.05.5.3.2 5.05.5.3.3 5.05.5.3.4 Introduction Global Trend Challenges Facing the Power Industry Available Biomass Materials Wood-Based Fuels Energy Crops Short-rotation coppice Miscanthus Agricultural Residues Olive residues Oil palm residues Shea residues Rice husks Straw Grass Bagasse Processed Wood (Wood Pellets and Torrefied Wood) Liquid Biomass Tall oil Tallow Jatropha oil Sewage sludge Gaseous Biomass Combustion Technology Pulverized Coal Combustion Fluidized Bed Combustion Stoker Combustion Cyclone Boilers Gasification Direct gasification Indirect gasification Gasification Techniques Fixed bed gasifiers Fluidized bed gasifiers Entrained flow gasifiers Co-firing Methods Direct Co-firing Parallel Co-firing Indirect Co-firing Global Overview of Biomass Co-firing Plant United States McNeil generating plant Netherlands Amer Borssele Maasvlakte and United Kingdom Aberthaw power plant Didcot power plant E.ON (Kingsnorth) Drax Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00506-0 56 56 57 57 57 57 57 57 58 58 58 58 58 59 59 59 59 60 60 60 60 60 61 61 61 61 61 62 62 62 62 62 62 63 63 63 63 64 64 64 64 65 65 65 66 66 66 66 68 68 68 55 56 Case Studies 5.05.6 Health and Safety Issues Associated with Co-firing 5.05.6.1 Spontaneous Fires 5.05.6.2 Exposure to Biomass and Coal Dust 5.05.7 Technical Issues regarding Biomass Co-firing 5.05.7.1 Fuel Delivery, Storage, and Preparation 5.05.7.2 Supply of Biomass 5.05.7.3 Properties of Biomass and Their Effects on Plant Operations 5.05.8 Conclusions References Further Reading 68 68 69 69 70 70 70 72 72 73 5.05.1 Introduction The global demand for energy has increased drastically over the last decade Coal is regarded as a major source of pollution, but continues to be one of the most reliable and widely available sources of energy in most countries This, coupled with the changing phase of the BRIC countries (Brazil, Russia, India, and China) toward industrialization, contributes enormously toward global carbon emissions, thereby causing global warming and its associated problems There is a growing acceptance that energy from renewable resources must replace the use of fossil fuels, in order to reduce the rate of global climate change There are, however, technologies that have the potential to mitigate these emissions, whether from coal, biomass, or other resources Carbon capture and storage (CCS) and renewable energy technologies have been identified as carbon abatement technologies which could drastically reduce the carbon emissions from power plants CCS, as innovative as it may be, still has a lot of technical challenges to overcome before it can be properly commercialized However, carbon capture, while novel in the power generation sector, has already been employed in other sectors such as the oil and gas industry, where carbon is captured, transported, and stored in depleted oil and gas fields to increase the pressure and the flow of oil beneath the ground – this gives grounds for optimism for coal CCS [1] This chapter limits itself to biomass co-firing as a renewable energy option and will not discuss other technologies in detail Biomass co-firing, as the name suggests, is the burning of biomass along with other fuels The principal objective of adding biomass as a partial substitute fuel in high-efficiency coal boilers is that the combustion of biomass is carbon neutral if the biomass is grown in a regenerative manner Moreover, there should be minimal changes in total boiler efficiency as a result of co-firing Currently, co-firing is the most effective use of biomass for power generation, with efficiency ranging between 35% and 45% [2] It has also been shown that the introduction of 5–10% biomass in co-firing requires only minor alteration of the handling equipment If larger amounts of biomass are used (exceeding 10%), modifications in the mills, burners, and dryers may be needed [2] Other advantages are that it promotes the use of renewable and/or other waste organic materials, thus augmenting the global effort to limit the use of land for landfilling activities Legislation around landfilling may be an effective means of promoting the use of biomass in the fuels market While there are many forms of renewable energy, from tidal power to solar power, biomass is an important source of energy that can be transported and used as a solid, liquid, or gaseous fuel Bioenergy, or energy from biomass, has huge potential, especially in countries with renewable forest resources, in wealthier countries with an excess of agricultural land, and in countries where specialized high-yielding biomass species can be grown 5.05.1.1 Global Trend Globally, about 10% of the total primary energy demand is met by biomass Biomass dominates predominantly in developing countries where it is used for cooking and heating [2] In industrialized countries, the use of biomass is below average although Finland, which is part of the International Energy Agency (IEA) member countries, meets 11% of its energy demand with solid biomass The United States, which is the largest producer of biomass, meets only 1% of its energy demand with this renewable feedstock Other European countries such as Sweden, Austria, and Portugal generate more than 2% of their total energy from biomass [3] In recent years, many governments have encouraged the use of biomass for power generation by initiating incentives to entice power generators The relatively high percentage of biomass usage enjoyed by Finland is as a result of government policy which saw the use of solid biomass being exempt from carbon tax on fossil fuels during the period 1990–97 In the United Kingdom, as part of the government effort to meet its stringent emission target, the government has set a target to increase the proportion of electricity generated from biomass from 3% in 2003 to 10.4% in 2011 [3] The final target is 15% by 2015 [2, 4] This is backed by a generous incentive package termed the Renewables Obligation, where power generators are issued with a Renewables Obligation Certificate (ROC) after generating 0.5–4 MWhe from renewable sources, depending on the ROC band, which can be sold to generate further revenue for the industry Biomass Co-Firing 5.05.1.2 57 Challenges Facing the Power Industry The major technical challenges in biomass co-firing lie in the storage, preparation, and handling of biomass Many countries lack the necessary infrastructure to transport biomass to the power plants since such crops are generally grown over a large geographical area The seasonal nature of biomass means that abundant supplies exist only around harvesting, with significantly reduced quantities being available during cultivation and growth The high moisture content of biomass (up to 70%), coupled with its low bulk density and heating value [5], adversely affects the behavior of the fuel during combustion, handling, and transport This also means that higher volumes of fuel will need to be collected, transported, stored, milled, and burned to achieve the same thermal heat output as coal, which has a very low moisture content Many of these drawbacks are reduced if biomass is co-fired with coal Co-firing can avoid the need for the high capital cost of building a new plant Retrofitted boilers can be altered to fire varying amounts of biomass with coal while maintaining its originally designed capacity The energy conversion efficiency of biomass is significantly increased when co-fired in larger plants This efficiency ranges between 35% and 45%, which is far higher than the efficiency in biomass-dedicated plants [2] Apart from reduced carbon dioxide (CO2) emissions, the sulfur and nitrogen content in biomass is very low, which subsequently reduces NOx and SOx emissions by diluting the contributions from coal [6] In other scenarios, the operating cost associated with co-firing is likely to be higher due to the higher cost of some biomass fuels compared with coal; nevertheless, co-firing is usually the cheapest form of ‘renewable’ energy, which is another important factor that favors the use of biomass when seeking to meet the European Union (EU)-level regulations on emissions 5.05.2 Available Biomass Materials Traditionally, solid biomass fuels have been the main form of biomass used to generate energy In most of the developing countries, solid biomass fuels have been utilized to provide heating and also for cooking In co-firing, other forms of biomass materials have also been used, such as liquid and gaseous biomass fuels While solid biomass still dominates the market, the liquid and gaseous biomass fuels are gaining some impetus as highly efficient energy sources 5.05.2.1 Wood-Based Fuels The characteristics, both physical and chemical, of wood-based fuels vary significantly in the form of sawdust, shavings, bark, and chips Residues from forestry, sawmills, and the furniture industry have been identified as viable options for co-firing The heating value of oven-dry sawdust is around 20.5 MJ kg−1 (high heating value (HHV)) depending on the type and the percentage presence of bark [7] Wood is one of the most widely spread sources of fuel for residential, commercial, or industrial utility boilers or furnaces for producing thermal and/or electrical energy Locally, the United Kingdom produces large amounts of wood residue – it is estimated that it produces 10 Mt of waste wood each year [41] While this amount is not significantly higher than in many countries, the United Kingdom does not have a large-scale producer of wood pellets, which is needed to maximize the potential of this fuel source Wood pellets are mostly imported from Russia, North America, Scandinavia, and other northeastern European countries Large-scale production facilities are under development, however, in Scotland [41] The type of wood used varies from country to country, but the United Kingdom has indigenous supplies of pine and spruce, wood from fruit trees like apple and pear, and eucalyptus wood among others Eucalyptus is a nonnative species and was first introduced into the United Kingdom in the late eighteenth century 5.05.2.2 Energy Crops Energy crops are specifically grown for use as a fuel and are therefore designed to maximize energy yields per hectare at the lowest cost possible The commercial ones are usually densely planted monocultures with high yields Short-rotation coppice (SRC) and miscanthus are popular choices for co-firing applications 5.05.2.2.1 Short-rotation coppice SRC cultures are high-density plantations with high-yielding varieties of willow and poplar The shoots are harvested every 2–5 years, but the roots are left intact in order to avoid replanting Approximately 3000 are planted with SRC across the United Kingdom The shoots from SRC are produced in the form of rods, chips, or billets The inherent moisture content, depending on the form, can be between 45% and 60% A typical yield from a UK-based SRC can be between and 18 oven-dry tonnes per hectare per year (or odt