How To Power Generation From Biomass

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1 Overview of Power Generation from Biomass Folke Engström Foster Wheeler Development Corporation Livingston, New Jersey 1999 Gasification Technology Conference San Francisco, California October 19-20, 1999 ABSTRACT The advances in Fluidized Bed Combustion and Gasification technologies over the last few decades have enabled a significant increase in the utilization of biomass for power and heat production. The forerunner of this application has been the pulp and paper industry, because on-site fuel supply and energy demand. Municipalities and utilities now see biomass as a significant fuel to meet their energy demand and emission requirements. The range of available biomass fuels include wood-based fuels products such as wood chips, saw dust and bark; agricultural wastes such as straw, olive waste and rice husks. Sludges from paper mills and deinking plants and municipal sludges are also attractive biomass fuels. Recent environmental regulations have resulted in an increased interest in utilization of biomass in energy production. Today, Fluidized Bed Combustion is the leading technology in co-combustion applications due to its high fuel flexibility. Cofiring biomass in pulverized-coal-fired boilers has inherently low capital cost and will result in a near-term increase in the use of biofuels in electric power generation. Fluidized Bed Gasification of biomass was employed in the mid-1980s in lime kiln applications to replace heavy fuel oils in the calcination process. Today, Fluidized Bed Gasification is seeing its renaissance after successful commissioning of the biomass Atmospheric Circulating Fluidized Bed (ACFB) Gasifier at the Kymijarvi Power Station in Lahti, Finland. This paper presents an overview of Power Generation Technologies from Biomass including experiences from the Lahti ACFB Gasifier after 1 year of operation. INTRODUCTION Approximately 13% of world energy demand is supported by biomass fuels at a rate of about 1200 Mtoe per year, as presented in Figure 1. While it is the main energy source (33%) in developing countries, it represents only 3% of the energy consumption in industrialized countries.[1] Biomass represents 4% of the primary energy in the United States, whereas the corresponding figures are 21% in Finland and 17% in Sweden, which have a large forest-based pulp and paper industry. 1

2 The World Total: 386 EJ, 9206 MTOE Population: 5.3 billion Hydro 6% Nuclear 5% Biomass 13% Oil 33% Industrialized Countries Total: 257 EJ, 6131 MTOE (67%) Population: 1.3 billion (25%) Hydro Biomass 3% 6% Nuclear 8% Oil 36% Gas 24% Coal 23% Oil Coal Gas Nuclear Hydro Gas 19% Biom ass Developing Countries Total: 129 EJ, 3074 MTOE (33%) Population: 4.0 billion (75%) Biomass 33% Coal 24% Hydro 6% Nuclear 1% Gas 8% Oil 25% Coal 27% Figure 1 Primary Energy Consumption (1990) Currently, the major sources of biomass in industrial countries consist of residues from forestry, agriculture, industry and domestic wastes, and sludges. While these residue fuels provide an important initial feedstock for the bioenergy industry, large-scale energy production from biomass has to rely upon energy crops such as sugar cane, rapeseed, switchgrass and short rotation forestry. Efforts to forecast utilization of biomass fuels for power production have been undertaken by the World Energy Council, Food and Agricultural Organisation (FAO), the United Nations Statistical Office, and others. The Existing Policies Scenario (EPS) developed by Hislop and Hall [2] is a more conservative approach that assumes less ambitious measures based upon an extension of current policies. It forecasts that globally 80% of sugar cane and kraft pulp residues will be used, except in Africa where only 10% of these fuels will be utilized. For biomass residues, 5% of the potential is in Africa, 10% in Europe, and 40% in the remaining countries will be used. For energy plantations, it predicts 50% utilization in the U.S. and Organisation for Economic Cooperation and Development (OECD) in Europe, 20% in Latin America, 10% in Asia, and 5% in Africa. Hislop and Hall have also considered long-term opportunities for electricity from biomass. They have investigated various sizes of plants likely to be needed in different sectors of industry, region and type of biomass. The authors concluded that the potential annual market for power from biomass under the EPS scenario is 207 GW by the year They divided the power production potential into three size classes representing small-, medium- and large-scale plants, having an electric output of 0.5 MWe, 7 MWe and 40 MWe, respectively. The results of this analysis for different regions of the world, as well as for different types of biomass feedstock, are presented in Figure 2. 2