ha−1 yr−1 for short) 5.05.2.2.2 Miscanthus Miscanthus is native to Asia and is a perennial fast-growing grass It uses the C4 photosynthesis pathway and hence is more efficient in fixing carbon and in water use than the majority of native species in Europe It grows rapidly during the summer months to produce canes that can be harvested annually, rather than on a 2- to 5-year cycle as with SRC The calorific value (CV) of oven-dry 58 Case Studies Miscanthus is 19.0 MJ kg−1 (HHV), with a yield similar to the average SRC at 7–12 odt ha−1 yr−1 It can continue to grow from the same rhizomes for at least 15–20 years One advantage over SRC is that if the landowner decides to stop growing energy crops, Miscanthus can be removed easily by spraying a herbicide like glyphosphate [8] 5.05.2.3 Agricultural Residues Usually, agricultural processes produce waste residues, which can end up as compost or animal bedding Reuse is a cheap source of biomass and is more benign since landfill sites will inevitably generate methane and other gaseous emissions that increase the environmental burden Residues that have been used in co-firing applications include olive residues, oil palm residues, and shea residues Other agricultural residues of interest include wheat straw, corn stalks, nutshells, sugarcane bagasse, orchard prunings, and vineyard stakes 5.05.2.3.1 Olive residues Olive residues are produced globally, but the main sources are from around the Mediterranean, with Spain, Italy, and Greece accounting for 97% of total production [7] Olive oil production is the main source of olive residues, where only about 21% of the weight of an olive is actually oil, the rest being residues that are normally treated as waste [6] The residues include crushed olive kernel, shell, pulp, skin, water, and any remaining oil Olive plantations can produce between 500 and 10 000 kg olives per hectare [7] The advent of a three- and two-phase production system of olive oil has generated new types of residues In Spain, about 90% of olive production systems utilize the two-phase process, which produces residues called ‘alpeorujo’ Alpeorujo is a solid residue from olive oil extraction and is made up of stones, skins, flesh, water (50–60%), oil (2–4%), and ashes (2%) High moisture content and high alkali metal content of alpeorujo create problems for boilers due to the low melting temperatures of the ash, although co-firing can reduce the problem 5.05.2.3.2 Oil palm residues Oil palms are mainly grown in Southeast Asia, South America, and Africa Malaysia and Indonesia currently dominate the world market in the production of palm oil Palm oil is extensively used in the food and chemical industries Forty-five percent of the palm fruit remains after oil is extracted, which can be used as a fuel This residual material consists of the empty fruit bunches, kernel, shell, and fibrous material Palm residues can also be burned in oil palm processing mills to generate heat and power The production of palm oil creates a vast amount of waste biomass after the milling and the crushing of palm kernel Over the last decade, palm kernel expeller (PKE) was investigated by power generators as a potential biomass fuel for co-firing It has now been burned commercially for at least years by many generators as one of the most popular co-firing fuels in the United Kingdom However, price fluctuations make its popularity more ‘volatile’ than other choices of biomass feedstock PKE’s lower moisture content and higher CV (compared with other biomass types) make it an excellent choice for co-firing with coal (Table 1) The kernel shell could also be used as a fuel during co-firing, but it is hard and therefore difficult to mill, thus making it less popular than PKE 5.05.2.3.3 Shea residues The shea tree is native to Africa, where its butter is extracted from the kernel for use in cosmetics and foods After butter extraction, significant quantities of waste are produced in the form of shell, husk, and the fleshy mesocarp These residues can be processed and used as fertilizers, domestic fuels, or as a waterproofing agent Similar to palm wastes, the characteristics of the shea residue can vary according to the processing methods used to extract the butter This material has been used for co-firing at a number of power stations in the United Kingdom, but there have been reports of issues with self-heating and dust handling problems 5.05.2.3.4 Rice husks Rice husks are the waste materials after the rice grains have been removed and are predominantly composed of silica They can be used as an energy source, but the high ash content, relative to other biomass materials, makes their use problematic during Table Properties of palm biomass Fiber Shell Empty fruit bunches PKE cake Moisture content (wt.%) Calorific value (kJ kg−1) 37 12 67 19 068 20 108 18 838 18 900 Biomass Co-Firing 59 co-firing Ash contents of 15–20% (on a dry basis) are not uncommon with >60–70% SiO2 content Pellets made from a mixture of rice husks and olive residues have also been marketed, but it is unclear as to whether this product has been commercially successful 5.05.2.3.5 Straw Straw is a product that is available in abundance in most farming-intensive countries Denmark has been one of the leaders in the use of straw as a power station fuel Denmark introduced legislation banning the practice of burning straw in the fields in 1990, which made straw a more practical fuel source There are disadvantages with using straw in combustion processes since its ash has an extremely low melting point, which can result in slagging problems, and its fibrous nature makes it very difficult to handle during milling and transportation Either specialist equipment is required for handling and grinding or the boiler needs to be reconfigured to burn straw bales 5.05.2.3.6 Grass Grass can be used as a biomass fuel since it is abundant in many countries Switchgrass and reed canary grass are examples of popular varieties of rapidly growing grasses As with straw, blending grass with coal can be problematic in terms of handling and milling, particularly without retrofitting existing equipment, thus making it less popular than other biomass types Grasses have the advantage that they can be grown outside the general harvest season and can also be harvested more than once a year 5.05.2.3.7 Bagasse Bagasse is the residue after sugarcane or sorghum stalks are crushed to extract their juice It contains high amounts of fixed carbon due to the high bioconversion by the sugarcane plant during photosynthesis Sugarcane is a major commercially grown agricultural crop in the vast majority of countries in Africa and in the southern part of America, in particular Mauritius and Brazil Table shows details for different types of solid fuels in terms of elemental and proximate composition 5.05.2.4 Processed Wood (Wood Pellets and Torrefied Wood) Processed wood fuels, such as wood pellets and torrefied wood, can be generated from a variety of wood residues Wood pellets have a higher and more uniform CV than raw wood The production of uniform shape and size means that handleability problems are predictable They also have a moisture content of 5–10% [7], which is considerably lower than the 60–70% moisture that is present in the fresh ‘parent’ material Torrefaction is a process of improving, or upgrading, the properties of lignocellulosic materials like wood (Table 3) The process involves slow pyrolysis of the feed material with a hold temperature between 200 and 300 °C The process lowers the moisture content and increases the CV (around 21 MJ kg−1, which is similar to subbituminous coal); removes the volatiles that cause smoke during combustion, resulting in a product that is still approximately 70% of its initial weight but with 80–90% of the original CV; and also increases the hydrophobicity, making it more durable while improving grinding properties The additional investment and Table Properties of different solid fuels [6] Property Coal Peat Wood without bark Ash content (db) Moisture content Net CV C (% db) H (% db) N (% db) O (% db) S (% db) Cl (% db) K (% db) Ca (% db) 8.5–10.9 4–7 0.4–0.5 2–3 1–3 1.1–4 6.2–7.5 2–7 6–10 40–55 5–60 45–65 50–60 50–60 17–25 15–20 60–70 26–28.3 76–87 3.5–5 0.8–1.5 2.8–11.3 0.5–3.1 < 0.1 0.003 4–12 20.9–21.3 52–56 5–6.5 1–3 30–40 < 0.05–0.3 0.02–0.06 0.8–5.8 0.05–0.1 18.5–20 48–52 6.2–6.4 0.1–0.5 38–42 < 0.05 0.001–0.03 0.02–0.05 0.1–0.5 18.5–23 48–52 5.7–6.8 0.3–0.8 24.3–40.2 < 0.05 0.01–0.03 0.1–0.4 0.02–0.08 18.5–20 48–52 6–6.2 0.3–0.5 40–44 < 0.05 0.01–0.04 0.1–0.4 0.2–0.9 18.4–19.2 47–51 5.8–6.7 0.2–0.8 40–46 0.02–0.1 0.01–0.05 0.2–0.5 0.2–0.7 17.4 45–47 5.8–6 0.4–0.6 40–46 0.05–0.2 0.14–0.97 0.69–1.3 0.1–0.6 17.1–17.5 45.5–46.1 5.7–5.8 0.65–1.04 44 0.08–0.13 0.09 0.3–0.5 17.5–19 48–50 5.5–6.5 0.5–1.5 34 0.07–0.17 0.1 (in ash) 30 (in ash) No data db, dry basis; % on a weight basis Bark Forest residues Willow Straw Reed canary grass Olive residues 60 Case Studies Table Properties of torrefied wood compared with others [9] Moisture content (%) NCV (MJ kg−1) Bulk density (kg m−3) Energy bulk density (GJ m−3) Hygroscopic nature Behavior in storage Wood chips Wood pellets Torrefied wood 35 10.5 550 5.8 Wet Gets moldy, dry matter loss 5−10 17 600 Wet Deteriorates, get moldy 21 800 16.7 Hydrophobic Stable loss of product during torrefaction are outweighed by the lower transportation costs and higher CV Torrefaction also allows for a wider use of source material (including grasses and roots) 5.05.2.5 Liquid Biomass Liquid biomass fuels are generally grouped into biodiesels and ethanol Some of the liquid biomass fuels specifically used for co-firing operations include palm oil, raw vegetable oil, tall oil, waste vegetable oil, rapeseed oil, and jatropha oil These fuels are converted into biodiesel by the process of transesterification, which is the chemical process of converting animal fat and vegetable oil into biodiesel Ethanol is produced by fermentation of sugar-bearing and starch crops such as wheat, maize, potato, and sugar beet In 2005, it was estimated that about 17% of biomass used in co-firing was liquid [10] Disadvantages include lower CV and potentially higher NOx emissions However, liquid biomass has advantages over conventional liquid fuels like petrol and diesel, particularly in that liquid biomass is renewable, biodegradable, and a superior lubricant (in the case of biodiesel) and has better solvent properties Another vital advantage of bioethanol and biodiesel is that they can be mixed with conventional petrol and diesel, respectively, which allows the use of the same handling and distribution infrastructure [11] Liquid biomass fuels have been tried, tested, and proven to be a very useful substitute for petrol and diesel in the transport industry; however, their application in the power generating sector is still in its infancy 5.05.2.5.1 Tall oil Tall oils are essentially by-products from the kraft pulping process in the papermaking industry During this process, pulp is created by the digestion of wood, which is influenced by a combination of factors including the high cooking temperature of the chemicals and its elevated pH level Black liquor is produced as a by-product of the pulping process, which contains cooking chemicals, residual pulp, and resin or pitch from the trees Tall oil is therefore produced from the refined form of the resin or pitch and is used in the manufacture of soaps, lubricants, and emulsions Moreover, it can easily replace, or be blended with, current energy fuels to supply energy Its physical properties vary based on the type of tree it is obtained from and the processing method employed Tall oil can be very corrosive and aggressive to low-grade steel Care must therefore be taken before it is introduced to ensure that the integrity of the combustion system is not compromised 5.05.2.5.2 Tallow Tallow is a product from rendered animal by-products The rendering process drives off water at high temperatures to separate the fat (or tallow) from the protein About 30–35% of an animal’s mass can be rendered, and of this, 24% is tallow Tallow is generally used in the food and chemical industries, with around 250 000 t of tallow being produced annually in the United Kingdom with an average CV of 40.0 MJ kg−1 [41] All grades of tallow can be used as liquid fuel in place of fossil fuel since the CV of tallow is just over 90% that of fuel oil and very little modification of combustion equipment is needed to burn it [12] However, the use of tallow in co-firing applications has been prevented as a result of its classification as a nonwaste material according to the UK interpretation of the European Waste Incineration Directive (WID) It only becomes classed as a waste material if it is burned 5.05.2.5.3 Jatropha oil The jatropha plant is pest and drought resilient with a high tolerance to poor soil conditions Enhanced growth rates have been shown to be achievable by applying fertilizers containing minerals such as magnesium, sulfur, and calcium It is estimated that a hectare of land can grow 2200 jatropha plants producing t of jatropha seed yielding 2.2–2.7 t of jatropha oil The oil content of jatropha kernel is 63% [13], which is higher than palm oil (up to 45%) The fact that the plant cannot be used as a food source without detoxification makes it very attractive as an energy fuel source Drax, a power generation company in the United Kingdom, has recently announced plans to develop and use jatropha oil in its plans for developing biomass-only power stations and units [14] 5.05.2.5.4 Sewage sludge Sewage sludge is the final solid component produced during wastewater treatment Approximately 1.5 Mt of sewage sludge is produced in the United Kingdom each year, which when processed is suitable for co-firing After the sludge component has been separated from the water fraction, it is dried and pelletized The drying is energy intensive since producing t of sewage sludge Biomass Co-Firing 61 pellets requires 20 t of sewage sludge The CV of dry sewage sludge is highly variable, with an average of 12 MJ kg−1 As a waste product, this falls under the control of the WID of the EU and can only be burned in a WID-compliant plant, which prevents its use in most co-firing plants as they tend to be not WID compliant 5.05.2.6 Gaseous Biomass Solid biomass can further be processed into gaseous forms by the process of gasification The gasification of solid biomass implies incomplete combustion of the material resulting in production of combustible gases, such as carbon monoxide (CO), hydrogen (H2), and traces of methane (CH4) The technique of converting solid biomass into gaseous forms has been described in detail in subsequent sections 5.05.3 Combustion Technology Biomass co-firing relies on existing coal technologies to function, as it is not a stand-alone technology This is achieved with slight or, in some cases, no modification at all to the parent plant Common technologies employed in biomass co-firing are pulverized coal combustion (PCC), fluidized bed combustion (FBC), cyclone boiler, stoker combustion, and gasification 5.05.3.1 Pulverized Coal Combustion PCC technology is a widely utilized technology to generate energy from fossil fuel, especially coal [15] In this technology, pulverized coal is injected to combust in a furnace in the presence of a controlled level of air The heat generated is used to produce high-pressure steam driving a steam turbine to generate electrical power The average efficiency for such plants is about 36% in the OECD (Organisation for Economic Co-operation and Development) countries and 30% in China [16] The concept of PCC has been enhanced to operate at higher temperatures and pressures to produce supercritical (SC) steam and also ultra-supercritical (USC) steam (>374 °C and 218 atm) These two advanced technologies have efficiencies far greater than PCC, with efficiency ranging between 40% and 55% However, the full commercialization of SC technology has been limited by the need for materials that can withstand high temperatures and pressures These technologies are seen as a major carbon mitigation route, as it is estimated that a percentage point increase in plant thermal efficiency can lead to a double reduction in CO2 emissions [17] Therefore, replacing old pulverized fuel (PF) plants with SC pulverized coal plants has the potential of reducing emissions by 10–25% [15] 5.05.3.2 Fluidized Bed Combustion FBC can be either a bubbling bed (BFBC) or a circulating bed (CFBC) BFBC is achieved by combusting the fuel with a bed material that has a depth of around m operating at gas velocities sufficient to fluidize the fuel and the bed material CFBC operates at higher gas velocities, high enough to entrain the fuel and bed particles in the gas flow leaving the combustion chamber, where the particles are separated in a cyclone or beam separator and recirculated to the combustion chamber These technologies result in lower NOx and SOx emissions than PF technology, which is due to the fact that FBC operates at temperatures (800–900 °C) below the temperatures required for thermal NOx formation [15] and also there exists an intimate contact between the fuel and the bed material Moreover, SO2 can be totally removed, negating the need for flue gas desulfurization or recirculation, by addition of limestone to the bed material FBC technologies are ideal for high-ash coals or coals with poorer burnout properties Their thermal efficiencies are normally about 3–4% below that of PF combustion However, with the advent of pressurized fluidized bed combustion (PFBC) technology, which employs the same processes but with higher pressures, thermal efficiencies can exceed 40% There is also the possibility of improving upon the PFBC by the application of combined cycle technology [15] 5.05.3.3 Stoker Combustion In the stoker or grate-fired boiler system, the fuel is fed onto a moving grate while air is blown through the bed of fuel Smaller particles burn out suspended above the grate while larger particles burn on the grate, as the fuel moves from the back to the front of the boiler These boilers are capable of firing a wide range of fuels, including coal, peat, straw, waste, and wood residues in fairly large pieces (not more than cm) This technology has a low maintenance and operational cost, but it is limited to a maximum capacity of about 100 MWe and has a lower efficiency compared with PCC and FBC [18] Modern stoker units are equipped with cyclones, electrostatic precipitators, or baghouses, sometimes with gas scrubbers to remove particulate from the stack phases There are often problems when firing low-melting fuels in stokers, but these can be reduced by using mechanical or water-cooled grates and by avoiding the use of preheated combustion air in the final burning region 62 Case Studies 5.05.3.4 Cyclone Boilers Cyclone boilers are another combustion technology suitable for biomass co-firing Here the mineral matter in the fuel forms a slag that holds and captures the large particles, allowing the volatile and fine particles to burn in suspension providing intense radiant heat for slag layer combustion The burners for cyclone boilers are generally large, water-cooled, and horizontal with the combustion temperature in the external furnace ranging between 1650 and 2000 °C For optimum cyclone performance, the fuels are specified to meet certain requirements, such as ash content must be greater than 6% (too high for many pure biomass types), volatiles must be greater than 15%, and the moisture content must be less than 20% unless the fuel is dried [41] Cyclone boilers only need fuels to be crushed and not pulverized making them suitable for co-firing in that they require minimal modification for feeding and mixing the biomass and the coal [42] 5.05.3.5 Gasification Gasification converts solid or liquid carbon-based fuels into a gas (syngas or biosyngas) in a high-temperature environment The process is initiated in the presence of oxygen, air, or steam and also heat The gas produced is mainly made up of CO and H2, and other components including CO2, H2O, and CH4 The percentage of these gases (CO, H2, CO2, H2O, and CH4) depends on the composition of the raw materials and the gasification conditions such as pressure and temperature To obtain the highest efficiency for this technology, the gasification process is integrated with a combined gas turbine set, called the integrated gasification combined cycle (IGCC), where efficiency is maximized by using the syngas to drive a gas turbine as well as powering a steam turbine by utilizing the exhaust heat generated from the gas turbine Work has been done to suggest that efficiencies of up to 56% can be achieved under IGCC [16]; however, it is understood that the practical thermal efficiency is about 40% [43] 5.05.3.5.1 Direct gasification In direct gasification, reaction temperatures are produced by partial combustion of the feedstock in the presence of air or oxygen in the reactor Usually, the syngas produced from this process is very dilute when air is used due to the high amount of nitrogen in air The use of pure oxygen produces syngas with high CV, which is good for combustion purposes, but the process becomes considerably more expensive 5.05.3.5.2 Indirect gasification Here, the heat required for gasification is supplied by an external source outside the main gasifier usually by the use of steam Steam contributes to increasing the CV of the product gas due to its high hydrogen content and, moreover, it is very attractive due to its low cost and easy production An example of an indirect gasifier using gas as heat source is fluidized bed gasifier equipped with heat exchanger tubes Here, part of the product gas is burned with air as oxidizing agent in a pulse combustor The resulting heat is used to gasify the fuel that is fed into the reactor [19] Two separate reactors