3 Number of Plants Sugar Industry Residues 2248 Number and Size of Plants (MWe) by 2025 Residues from Pulping Industry Other Residues Planatation MWe 40 MWe 7 MWe 40 MWe 7 MWe 40 MWe 7 MWe 40 MWe Europe US/Canada Ex-USSR Australia/New Zealand Latin America Africa Asia Reference: HisloP & Halll, Biomass Resources for Gasification Power Plants, 1994 Figure 2 Potential Markets for Energy from Biomass Systems under Existing Policies Scenario It is shown in Figure 2 that the potential market for biomass power plants is indeed very large. The market for small-scale plants (0.5 MWe) is huge (over 10,000 plants) and has been excluded from this presentation due to lack of reliable cost and operating data. RENEWABLE ENERGY POLICIES AND PROGRAMS The global warming debate and the need to develop a viable strategy for stabilizing CO 2 emissions makes renewable energy sources (RES) more attractive. The Kyoto Protocol mandates the industrialized countries to reduce Greenhouse Gas (GHG) emissions by roughly 6% below 1990 levels by The European Commission has recognized the urgent need to tackle the climate change issue and has adopted a 15% GHG reduction target by the year 2010 from the 1990 level. To meet this goal, the European Commission has set as a target in its White Paper to double the use of renewable energy from the present 6% to about 12% by the year Similarly, President Clinton has set a goal of tripling the use of bioenergy and bioproducts by 2010 to spur bio-based technologies, enhance U.S. energy security, and meet the environmental challenges of global warming. Without a clear and comprehensive strategy, accompanied by legislative measures, the necessary deployment of renewable energy will not occur and the targets will be missed. A long-term stable framework is required for the development of renewable sources of energy, covering legislative, administrative, economic and marketing 3

4 aspects. The European Commission is formulating its strategy in an Action Plan for implementation by its member states. Likewise, President Clinton has proposed additional research funding and tax credits to promote energy efficiency, bioenergy, and other clean energy sources. The proposal suggests extending for 5 years the current 1.5% per kilowatt hour tax credit for electricity produced from biomass. It also expands the types of biomass fuels eligible for the credit to include certain forest-related, agricultural and other resources. POWER GENERATION THROUGH DIRECT COMBUSTION OF BIOFUELS Sugar, pulp and paper industries traditionally used grate-fired boilers for power and steam generation. Biomass fuels range from wood byproducts, sugar cane and other agricultural residues to domestic/industrial wastes. This combustion technology is well established, with over 140 grate-fired boilers in operation by Foster Wheeler alone. In Europe and especially in Scandinavia, the major driving force that led to the development of Fluidized Bed Combustion was the need for more efficient technologies for the utilization of low-grade fuels such as biomass. Both Bubbling Fluidized Bed (BFB) and Circulating Fluidized Bed (CFB) technologies have received significant interest since the early 1970s for various biomass and waste applications. In the pulp and paper industry, BFB is the preferred technology for combustion of wood wastes and bark in smaller cogeneration plants. Higher environmental awareness, larger plants, and multi-fuel capabilities have favored the CFB boilers for power and steam production from biomass fuels. Foster Wheeler is the pioneer in the development and commercialization of BFB and CFB boilers and has maintained this leadership with 50% of the world s market share since the early 1980s. The development of next-generation CFB technology (i.e., Foster Wheeler Compact) offers lower investment costs and high efficiencies with excellent environmental performance as well as opportunities to utilize difficult biomass fuels such as agro wastes with low emissions (Figure 3). Cofiring is a highly promising technology for using biomass in large-scale utility boilers. This combustion technique is commonly used in the power boilers of pulp and paper industries, where combustion occurs on traveling grates in spreader-stoker boilers. This is especially popular in the United States as an immediate low-cost approach to greenhouse gas mitigation with utility boilers. Research to develop techniques for cofiring biofuels began in 1992, sponsored by EPRI, TVA, USDOE, and large U.S. utilities. Two distinct techniques are available to cofire biofuels in utility boilers: (1) biomass and/or other opportunity fuels are blended with coal in the coal yard, and the blend is transported through the crusher and further to the firing system; or, (2) the biofuel is prepared separately from the coal and injected into the boiler without impacting the coal supply or delivery process. The first approach has been used with less than 5% cofiring on a mass basis for pulverized coal (PC) boilers, and at moderate percentages (<20% 4