are required when char is used as the heat source: a circulating fluidized bed steam gasifier converts fuel to produce gas and a circulating fluidized bed combustor burns residual char to provide heat which is needed to gasify the fuel Bed material, usually silica sand, is circulated between the two reactors to facilitate better heat transfer 5.05.3.6 Gasification Techniques Based on the type of technique and equipment used, three basic types of gasifiers can be distinguished: fixed bed, fluidized bed, and entrained flow gasifiers 5.05.3.6.1 Fixed bed gasifiers Fixed bed gasifiers require mechanically stable fuel of smaller particle size (1–3 cm), such as pellets or briquettes [20], to ensure free and easy passage of gas through the bed Depending on the direction of flow of the feedstock and the gas, these gasifiers are classified as updraft and downdraft gasifiers In updraft fixed bed gasifiers, the fuel is fed from the top while air is blown into the bottom of the reactor This arrangement can withstand biomass of higher moisture content (up to 40–50%) [19] This is because the hot gas exiting the gasifier initiates the combustion process by drying and pyrolyzing the fuel as it moves down the gasifier until finally undergoing gasification and combustion at the bottom The product gas is generally useful for heat and power generation through a steam turbine and not particularly applicable for synthetic fuel, chemical, or gas turbine applications, due to the amount of higher hydrocarbons (e.g., aromatic hydrocarbons and tars) contained in it Alternatively, in downdraft fixed bed gasifiers, the fuel is fed in from the top while air is introduced at the sides above the grate and combustible gas blown through the grate The setup is very simple and of low cost The gas produced (mainly CO, H2, CH4, CO2, and N2) is relatively clean compared with that produced in updraft fixed bed gasifiers and contains no or low amounts of tars or oils, making it suitable for application in heat and power generation using gas turbines Biomass Co-Firing 5.05.3.6.2 63 Fluidized bed gasifiers For biomass gasification in a fluidized bed, the temperature for successful gasification is at least 750 °C [44] while maintaining the bed temperature below the ash melting point of the fuel Failure to adhere to this standard may produce sticky ash that might glue together with bed particles causing agglomeration and breakdown of fluidization Hence, fluidized bed gasifiers are appropriate for woody biomaterials, which have a higher ash melting point (above 1000 °C) than herbaceous biomaterial (e.g., straw), whose ash melting point can be as low as 700 °C [21] 5.05.3.6.3 Entrained flow gasifiers Entrained flow gasifiers operate at a very high temperature (1200–2000 °C) and pressure (about 50 bar) and turn the mixture of fuel and oxygen into a turbulent dust flame, producing liquid ash, which deposits on the walls of the gasifier This is sometimes very problematic, especially when analyzing the ash melting behavior of solid biomass feedstock; another drawback is the high cost associated with oxygen production and the milling of the fuel to suitably fine sizes for easy entrainment [44] Due to the operating conditions of this type of gasifier, only specific types of biomass are suitable to be applied The technology is relatively mature and has been commercially utilized in the petroleum industry for the gasification of petroleum residues 5.05.4 Co-firing Methods There is no dedicated technology for co-firing, which implies that it utilizes the existing technology for generating power from fossil fuels Some of these technologies have been described in the preceding sections However, based on the different routes by which the coal and biomass blend can be introduced into the boiler, three other techniques have been identified: direct, indirect, and parallel co-firing 5.05.4.1 Direct Co-firing This is the most popular, simplest, and cost-effective way of co-firing coal with biomass Here, the combustion of coal and biomass takes place in the same boiler producing blended coal and biomass ash There are four possible ways to this technique [4]: Co-milling of coal and biomass with existing coal mill equipment or with dedicated individual milling equipment and firing them through the existing coal feeding system Direct injection of premilled biomass and firing of the biomass material through existing coal injection systems and burners Installation of new, dedicated biomass milling equipment and firing the coal and the biomass through separate injection systems Utilizing the biomass as a reburn fuel Option can be achieved in different ways depending on the milling system: • Milling and firing of blended biomass and coal fuel through existing coal milling and firing equipment Here, coal and biomass are milled and dried together in existing equipment to achieve the desired particle size The blended fuel is then fired into the furnace for operation of the plant It is the cheapest and most straightforward option The main disadvantage of this technique is that the grinding performance of the coal mill degrades due to the presence of the biomass It also carries the highest risk of malfunction of the fuel feeding system [42] This option is suitable for biomass fuels such as olive/palm kernels or cocoa shells as well as sawdust [45] but not for herbaceous biomass • Modifying existing coal mills on each boiler to mill biomass materials separately and firing the milled material through existing pulverized coal pipework and burners [46] • Milling of the biomass in dedicated biomass mills and the introduction of the milled fuel into the existing coal-firing systems With option 2, for the introduction of the premilled fuel into the furnace, three direct co-firing options can be applied: • Injection of the biomass directly into the furnace, with no flame stabilization and no additional combustion air This is relatively inexpensive and simple to install as it involves direct injection through the walls of the furnace, albeit it does involve the installation of new, small-diameter pipework for better furnace penetration [4] The drawback of this process is that its application is limited by conventional wall- or corner-fired furnaces This approach has been used in downshot boilers at RWE npower’s Aberthaw power station in the United Kingdom • Installation of new, dedicated biomass burners, with a combustion air supply Here, the premilled biomass is fed into the same boiler as the coal but through separate feeding systems This option requires the installation of a number of biomass transport pipes across the boiler front, which may already be congested, creating difficulties in maintaining an adequate burner performance over the normal load curve This is more capital intensive than co-milling, since it requires greater modification to existing coal plants Again, there are a number of potential problems that need to be resolved such as the location of new burner, alterations to accommodate secondary air supply, and the lack of experience in large-scale biomass burners [4] 64 Case Studies However, option presents a relatively simple and cost-effective way of increasing the proportion of biomass co-fired in a typical coal plant Moreover, unlike co-milling, where there is an undue pressure from the biomass on the mill and feed system, this option ensures that there is no interference on the existing coal milling and feeding system • Pneumatic injection of premilled biomass into existing coal pipework downstream of the coal mills or at the burner and firing through the existing burners The introduction of additional fuel and air reduces the mill primary air and coal flow rate accordingly to maintain both the coal mills and the burners within their normal operating envelope Albeit relatively inexpensive and simple to install, there are significant interfaces with the mill and combustion control system, which have to be carefully managed The available options for the biomass injection points are directly into the burner, just upstream of the burner into the pulverized coal pipework, and into the mill outlet pipework 5.05.4.2 Parallel Co-firing Here, the biomass and the coal are separately combusted producing individual ashes, supplying steam to a common header The fuel preparation and feeding are physically independent The only potential limiting factor to this technique is the capacity of existing downstream infrastructure, such as the steam turbine The amount of steam that could be co-fired could be limited by the capacity of the steam generator For this technique to be successful, there should be sufficient overcapacity of the steam turbine to accommodate the extra power from biomass or a reduction in the coal boiler capacity to make room for the biomass Since the coal and the biomass are converted independently, an optimal system for each fuel can be chosen, for example, CFB for biomass and PC for coal The capital investment of installing parallel co-firing is significantly higher than for direct co-firing; however, this technique attracts some interest due to its potential to optimize the combustion process, the ability to use fuels with high alkali and chlorine content, and the possibility of producing separate ashes 5.05.4.3 Indirect Co-firing The process of indirect co-firing involves gasifying biomass separately and injecting the produced gas into a coal boiler to be burned Like parallel co-firing, this technique produces separate ashes while allowing very high co-firing ratios Its main disadvantage is the high capital investment of installation However, this approach may suit the future of co-firing, which will see, almost certainly, an increase in the biomass/coal ratio as well as in the range of different biomass fuels considered Therefore, high capital investment could pay off in the future with more advanced co-firing configurations giving better operability and flexibility 5.05.5 Global Overview of Biomass Co-firing Plant The huge benefits of biomass co-firing as the most efficient renewable route and the corresponding incentives enjoyed by companies for generating electricity from renewables have globally stimulated the development of the technology at a considerable rate This rate has been intensified in the past 5–10 years, which has seen the modification of existing plant to accommodate the biomass and the introduction of new plant designs with the capacity to co-utilize biomass with fossil fuels [22] According to the IEA Bioenergy database (http://www.ieabcc.nl), a significant number of these plants are in the United States, with about 100 in Europe and 12 in Asia 5.05.5.1 United States Biomass co-firing has been practiced in the United States for a long period of time It, however, gained impetus from the 1990s, where energy generators started enjoying incentives from the government for generating electricity from renewable sources [23] A large range of biomass fuels, including residues, energy crops, and herbaceous and woody biomass, have been fired using PCC, stokers, and cyclone boilers The introduction of the energy generated tax credit in 1992, which was exclusively targeted to support electricity generated from wind and closed-loop biomass, attracted electricity generators to invest in these technologies Initially, the incentive package covered only closed-loop biomass – that is, energy or forest crops where what is harvested is regrown – but changes in legislation in 1999 allowed for extension and expansion of the existing incentive for electricity generated from biomass The qualified biomass fuels for incentives included any solid, nonhazardous cellulosic waste material, pellets, crates, trimmings, and agricultural by-products and residues Other incentives were introduced to encourage renewable technologies in the United States including exemptions from property tax, state sales tax, and income tax, with offers for loan and special grant programs, industry recruitment incentives, accelerated depreciation allowance, and net-metering provisions [3] Over 40 plants in the United States have co-fired biomass and coal for several years mostly for testing and demonstration purposes Five plants currently operate continuously for testing wood or switchgrass, and one plant has been operating Biomass Co-Firing 65 commercially for the past years burning wood-based fuels and coal [24] Almost all the co-firing plants in the United States utilize the direct co-firing process, apart from the McNeil generating plant, which employs the indirect options (Table 4) 5.05.5.1.1 McNeil generating plant The construction of this plant was by popular acclamation where residents in the city of Burlington, Vermont, USA, approved the construction of a 50 MW wood-fired power plant Electricity was initially supplied by an aging coal-fired plant with poor emission records, and hence the idea of using locally available and environmentally friendly feedstock was widely accepted The McNeil generating plant operates by the indirect co-firing techniques, with an installed capacity of 50 MWe Wood chips are the main biomass fuel and contribute about 15% by heat to the co-firing plant It utilizes a low-pressure Battelle gasification process that consists of gasification and combustion reactors The gasification reactor is heated by an indirect source to generate medium-CV fuel of about 17–18 MJ m−3 and residual char at a temperature of 700–850 °C [25] The combustion reactor burns the residual char to provide heat for gasification Sand is used as a medium of heat transfer between the reactors by circulating the sand between the gasifier and the combustor The product gas is co-fired in a stoker grate boiler for steam generation, which is used in steam turbines for power 5.05.5.2 Netherlands The Dutch government has set ambitious targets with 14% of its total energy from renewable sources by 2020 [26] For that reason, the government has entered into an agreement with six utilities companies, which operate coal-fired power plants, to commit themselves to reduce CO2 emission by an equivalent of 5.8 Mt yr−1 in the period 2008–12 It is envisaged that more than half of this target (3.2 Mt yr−1) will be achieved by the substitution of coal by biomass [27] In The Netherlands, GWe out of the total power production of 14 GWe is generated by coal-fired power plants, which co-fire biomass Table shows the total number of coal-fired power plants and the types of biomass used Apart from Amer 9, all co-fired power plants in The Netherlands utilize the direct injection technique 5.05.5.2.1 Amer The Amer plant utilizes both direct and indirect co-firing configurations The plant co-fires biomass pellets up to a maximum of 1200 kt yr−1, generating 27% by heat through two modified coal mills Only wood-based fuel has been used since 2006, due to reduced subsidies for agricultural by-products For the indirect co-firing option, low-quality demolition wood is gasified in a CFB gasifier at atmospheric pressure and a temperature of approximately 850 °C The raw fuel gas is cleaned extensively and combusted in a coal boiler via specially designed low-CV gas burners An advantage of this concept is that there is no contamination of the fuel gas as it enters the coal-fired boiler This allows a wide range of fuels to be co-fired within existing emission constraints while avoiding problems with ash quality The Table Major US biomass co-firing demonstration plants [3, 22] Utility Plant Boiler type Boiler size (MWe) Biomass type Biomass heat input (%) Alabama Power Allegheny Energy Allegheny Energy Alliant Energy Corporation GPU GPU GPU/RE La Cygne NRG Energy NRG Energy NIPSCO Nisource NYSEG/AES Madison G&E Otter Tail Santee Cooper Southern Company Services, Inc Southern Company Services, Inc Tampa Electric TVA TVA TVA FERCO Gadsden Albright Willow Ottumwa Shawville Shawville Seward KCP&L B L England Dunkirk Michigan City Bailly Greenidge Blount St Big Stone Jefferies Hammond Kraft Gannon Allen Colbert Kingston McNeil Tangentially fired Tangentially fired Cyclone Tangentially fired Wall-fired Tangentially fired Wall-fired Cyclone Cyclone Tangentially fired Cyclone Cyclone Tangentially fired Wall-fired Cyclone Wall-fired Tangentially fired Tangentially fired Cyclone Cyclone Wall-fired Tangentially fired Stoker grate 70 150 188 704 130 160 32 840 120 100 425 160 105 50 450 165 120 55 165 270 190 160 50 Switchgrass Sawdust Sawdust Switchgrass Wood Wood Sawdust Wood Wood Wood Wood Sawdust Wood Switchgrass Seed corn Wood Wood Wood Wastepaper Sawdust Sawdust Sawdust Wood chips 7 5–10 1.5 1.5 10 12 (mass) 10–15 5.5 10 10 10 1–4 10–20 (mass) 5–14 (mass) 20–50 (mass) 10 1.5 2.5 15 66 Case Studies Table Biomass fuels power plants in The Netherlands [22] Plant Type of co-firing Co-firing Co-firing (%, thermal) Status Burner configuration Gelderland Amer Amer Borssele 12 Maasvlakte Maasvlakte Hemweg Direct Direct Indirect Direct Direct Direct Direct Demolition wood Wood pellets Demolition wood Kernels, shells, fibers, paper sludge Biomass pellets Poultry litter Sewage sludge 10–12 27 10–15 5 Commercial Commercial Commercial Commercial Commercial Commercial Test phase Wall-fired Tangentially fired Tangentially fired Tangentially fired Tangentially fired Tangentially fired Wall-fired challenge, as always, is working within the relatively stringent fuel constraints while avoiding the inevitable high investment costs [22] Amer also co-fires at high biomass feed levels but uses a standard hammer mill configuration 5.05.5.2.2 Borssele Borssele is a 420 MWe, tangentially fired PCC unit, equipped with low-NOx burners with overfire air This plant co-fires coal and phosphor oven gas, which is transported from a nearby phosphor production plant through a pipeline About 80 000 t of gas are fired annually, which is equivalent to a 3.5% coal replacement Initially, there were concerns over the quality of the fly ash, due to risk of contamination by the presence of phosphate; however, the concentration of phosphorus is sufficiently low, and as such, no adverse effects have been observed [27] Lately, paper sludge, olive pulp, cocoa shells, palm kernels, and wood pellets have been co-fired over the coal belt through existing mills It is also possible to co-fire these fuels through separate mills and burners Sewage sludge has also been fired at Borssele 12, but it is no longer used due to odor problems [28] 5.05.5.2.3 Maasvlakte and Liquid organic waste from the petrochemical industry is co-fired with coal in the 518 MWe PCC, a tangentially fired plant fitted with flue gas desulfurization The liquid waste is handled and fired separately from the coal and constitutes about 1% of the output of the plant The disposal of the fly ash and the bottom ash produced is problematic due to molybdenum contamination of the liquid waste At Maasvlakte 2, the co-fired biomass pellets consist of a mixture of wood, composted sewage sludge, and paper sludge The pellets are mixed with the raw coal and milled in the existing milling equipment The pellet production plant is capable of manufacturing 150 000 t of pellets with a heating value of 16 MJ kg−1, and is situated adjacent to the power station During the demonstration trials, the operation of the mills was found to be normal and inspections revealed no damage or accumulation of woody material in the mills or transport lines The quality of the fly ash, bottom ash, and gypsum produced was tested and they fulfilled their regular quality specifications Also, no adverse effects on atmospheric emissions or wastewater effluent were observed Co-firing started commercially in 1998 and the amount co-fired is equivalent to 5% of the output of the plant [3] 5.05.5.3 United Kingdom The United Kingdom has, by law, set a stringent target to reduce its carbon emissions by 80% by 2050, with a preceding short­ to-medium target, where 20% reduction is expected by 2020 and 10.4% and 15.4% by 2011 and 2016, respectively [41] To meet this target, it is expected that about 30–40% of electricity would be generated from renewables, which is made attractive by the introduction of the Renewables Obligation, as power generators are awarded ROCs that are tradable Table shows the number of ROCs awarded for the production of MWh of electricity by different technologies The certificates were traded at a price of around £45 in 2010 [31] If a supplier cannot provide the required number of ROCs, they have to pay a buyout fee, which was £37 per ROC in 2010 [32] Currently, all the 15 coal-fired power plants have co-fired biomass, but several of them have stopped doing so A wide variety of biomass fuels have been utilized, including baled straw, woody materials, poultry litter, residues, tallow, and bonemeal (Table 7) 5.05.5.3.1 Aberthaw power plant This plant is located on the coast of south Wales in the Vale of Glamorgan It is one of the three RWE npower power stations co-firing a range of biomass Originally, the plant was designed to fire only coal and has been operating at full scale since 1971 with three operation units It has the capacity to generate 1500 MW of electricity As part of the company’s effort to invest in lower carbon technologies, it has invested over £9.5 million in biomass co-firing technology to allow the substitution of some of the coal burned Biomass Co-Firing Table The ROC bands for different approaches that use biomass [29, 30] ROCs (MWh−1) Approach Fuel type Co-firing Biomass 0.5 Co-firing of energy crops Energy crop which is one of the following: (1) Miscanthus giganteus; (2) Salix (also known as SRC willow); or (3) Populus (also known as SRC poplar) Co-firing with combined heat and power (CHP) Biomass by a qualifying CHP generating station and where the fossil fuel and biomass have been burned in separate boilers Co-firing of energy crops with CHP Energy crop which is one of the following: (1) Miscanthus giganteus; (2) Salix (also