5 cofiring on a mass basis) for cyclone boilers. The second approach is preferred for PC boiler firing >10% biomasses. Figure 3 The Compact Separator of the New Foster Wheeler Principle of Operation POWER GENERATION BY GASIFICATION OF BIOMASS The primary advantage of CFB gasification is that it enables the substitution of expensive fossil fuels with cheaper fuels such as wood wastes. Gasification also has the advantage in separating noxious ash and gas constituents from the fuel gas prior to combustion in the boiler, and as such, provides opportunities for replacement of natural gas in heat-recovery steam generators (HRSGs). Finally, gasification of biofuels provides the means to reduce SO 2 and NOx emissions, and can further be used as a reburn fuel for a more dramatic reduction of NOx emissions. Foster Wheeler s CFB Gasification Technology was developed in the early 1980s for the pulp and paper industry to reduce the dependence on fossil fuels after a dramatic increase in oil prices. The first commercial CFB gasifier was supplied by Foster Wheeler Energia Oy in 1983 to Wisaforest Oy pulp and paper plant in Pietarsaari, Finland, to replace fuel oil in the lime kiln. Since then, similar gasification plants have been installed at two pulp mills in Sweden and one mill in Portugal (Table 1). 5

6 Table 1 Reference List of Foster Wheeler Atmospheric CFB Gasifiers Site Delivery Year Gasifier Capacity Drying Oy Wisaforest Ab Pietarsaari, Finland Norrsundet Bruk Ab Norrsundet, Sweden ASSI Karlsborgs Bruk, Sweden Portucell Rodao Mill, Portugal MW MW MW MW Flue gas recycling, Gas combustion Flue gas recycling, Gas combustion Flue gas recycling, Gas combustion Flue gas recycling, Gas combustion These atmospheric CFB gasifiers produce low-btu gas from bark and wood wastes for the lime kiln and utilize a part of the generated gas to dry biofuels from 50% to 15% moisture content prior to gasification. Lower oil prices in the late 1980s undermined the market for the ACFB gasifiers. A renaissance of the ACFB Gasification Technology was recognized in 1996 by Foster Wheeler Energia Oy as a means to produce Green Power for Lahden Lämpövoima Oy and at the same time, improve the environmental performance of the utility boiler. In 1997, the customer authorized Foster Wheeler to demonstrate, on a commercial scale, the gasification of biofuels to substitute about 15% of the total fuels burned in the main boiler, equaling up to 30% of the coal feed. FOSTER WHEELER ACFB GASIFICATION PROCESS The atmospheric CFB gasification system consists of a reactor, a uniflow cyclone to separate the circulating bed material from the gas, and a return pipe for conveying the circulating material to the bottom of the gasifier (Figure 4). From the uniflow cyclone, hot product gas flows into the air preheater, which is located below the cyclone. The gasification air enters the bottom of the reactor via an air distribution grid and fluidized the particles in the bed. At this stage, the bed expands and all particles are in rapid movement. The gas velocity is so high that a majority of bed particles are conveyed from the reactor to the uniflow cyclone. The fuel is fed into the lower part of the gasifier above a certain distance from the air distribution grid. The incoming biofuels contain 20-60% water, 78-39% combustibles, and 1-2% ash. The operating temperature in the reactor is typically ºC depending upon the fuel and the application. After entering the reactor, the biofuel particles dry rapidly and a first primary stage of reaction (namely, pyrolysis) occurs. During this reaction, fuel converts to gases, charcoal and tars. These products flow upward in the reactor and a secondary reaction takes place. This reaction can be divided into a series of 6