known as SRC willow); or (3) Populus (also known as SRC poplar) by a qualifying CHP generating station and where the fossil fuel and energy crops have been burned in separate boilers 1.5 Dedicated biomass power generation Electricity generated solely from biomass 1.5 Dedicated energy crops power generation Energy crop which is one of the following: (1) Miscanthus giganteus; (2) Salix (also known as SRC willow); or (3) Populus (also known as SRC poplar) generating electricity solely from energy crops Electricity generated from biomass by a qualifying CHP generating station in a calendar month in which it is fueled wholly by biomass As above but with energy crops Dedicated biomass with CHP Dedicated energy crops with CHP Table 67 Information on UK co-firing power plants [33] Company Power station Installed capacity (MW) Primary fuel AES Alcan British Energy Kilroot Lynemouth Eggborough 390 420 2000 Coal/oil Coal Coal Drax Power Limited EDF Energy EDF Energy E.On UK E.On UK E.On UK International Power RWE npower RWE npower RWE npower Scottish and Southern Energy Scottish and Southern Energy ScottishPower ScottishPower Uskmouth1 Power Company Drax West Burton Cottam Kingsnorth Ironbridge Ratcliffe Rugeley Aberthaw B Tilbury B Didcot A Ferrybridge C 4000 2000 2000 2034 964 2000 1000 1553 1029 1940 2034 Coal Coal Coal Coal/oil Coal Coal Coal Coal Coal/oil Coal/gas Coal Olive pellets Wood pellets, olive pellets 2.5% PKE, olive pellets and pulp, shea pellets and meal 3% Energy crops/wood pellets 5% Biomass blend – wood pellets, shea, miscanthus 5% Biomass blend – wood pellets, olive cakess Cereal residues Wood chips, PKE None None Tallow, sawdust, PKE, wood chips, small roundwood 3% PKE, sawdust, olive residues PKE, sawdust, olive residues, shea, wood pellets 10% Biomass – wood, olives, shea residues, PKE Fiddler’s Ferry Cockenzie Logannet Uskmouth Littlebrook 1995 Coal 10% Biomass 1200 2400 393 685 Coal Coal Coal Wood pellets Sewage sludge/wood pellets Shea meal Palm oil Co-firing fuel 68 Case Studies with biomass fuels such as sawdust, PKE, and wood chips in a 55 MW existing generating plant [34] It employs the direct injection technique, with 5% (thermal basis) of biomass The biomass is milled in a separate biomass milling plant using hammer mills and then injected into the boilers through dedicated injectors and burns in the coal flames, rather than milling the biomass and coal together This has been achieved without significant modification of the existing boiler Liquid biomass fuels such as tallow and tall oil have also been co-fired at the plant for a number of years 5.05.5.3.2 Didcot power plant RWE npower invested over £3.5 million in the Didcot power plants to provide enough renewable electrical energy, through biomass co-firing, to over 100 000 homes each year [34] It is estimated that this investment could replace about 300 000 t of coal and avoid 700 000 t of CO2 emissions into the atmosphere Didcot A has an installed capacity of about 2000 MW, which was originally designed to fire coal or gas It has been modified to fire biomass materials such as PKE, sawdust, olive residue, shea residue, and wood pellets alongside coal in its co-firing operations The biomass is co-milled with the coal at Didcot and burned through the existing unmodified coal burners 5.05.5.3.3 E.ON (Kingsnorth) Pelletized straw materials have been co-fired at the plant’s four 500 MWe, tangentially fired, low-NOx units The pelletized biomass fuel is preblended with the coal in the coal handling system and is fired through the existing handling and firing system Before the commissioning of the plant, pretrials and health and safety checks had been completed, which led to the installation of temporary blend facilities and additional explosion suppression systems During the pretrials, it was observed that for every percentage increment of biomass in the co-firing mixture, there was a corresponding 2% reduction in mill capacity, even though this depended largely on the type of biomass There were also noticeable reductions in carbon, NOx, and SOx emissions, and the ash produced met salable standards [3] 5.05.5.3.4 Drax Drax has invested in a multimillion-pound co-firing facility to allow the company to meet its set target of generating 12.5% of the total electricity from biomass co-firing, reducing carbon emissions by about 15% and saving over 2.5 million tonnes of CO2 annually This investment puts the company in the forefront of developments to establish alternative fuels technology for power generation in the United Kingdom The company has again announced plans to build three biomass co-firing plants, with each having an installed capacity of 300 MW This could result in the company being responsible for the supply of at least 15% of UK renewable energy and up to 10% of total UK electricity [35] Drax Power Limited has been burning biomass since 2003, with its main fuel being sustainable wood-based products and residual agricultural products such as sunflower seed husks and peanut husks, which are secured through a supply agreement for biomass with local producer groups and supplier groups for all their required biomass fuels 5.05.6 Health and Safety Issues Associated with Co-firing 5.05.6.1 Spontaneous Fires Spontaneous combustion has always been one of the most serious hazards in the power generation industry, as recorded in the United States, France, Great Britain, and Australia from the 1950s to the 1990s [36] Spontaneous combustion is most likely when deep deposits of coal have been heated and moistened by steam purges For vertical-spindle mills, deposits of coal in the air inlet to the mill are particularly dangerous because the air inlet temperature is higher than the temperature in all other parts of the mill In addition, smoldering could take place, without causing excessive increase in mill outlet temperatures, which could remain undetected for a considerable period of time Biomass, like all materials with high-volatile matter, is often prone to spontaneous combustion, which makes storage, milling, and transport often problematic [37] A combination of low humidity levels and dust particulate accumulation can expose the plant to a high fire risk due to the occurrence of combustion and deflagration It has, however, been found that for an air/gas mixture, deflagration may fail if the ratio of gas to oxygen is too high (fuel-rich mixture) or too low (fuel-lean mixture) For a coal dust/air mixture, the lower explosive limit for lignite and bituminous coals is 30 and 140 g m−3, respectively Biomass fuels have a lower explosion limit similar to lignite at around 30–40 g m−3 [37] The concept of a fuel-rich upper explosive limit for any PF milling system is questionable if air is used as the transport medium due to the sufficiently energetic source of ignition It has been suggested that PF particles can use the available air to continue to react, even in the fuel-rich region Generally, for bituminous coals, the risk of deflagration occurring can be reduced if the air/fuel ratio within the PF supply system is limited It has also been found, based on empirical data and laboratory analysis, that applying a similar operating restriction when co-milling biomass should not pose any additional threat during normal operation, provided that blend concentrations are kept within well-defined limits This is due to the fact that the explosion characteristics of biomass/coal blends containing up to 15 wt.% biomass are dominated by the coal blend [37] Work by Caini and Hules [38] found that suppression systems could be used to minimize or prevent the occurrence of fires and deflagration, especially in coal bunkers, feeders, and pulverizers This system injects inert gases or particles like sodium bicarbonate to dilute the oxygen concentration to a level where fire and deflagration of the mixture are not possible [43] It is very important that Biomass Co-Firing 69 the oxygen level be reduced to a level below 15% and above 12%, as below 15% oxygen fire cannot thrive and below 12% visible signs of oxygen depletion might set in [39] 5.05.6.2 Exposure to Biomass and Coal Dust Workers subject themselves to high risk of exposure to dust when working with biomass and coal Dry biomass particles are easily suspended in air due to their low density and large drag coefficient Exposure to dust at work contravenes the Control of Substances Hazardous to Health (COSHH) regulations, which clearly stipulate that workers should not unnecessarily be exposed to chemical, physical, or biological agents that may harm their health Where it is impossible to prevent exposure, steps must be put in place to control the situation to reduce as much as possible the level of contamination and exposure Moreover, workers must be given the necessary training and protective equipment to protect themselves from unnecessary exposure when it becomes inevitable Information on suitable protective equipment against exposure can, in the UK, be provided by the Health and Safety Executive (HSE) where recommendation has been made for suitable protection against exposure to specific materials such as wood dust and grains This information has been found to be applicable to biomass materials such as cereal pellet and wood pellet [37] The effects of coal or biomass dust on health are very diverse, depending on the type of dust But dust mainly affects the lungs and the respiratory systems through inhalation, creating the risk of nasal cancer Other dust-associated health problems include dermatitis and soreness of the eyes, abrasion, and conjunctivitis It is strongly advised that people who are allergic to dust must avoid exposure completely Palm and shea residues originally contained nuts; hence, it is advisable for people who have a nut allergy to avoid working in storage areas or areas where dust inhalation is possible In general, the use of a disposable filtering facepiece respirator would be enough to provide adequate protection, unless personnel are exposed to high concentrations of dust that might be beyond the designed limit of the respirator In such cases, dust levels must be continuously monitored to determine the risk of exposure Dust formation can be suppressed or prevented by the use of suppression systems such as moisture or foaming agents This system of dust prevention requires very low capital investment compared with full dust extraction systems However, misting systems can also generate conditions that are favorable for mold growth on stored biomass This problem clearly depends on biomass type, plant design, atmospheric conditions, and cleaning regimes Currently, there is limited information regarding the effects of mold on health, as well as human susceptibility which varies considerably Even though the majority of the molds found on biomass are common species and pose no known harm to health, persons with impaired immune systems may be at risk when exposed to high levels of airborne molds and fungal spores Good housekeeping such as minimizing storage times, the immediate cleaning up of spillages, and minimization of dust and moisture levels can reduce the ambient spore loading in areas where biomass is handled Currently, there is no regulatory limit on airborne levels for spores, even though the level of bacteria and microbiological organisms is covered by COSHH This, however, does not negate the potential health hazards of working with biomass; hence, good housekeeping procedures and personal protective measures must be adhered to in order to mitigate these risks Under COSHH, hardwood dust is classified as a carcinogen which has the potential to cause lung and respiratory problems However, only softwoods are usually used as biomass fuels and they present a significantly lower risk than hardwoods [37] Mycotoxins and endotoxins are by-products from the growth of mold and breakdown of bacterial cells, respectively They contribute to naturally occurring aerosols which are generally noninfectious Nevertheless, they may cause irritations mainly of the respiratory tract such as mucous membrane irritation (MMI), immunotoxic diseases, and allergic diseases (e.g., asthma and allergic rhinitis) Like dust, there is no exposure limit; the general approach for protection against endotoxins and mycotoxins has been to limit exposure The release of volatiles from many volatile organic compounds (VOCs) in biomass materials gives them their characteristic smell Some of these biomass materials, especially olive, may release substances such as carbon monoxide, hydrogen sulfide, methane, CO2, and volatile fatty acids (e.g., acetic acid, propanoic acid, and butyric acid) In general terms, this may not be harmful, but there may be potential health and safety hazards when personnel are exposed in a confined space with poor ventilation An employee from a power station in The Netherlands suffered carbon monoxide poisoning after unloading a consignment of olive cakes in a confined space Risk could be reduced by conducting an appropriate laboratory screening of the fuel prior to delivery 5.05.7 Technical Issues regarding Biomass Co-firing There are vast characteristic differences between coal and biomass largely influenced by the behavior of coal and biomass blends under co-firing conditions Apart from the heating value of coal being almost twice that of biomass and the corresponding bulk density of biomass being significantly less than that of coal, the moisture content of biomass is usually much higher than that of coal, ranging from 25% to over 50% The ash content of biomass can also vary from less than 1% to over 20% Moreover, the fuel nitrogen of biomass can vary from 0.1% to over 1%, but its sulfur content is usually very low [3] 70 Case Studies 5.05.7.1 Fuel Delivery, Storage, and Preparation Work on the delivery, storage, and preparation of coal for power generation has been well documented However, when coal is co-fired with biomass, new challenges arise, which are primarily due to the differences in their properties The low bulk density of biomass requires different types of handling, storage, and preparation compared with coal The low energy and high shear strength require a receiving pit as open as possible to allow sufficient unloading for the boiler capacity, and a screening device designed to meet the irregular shapes of the biomass material Storing biomass with moisture content greater than 20% for a long period of time can cause problems, which could lead to the growth of biological activity causing self-heating of the storage piles, loss of dry matter, and significant deterioration of the physical quantity of the fuel [4] This means that moist biomass cannot be stored on-site for a long period of time; hence, co-milled mixtures of biomass and coal must be prepared shortly before the fuel is fired or else the quality of the feedstock will be deteriorated due to the degradation of the biomass [47] There is also the possibility that high dust and spore concentration in the stored fuel can create health and safety issues during subsequent fuel handling operations [37] Taking appropriate steps prior to delivery can minimize biological activity during long-term storage but significantly add to the cost of the fuel Some of these steps are as follows [5]: • • • • Storage of biomass in billets or larger pieces, if possible, to reduce the surface area available for biological activity Using fungicides and other chemical agents to suppress biological activity Predrying of the fuel to a moisture level where biological activity cannot flourish Cooling the stored fuel by forced ventilation to temperatures where biological activity can be minimized On the other hand, some biomass materials (such as PKE and wood pellets) have moisture content below 20% and are not affected by biological activity to the same extent as wet fuels Due to the nature of biomass and its high moisture content, there is a risk of spontaneous combustion occurring This can be prevented by adhering to the following guidelines [3]: • Storage piles should consist of a homogeneous material • Biomass piles should not be compacted • Temperature and the gas composition in the pile should be monitored Co-milling of blended coal and biomass with already existing coal mill equipment may require significant modification, since the equipment was designed to suit the brittle nature of coal Since biomass is not brittle, the breakage mechanisms are different Biomass/coal ratios may be limited if the biomass is not milled to the required specification Moreover, the use of wet biomass can alter the heat balance in the mill and wet biomass also has the tendency to accumulate in the mill, which can be problematic during normal operation and when emptying During milling and processing, biomass releases combustible volatiles at lower tempera­ tures than coal, which can result in health and safety issues that need to be considered especially during start-ups, shutdowns, mill trips, and restarts [3] 5.05.7.2 Supply of Biomass To meet the quantities of biomass suitable for co-firing, power generators usually rely on imported biomass fuels Dried or pelletized wood is widely available in countries such as North America, Scandinavia, Russia, and specific European countries Other biomass materials such as olive and palm residues can be sourced from countries with large olive or palm oil production such as Spain, Italy, Greece, Turkey, Tunisia, Portugal, Malaysia, and Thailand [40] Oil, sugar, and starch energy crops can be used for the production of liquid fuels with high energy content as in biodiesel and bioethanol However, they are a primary food stock; hence, their full-scale utilization may compete with and defeat their main purpose of serving as food for human consumption 5.05.7.3 Properties of Biomass and Their Effects on Plant Operations As mentioned earlier, biomass and coal have diverse characteristics Generally, biomass has a higher moisture content (about 50–70% in fresh wood) than coal (about 3% in bituminous coal), resulting in low CV of the biomass fuel The volatile matter content of biomass is close to 80% and 20% fixed carbon (on a moisture-free and ash-free basis), whereas bituminous coal has around 20–30% volatile matter and 70–80% fixed carbon [5] Moreover, both particle size variations and the high fiber content of biomass contribute to the poor flow properties of biofuels This means that, with the exception of pelletized fuel made from dry raw materials, high internal and external frictions will occur during movement of the material, making it more abrasive and somewhat corrosive [6] Apart from the physical properties, the chemical properties of wood biomass set demanding requirements for power plant operation These properties include total ash content, ash melting behavior, and the chemical composition of ash Alkaline metals present in the ash are generally responsible for fouling the heat transfer surfaces and are abundant in wood fuel ashes which will be easily released in the gas phase during combustion [6] It is known that a small concentration of chlorine in the fuel can result in the development of harmful alkaline and chlorine compounds on boiler heat transfer surfaces [6] Biomass Co-Firing 71 EUBIONET concluded that most of the problems with boiler performance when co-firing arise from the difference in properties between the coal and the biomass fuel (Table 8), which can be summarized as follows (Maciejewska 2006): • Biomass has a higher inherent moisture content • Pyrolysis starts at lower temperatures with biomass than with coal • The volatile matter content of biomass is higher than that of coals, even that of high-volatile coals • The proportion of heat that is generated from the volatile fraction of biomass is approximately 70% compared with 30–40% for coal • The CV of the volatile matter from biomass is significantly lower than that from coal • Biomass char contains more oxygen than coal char and is also more porous and reactive • Biomass ash tends to be more alkaline, which increases the chances of fouling in the boiler • Biomass can have high chlorine content, but typically low sulfur and ash content These variations mean that if biomass is blended with coal, the following implications may be expected: • • • • • • Increased rate of deposit formation More frequent soot blowing Higher risk of corrosion of heat transfer surfaces Bed material agglomeration (in fluidized beds) Higher in-house power consumption, particularly with the mills Higher flue gas temperature Table Properties of biomass and their effects on power plants Properties Physical Moisture content Bulk density Particle dimension and size distribution Chemical Carbon (C) Hydrogen (H) Oxygen (O) Chlorine (Cl) Nitrogen (N) Sulfur (S) Fluorine (F) Potassium (K) Sodium (Na) Magnesium (Mg) Calcium (Ca) Phosphorus (P) Heavy metals Impacts Storage durability Dry matter losses Low CV Self-ignition Fuel logistics (storage, transport, handling) Determines fuel feeding system Determines combustion technology Drying properties Dust formation Operational safety during fuel conveying Gross CV (GCV; positive) GCV (positive) GCV (negative) Corrosion NOx, N2O, HCN emissions SOx emissions, corrosion HF emissions, corrosion Corrosion (heat exchangers, superheaters) Lowering of ash melting temperature Aerosol formation Ash utilization (plant nutrient) Corrosion (heat exchangers, superheaters) Lowering of ash melting point Aerosol formation Increased ash melting temperature Ash utilization (plant nutrient) Increased ash melting temperature Ash utilization (plant nutrient) Increased ash melting temperature Ash utilization (plant nutrient) Emission of pollutants Ash utilization and disposal issues Aerosol formation 72 Case Studies While the magnitude of these implications depends on the quality and the proportion of biomass in the fuel blend, the overall result is that operating