7 heterogeneous reactions, where char is one ingredient in the reaction and homogeneous reactions where all the reacting components are in the gas phase. Due to these reactions, a combustible gas is produced that is cleaned in the uniflow cyclone. Most of the solids in the system are captured in the cyclone and returned to the lower part of the gasifier reactor. These solids contain char, which is combusted in the fluidized the bed. This combustion process generates the heat required for the pyrolysis process and endothermic reactions. The circulating bed material serves as the heat carrier and stabilizes the temperatures in the process. REACTOR BIOFUEL FEED COOLING WATER 850 C 900 C CFB GASIFIER RE TU RN LE G UNIFLOW CYCLONE GASIFICATION AIR FAN AIR PREHEATER HOT LOW CALORIFIC GAS ( C) BOTTOM ASH COOLING SCREW Kymijarvi Power/Gasification Plant BOTTOM ASH Lahden Lämpövoima Oy operates the Figure 4 CFB Gasifier Concept Kymijarvi Power Plant, located near the City of Lahti in southern Finland. Originally, the power plant was oil-fired; however, it was modified for coal firing in The boiler is a Benson-type once-through boiler with steam conditions of 125 kg/s, 540ºC/170 bar and 540ºC/40 bar (204,000 lb/hr, 1004ºF/2465 psig, 1004ºF/580 psig) reheat. The maximum power output is 167 MWe and district heat production is 240 MW th. In the summer, when the heat demand in the city is low, the boiler is shut down. In the spring and fall, the boiler operates at a lower capacity with natural gas as the main fuel. The boiler uses 1200 GWh/a (180,000 ton/a) of coal and about 800 GWh/a of natural gas. The boiler does not have any sulfur removal system and burns low-sulfur ( %) coal. The burners are provided with flue gas recirculation and stage combustion for NOx control. Different types of biofuels and wastes, corresponding to 300 GWh/a, are available in the Lahti area to substitute for about 15% of the fuels burned in the main boiler. The design basis of the feedstock to the gasifier is shown in Table 2. The recycled fuel (REF) is produced from classified refuses from households, offices, shops and construction sites. In addition to the fuels listed above, peat, demolition wood waste, and shredded tires are used as fuels in the gasification plant. 7

8 Table 2 Gasifier Feedstock, on Average Fuel Amount % weight of total Moisture % weight Saw dust Wood residues (bark, wood chips, wet and fresh wood residues, etc.) Dry wood residues from the wood working industry (plywood, particle board, cuttings, etc.) Recycled Fuel (REF) No drying of the feedstock is required, which reduces the investment costs and simplifies the overall system (Figure 5). The incoming fuel is prepared in two receiving stations. REF and wastes from the wood working industry are crushed, screened and fed into a large storage silo, where homogenegation occurs with the biofuels and peat from the other receiving station after first screening out larger particles on a disk screen. BIOMASS GASIFICATION - COAL BOILER - LAHTI PROJECT Biomass 300 GWh/a -15 % fuel input 350 MW 540 C/170 bar CO 2 Reduction - 10 % Processing 50 MW Pulverized coal flames Power * 600 GWh/a District Heat * 1000 GWh/a Gasifier Gas flame Bottom ash Coal Natural Gas 1050 GWh/a -50 % 650 GWh/a -35 % Fly ash Figure 5 Flowsheet of the Lahden Lämpövoima Oy Gasifier 8