and maintenance costs may increase However, this can be reduced or avoided with appropriate fuel blend control, where optimum amounts of the biomass fuel in the fuel blend can be defined with appropriate combustion tests together with bed material and deposit quality assessment [6] 5.05.8 Conclusions Coal will continue to play a critical role in the global energy mix for the foreseeable future, despite all the known impacts that come from its combustion There are many technologies that are destined to mitigate emissions from coal Co-firing of coal with biomass has been identified as a low-cost option for efficiently and cleanly converting biomass to energy by adding biomass as a partial substitute fuel in high-efficiency coal boilers Moreover, it makes good use of materials that would otherwise end up in landfill, thereby serving a dual purpose of mitigating emissions and contributing toward a cleaner environment However, biomass co-firing cannot thrive without adequate financial support to entice power generators and favorable legislation from the government to allow the technology to prevail This support is common to many new technologies; however, the huge benefits associated with biomass co-firing, in terms of its ability to use existing PF technologies and achieve direct and immediate results, make such support essential Biomass co-firing is not a stand-alone plant; it employs the already existing coal-fired technologies with slight modification, or no alteration at all if the biomass percentage is low This chapter has discussed in detail the common technologies employed in biomass co-firing It is found that, among the three main biomass co-firing options, the direct co-firing method is the simplest to operate and common among many biomass co-firing power stations The indirect and parallel co-firing options also have some advantages; however, the complexity and the cost involved in their operations deter power generators from embarking on that route Notwithstanding, there are pockets of plants, especially in Europe, where the technology is used Biomass co-firing technology, despite presenting numerous benefits, also presents significant challenges Health and safety issues associated with the handling of the biomass and the coal have been discussed in this chapter, and it is generally accepted that adequate measures are needed to protect personnel, such as the wearing of personal protective equipment (PPE), and to prevent spontaneous fires Coal bunkers, feeders, and pulverizers may be protected by injecting adequate amounts of inert gas by using suppression systems This would dilute the oxygen concentration to a level where deflagration of the mixture is not possible Another challenge faced by the technology is the effects of the properties of biomass on boiler equipment Some of these effects include increased rate of deposit formation and soot blowing, risk of corrosion of heat transfer surfaces, and bed material agglomeration These effects are found to be reduced in lower biomass percentage and very pronounced in higher biomass, and can be reduced if appropriate fuel blend controls are adhered to Progressive developments based on an increasing awareness of biomass behavior will lead to increases in the optimum amount of the biomass that can be introduced into the blend, thus achieving maximum environmental benefit with minimal impact on the plant References [1] McPherson B (2010) Development and application of carbon dioxide (CO2) storage for improving the environmental impact of advanced power plants In: Roddy D (ed.) 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Energy and Environment 16(5): 767–779 [17] World Coal Association (2011) Improving efficiencies http://www.worldcoal.org/coal-the-environment/coal-use-the-environment/improving-efficiencies/ (accessed 25 February 2011) Biomass Co-Firing 73 [18] Khan AA, De Jong W, Jansens PJ, and Spliethoff H (2009) Biomass combustion in fluidized bed boilers: Potential problems and remedies Fuel Processing Technology 90: 21–50 [19] Belgiorno V, De Feo G, Della Rocca C, and Napoli RMA (2002) Energy from the gasification of biomass Waste Management 23: 1–15 [20] Vamvuka D (2010) Overview of solid fuels combustion technology Handbook of Combustion 5: 31–84 [21] Dai J, Sokhansanj S, Grace JR, et al (2008) Overview and some issues related to co-firing biomass and coal The Canadian Journal of Chemical Engineering 86: 367–386 doi:10.1002/cjce.20052 [22] International Energy Agency (IEA) (2005) Biomass combustion and co-firing IEA Bioenergy Task 32 http://www.ieabcc.nl/database (accessed 20 November 2010) [23] Deal C (2007) Climate change technology transfer: Opportunities in the developing world http://www.wise-intern.org (accessed 11 February 2011) [24] Klara SM (2009) Biomass for thermal energy and electricity: A research and development portfolio for the future – Before the Committee on Science and Technology, Subcommittee on Energy and Environment, US House of Representatives National Energy Technology Laboratory (US DOE), Pittsburgh, USA [25] Maniatis K (2001) Progress in biomass gasification: An overview http://www.ec.europa.eu (accessed 11 February 2011) [26] Jansen JC, Uslu A, and Lako P (2010) What is the scope for the Dutch government to use the flexible mechanisms of the Renewables Directive cost-effectively? Energy Research Centre of the Netherlands http://www.ecl.nl (accessed February 2011) [27] Spliethoff H (2010) Power generation from biomass and waste In: Power Generation from Solid Fuels: Power Systems, ch 6, pp 361–467 London: Springer doi: 10.1007/978-3-642-02856-4_6 [28] Beekes ML, Gast CH, Korevaar CH, et al (1998) Co-combustion of biomass in pulverised coal fired boilers in the Netherlands Proceedings of the 17th Congress of the World Energy Council, 13–18 September 1998, Houston, TX, USA [29] Department of Energy and Climate Change (DECC) (2010) http://chp.decc.gov.uk/cms/roc-banding (accessed 28 February 2011) [30] Flower MF (2010) Combustion of Single Biomass Particles in a Heated Wire Mesh Apparatus with Video Based Measurements PhD Thesis, Imperial College London, 288pp [31] UK Powerfocus (2010) McCloskey 125 [32] Ofgem (2010) The renewables obligation buy-out price and mutualisation ceiling 2010–11 http://www.ofgem.gov.uk/Media/PressRel/Documents1/RO%20BuyOut%20price% 202010%2011%20FINAL%20FINAL.pdf (accessed 28 February 2011) [33] UK Department of Trade and Industry (DTI) (2006) The economics of co-firing Final Report No URN06/1959 [34] RWE npower (2011) About Didcot A power station http://www.npower.com/rwenpowercr (accessed 27 February 2011) [35] Drax (2011) Corporate and social responsibility: Co-firing http://www.draxpower.com (15 January 2011) [36] Kuchta JM, Rowe VR, and Burgess DS (1980) Spontaneous combustion susceptibility of US coals US Bureau of Mines, RI874 Washington, DC, USA [37] Colechin M (2005) Best practice brochure: Co-firing of biomass (main report) DTI Report No COAL R287, DTI Pub URN 05/1160 [38] Caini KC and Hules KH (1986) Coal pulverizer explosions Industrial dust explosions ASTM STP 958 Cashdollar and Hertzberg (eds.) American Society for Testing and Materials, Philadelphia, pp 200–216 [39] FS (Fire Suppression) (2011) http://www.fire-suppression.co.uk (accessed 22 January 2011) [40] Livingstone WR (2005) A Review of the Recent Experience in Britain with the Co-firing of Biomass with Coal in Large Pulverised Coal Fired Boiler Mitsui Babcock Copenhagen, Denmark: IEA Exco Workshop on Biomass Co-firing [41] DECC (2010) http://chp.decc.gov.uk/cms/roc-banding [42] Fernando (2010) Co-gasification and indirect co-firing of coal and biomass CCC/158, 37pp ISBN 978-92-9029-478-8 [43] Granatstein DL (2002) Case study on BioCoComb biomass gasification project, Zeltweg Power Station, Austria IEA Bioenergy Task 36 Report, September [44] Heinrich E and Weirrich F (2004) Pressurised entrained flow gasifiers for biomass Environmental Engineering Science 2153 [45] Maciejewska A, Veringa H, Sander J, and Peteves SD (2006) Co-firing of biomass with coal: Constraint and role of biomass pre-treatment European Commission Directorate General Joint Research Centre EU 22461 EN [46] Livingstone WR (2007) Advanced biomass co-firing technologies for coal-fired boilers Doosan Babcock, Technology and Engineering, Renfrew, Scotland Available at http://www.see.ed.ac.uk (accessed 10 November 2010) [47] Jenkins BM, Miles Jr TR, and Miles TR (1998) Combustion properties of biomass Fuel Processing Technology 54(1–3): 17–46 Further Reading [1] Cliff D, Rowlands D, and Sleeman J (1996) Spontaneous Combustion in Australian Coal Mine Queensland, Australia: SIMTARS [2] Department of Energy & Climate Change (DECC) (2009) Calculating the level of the Renewables Obligation http://www.decc.gov.uk/assets/decc/what%20we%20do/uk%20energy% 20supply/energy%20mix/renewable%20energy/renewable%20energy%20policy/renewables%20obligation/1_20091001145510_e_@@_calculatingtheleveloftherenewablesobligation.pdf (accessed 15 January 2011) [3] US Department of Energy (DOE) (2000) Biomass co-firing: A renewable alternative for utilities National Renewable Energy Laboratory, DOE/GO-102000-1055 http://www.nrel.gov/ docs/fy00osti/28009.pdf (accessed 15 November 2010) [4] Gouws MJ and Knoetze TP (1995) Coal self-heating and explosibility Journal of the South African Institute of Mining and Metallurgy 1995: 37–43 [5] Kiel J (2009) Biomass co-firing in coal fired power plants: Status, trend and R&D needs Energy Research Centre of the Netherlands Bioenergy Seminar, Brussels, Belgium http:// www.ieabcc.nl (accessed 15 February 2011) [6] Nasrin AB, Ma AN, Choo YM, et al (2008) Oil palm biomass as potential substitution raw materials for commercial biomass briquettes production American Journal of Applied Sciences 5(3): 179–183 [7] Scurlock JMO (1999) Miscanthus: A review of European experience with a novel crop US Department of Energy’s Environmental Science Division, ORNL/TM-13732, Publication No 4845 Tennessee, USA [8] Tillman D, Plasynski S, and Hughes E (2002) Biomass co-firing: Results of technology progress from co-operative agreement between EPRI and USDOE The 27th International Technical Conference on Coal Utilisation and Fuel Systems Clearwater, FL, USA, 4–7 March  ... 2–7 6–1 0 40 55 5 60 45 65 5 0–6 0 50 –6 0 1 7– 25 15 20 6 0–7 0 2 6–2 8.3 7 6–8 7 3 .5 5 0. 8–1 .5 2. 8–1 1.3 0 .5 3.1 < 0.1 0.003 4–1 2 20. 9–2 1.3 52 56 5 6 .5 1–3 3 0–4 0 < 0. 05 0.3 0.0 2–0 .06 0.8 5. 8 0. 05 0.1 18 .5 20... 18 .5 20 48 52 6. 2–6 .4 0. 1–0 .5 3 8–4 2 < 0. 05 0.00 1–0 .03 0.0 2–0 . 05 0. 1–0 .5 18 .5 23 48 52 5. 7–6 .8 0. 3–0 .8 24. 3–4 0.2 < 0. 05 0.0 1–0 .03 0. 1–0 .4 0.0 2–0 .08 18 .5 20 48 52 6–6 .2 0. 3–0 .5 4 0–4 4 < 0. 05 0.0 1–0 .04... 0. 1–0 .4 0. 2–0 .9 18. 4–1 9.2 47 51 5. 8–6 .7 0. 2–0 .8 4 0–4 6 0.0 2–0 .1 0.0 1–0 . 05 0. 2–0 .5 0. 2–0 .7 17.4 45 47 5. 8–6 0. 4–0 .6 4 0–4 6 0. 05 0.2 0.1 4–0 .97 0.6 9–1 .3 0. 1–0 .6 17. 1–1 7 .5 45. 5–4 6.1 5. 7 5. 8 0. 65 1.04