9 The gasification takes place at a temperature about 850ºC (1560 ºF) in a refractorylined steel vessel. The hot product gases flow from the gasifiers to a uniflow cyclone and are cooled down in the air preheater before being fed into the main boiler. Simultaneously, the gasification air is heated up in the air preheater before introduction into the windbox of the gasifier (Figure 6). Figure 6 Foster Wheeler CFB Biomass Gasifier, MW th The product gas from the gasifier flows to the boiler through two specially-designed burners at the lower coal burner level. Operating Experience The shakedown of the gasification plant was completed at the end of January 1998, and it was continuously operated until the power plant was shut down for summer maintenance on June 2 nd. The gasifier was recommissioned on September 21, 1998 and continued its operation through the end of the year. During 1998, 4,730 hours of operation in the gasification mode were logged with an availability of 82%. The operating experience during this first operational period was excellent. Only a few problems occurred at the gasification plant, and the availability of the plant has been high. Most of the problems in the beginning were related to the fuel processing plant. Lack of fuel and operational problems at the fuel processing plant decreased the availability of the whole plant during the first half of With regard to the gasification plant itself, the problems were related mostly to the use of shredded tires as a fuel in the gasifier. On several occasions, the wire content of tires (there is no additional separation of metal wires with magnet after shredding) was so high that 9

10 accumulated wires blocked the ash extraction system and the gasifier had to shut down. With all other fuel fractions, the operation of the gasification process was trouble-free. Regarding the gasification process itself, the results met expectations. At design operating conditions with regard to temperatures, pressures and flow rates, the process measurements with regard to product gas, bottom ash and fly ash composition were very close to the predicted values. Due to the high moisture content (up to 58%) of the gasifier fuels, the heating value of the product gas has been low, typically only MJ/m 3 n. The stability of the main boiler steam cycle has been excellent. Large openings for the low-btu gas burners have not caused any disturbances into the water/steam circulation. The product gas combustion has been stable, even with a high moisture content in the fuel. The stability of the main boiler coal burners behaved stably despite the fact that the product gas burners were installed very close to the lower level coal burners. In the beginning, the gasifier fuel consisted mainly of biofuels such as bark, wood chips, saw dust and uncontaminated wood waste. To-date, the supply of refuse fuel has been insufficient to meet the gasifier capacity. However, the fuel supply is expected to be increased in the future. Besides the above-mentioned fuels, railway sleepers (chipped on-site) and shredded tires have also been used as fuel in the gasifier. The following table summarizes the main fuels that were used during the first operating year. Furthermore, Table 3 presents the produced energy divided between different fuel fractions. Table 3 The Gasifier Fuels and Produced Energy during First Operating Year 1998 Fuel Amount (ton) Energy (GWh) Energy (%) Wood Residues 67, REF 8, Railway Sleepers 4, Shredded Tires TOTAL 79, The operating temperature of the gasifier was typically ºC and were consistent with the design temperature. The gasifier output varied between 35 and 55 MW depending upon the gasifier fuel moisture content and the required gasifier load. During 1998, the moisture content in the fuel mixture has been rather high, varying between 45 to 58%. Due to the high moisture content, the product gas heating value has been relatively low, typically MJ/m 3 n, as mentioned earlier. 10

11 The fuel gas quality more than met our expectations. Table 4 summarizes the typical measured values of the product gas main components. Table 4 Average Values of the Product Gas Main Components Gas Component Unit Average CO 2 %-vol (wet) 12.9 CO %-vol (wet) 4.6 H 2 %-vol (wet) 5.9 CxHy %-vol (wet) 3.4 N 2 %-vol (wet) 40.2 H 2 O %-vol (wet) 33.0 Main Boiler Flue Gas The main boiler emissions were of great interest with regard to the measurements during the monitoring phase. Briefly, the changes in the main boiler emissions with gasifier were very low. As indicated earlier, the main boiler is not equipped with DeNOx or DeSOx systems, and is subjected to the emission limits of 240 mg/mj for NOx (as NO 2 ) and 240 mg/mj for SOx. Table 5 summarizes the effect of the co-combustion of the gasifier product gas on the main boiler emissions. Table 5 The Effect of Gasifier to the Main Boiler Emissions Emission Change Caused by Gasifier NOx Decrease by 10 mg/mj (=5 to 10%) SOx HCl CO Particulates Heavy Metals Dioxins Furans PAH Benezenes Phenols Decrease by mg/mj Increase by 5 mg/mj* No change Decrease by 15 mg/m 3 n Slight increase in some elements, base level low No change * Low-chlorine coal in main boiler and REF + shredded tires used in gasifier. 11

12 The dust content in the flue gas after the ESP decreased by mg/m 3 n. The most likely reason for this is the increase in the flue gas moisture content, which enhanced the ESP operation. Perhaps, the most positive result has been the decrease in NOx emission. The NOx content of the main boiler decreased typically 10 mg/mj, representing a reduction of 5 to 10% from the base level. The most likely reasons for the decrease are the reburning effect of ammonia; and more importantly, the cooling effect of the low-btu, high-moisture product gas in the bottom part of the boiler. Due to this cooling effect, less thermal NOx was formed in the coal burners located at the lower part of the boiler. Because of the extremely low sulfur content of biofuels, the main boiler SOx emission decreased approximately mg/mj. In contrast, because of the very low chlorine content (0.1%) of the main boiler coal, the HCl content of the flue gas increased by approximately 5 mg/mj when the gasifier was in operation. This was caused by the use of REF fuel and shredded tires in the gasifier. Both fuels are known to contain chlorine. With regard to CO emissions of the main boiler, no changes were seen. PRESSURIZED GASIFICATION OF BIOFUELS The development and demonstration of the Biomass IGCC power plant is presented in another paper and not be repeated here. Some general comments are presented below: Biomass IGCC power plant technology is an advanced power generation technology for large-scale gasification in the range of MWe. Biomass IGCC technology is fully demonstrated in the Värnamo plant and offers higher efficiencies and lower emissions than conventional technologies. At present, the Biomass IGCC can not compete with natural gas combined cycles and low-cost conventional CFBs. Further further R&D investment is necessary to increase its competitiveness. Secured fuel supply for a Biomass IGCC plant over its lifetime is questionable. Electricity prices in today s deregulated market are too low to justify power generation from a Biomass IGCC plant. CONCLUSIONS The available renewable energy resources, including plantations, represent roughly 13 % of the primary energy consumption in the world. With the current cost structure of fuels and power, the targeted contribution (double or tripling the use of bioenergy) by 2010 will not be realized unless vigorous legislative actions are mandated within the next few years. 12

13 Conventional combustion techniques such as grate-fired, bubbling, and circulating fluidized boilers are proven and cost effective, and will steadily improve their efficiencies and emission performances. Cofiring is a low-cost approach for the utility-scale boiler to utilize Green power and achieve a reduction in greenhouse gas emissions. ACFB gasification is a very attractive approach for repowering existing utility and other boilers, with flexibility to meet regulatory stipulations for ash disposal. It has the potential to replace natural gas in many industrial applications. Biomass IGCC needs a secured, long-term fuel supply for large power plants ( MWe). A favorable change in today s power-to-heat tariff is required for it to effectively compete with the low-cost power generation technologies such as natural gas combined cycles. REFERENCES [1] World Energy Council, Survey of Energy Resources, 1995, Oxford: Holywell Press Ltd, [2] Hislop, D. and Hall, D.O., Biomass Resources for Gasification Power Plant. A report for the IEA Bio-Energy Agreement, Thermal Gasification Activity, London: IEA, 28 April