Biomass encompasses a wide variety of feedstocks, including solid biomass, i.e., forest product wastes, agricultural residues and wastes, and energy crops, biogas, liquid biofuels, and biodegradable component of industrial waste and municipal solid waste. Feedstock quality affects the technology choice, while feedstock costs, including transportation costs determine the process economics. Bioelectric plants are an order of magnitude smaller than coal-fired plants based on similar technology. This roughly doubles investment costs and reduces efficiency relative to coal. Biomass-based electricity generation is a base load technology and, provided that adequate supplies are available, is considered one of the most reliable sources of renewable-based power.
Methods for converting biomass to electricity fall into four main groups:
Combustion. The burning of biomass can produce steam for electricity generation via a steam-driven turbine. Current plant efficiencies are in the 30% range at capacities of around 20-50 MW. Using uncontaminated wood chips, efficiencies of 33-34% (LHV) can be achieved at 540°C steam temperature in combined heat and power (CHP) plants. Operated in an electricity-only mode this technology would generate at least 40% electricity output. With municipal solid waste, high-temperature corrosion limits the steam temperature that can be generated, thereby holding electric efficiency to around 22% (LHV). Plants with electric efficiencies of 30% (LHV) are in the demonstration phase. Supplying energy for district heating systems from municipal solid waste is expected to generate 28% electricity in CHP mode. Many parts of the world still have large untapped supplies of residues which could be converted into competitively priced electricity using steam turbine power plants. For example, sugar cane residues (bagasse) are often burned in inefficient boilers or left to decay in fields.
Stirling engines have received attention for CHP applications, but such systems are not yet competitive. Small-scale steam cycles also need to see cost reductions.1
Co-firing. Fossil fuels can be replaced by biomass in coal power plants, achieving efficiencies on the order of 35-45% in modern plants. Because co-firing with biomass requires no major modifications, this option is economic and plays an important role in several countries' emission reduction strategies (Box 5.1). To raise biomass shares above 10% (in energy terms), technical modifications and investments are necessary. Co-firing systems that use low-cost, locally available biomass can have payback periods as short as two years.
Gasification. At high temperatures, biomass can be gasified. The gas can be used to drive engines, steam or gas turbines. Some of these technologies offer very high conversion efficiencies even at low capacity. The biomass integrated gasifier/gas turbine (BIG/GT) is not commercially employed today, though the overall economics of power generation are expected to be considerably better with an optimized BIG/GT system than with a steam-turbine system. However, the costs are much higher than for co-combustion in coal-fired or fossil-fuelled power plants. Black liquor gasification, (discussed in Chapter 7), is economic for electricity and steam cogeneration. Other technologies being developed include integrated gasification/ fuel cell and bio-refinery concepts.
Anaerobic Digestion. Using a biological process, organic waste can be partly converted into a gas containing primarily methane as an energy carrier. This biogas can be used to generate electricity by means of various engines at capacities of up to 10 MW.2 While liquid state technologies are currently the most common, recently developed solid-state fermentation technologies are also widely used. Anaerobic digestion technologies are very reliable, but they are site-specific and their capacity for scaling-up is limited; thus, the market attractiveness of this approach is somewhat restricted. The increasing costs of waste disposal, however, are improving the economics of anaerobic digestion processes.
1A Stirling engine is a highly efficient, combustion-less, quiet engine that harnesses the energy produced when a gas expands and contracts as its temperature changes. Invented by Robert Stirling in 1816, the Stirling engine uses simple gases and natural heat sources, such as sunlight, to regeneratively power the pistons of an engine.
2 After purification, the gas can be used for production of transport fuels.
Box 5.1 Biomass co-firing potential for CO2 reduction and economic development
Biomass co-firing has been demonstrated successfully for most combinations of fuels and boiler types in more than 150 installations worldwide. About a hundred of these have been demonstrated in Europe, mainly in Scandinavian countries, the Netherlands, and Germany. There are about 40 plants in the United States and some 10 in Australia. A combination of fuels, such as residues, energy crops, herbaceous and woody biomass, has been co-fired. The proportion of biomass has ranged from 0.5% to 10% in energy terms, with 5% as a typical value.
Co-firing biomass residues with coal in traditional coal-fired boilers for electricity production generally represents the most cost effective and efficient renewable energy and climate change technology, with additional capital costs commonly ranging from USD 100 to USD 300 per kW.
The main reasons for such low capital costs and high efficiencies are (1) optimal use of existing coal infrastructure associated with large coal-based power plants, and (2) high power generation efficiencies generally not achievable in smaller-scale, dedicated biomass facilities. For most regions that have access to both power facilities and biomass, this results in electricity generation costs that are lower than any other available renewable energy option, in addition to a biomass conversion efficiency that is higher than any proven dedicated biomass facility.
Co-firing of woody biomass can result in a modest decrease of boiler efficiency. A typical reduction is 1% point boiler efficiency loss for 10% biomass co-firing, implying combustion efficiency for biomass that is 10% points lower than for the coal that is fired in the same installation. A coal-fired power plant with 40% efficiency would have an efficiency of 30% with co-firing, which is higher than for dedicated biomass-fuelled power plants. Biomass is either injected separately or it is mixed with coal. The challenges for wood co-firing are not so much in the boiler, but in wood-grinding mills. Co-firing of herbaceous biomass is technically possible, but results in a higher chance of slagging and fouling, and its grinding costs and energy use are higher than for other types of biomass.
Worldwide, 40% of electricity is produced using coal. Each percentage point that could be substituted with biomass in all coal-fired power plants results in a biomass capacity of 8 GW, and a reduction of about 60 Mt of CO2. If 5% of coal energy were displaced by biomass in all coal-fired power plants, this would result in an emission reduction of around 300 Mt CO2 per year. Furthermore, the biomass used in this process would be approximately twice as effective in reducing CO2 emissions as it would be in any other process, including dedicated biomass power plants. In the absence of advanced but sensitive flue gas cleaning systems commonly used in industrialized countries, co-firing biomass in traditional coal-based power stations will typically result
Box 5.1 (continued)
in lower emissions of dust, NOx, and SO2 due to the lower concentrations (ash, sulphur, and nitrogen) that causes these emissions. The lower ash content quantities of solid residue from the plant.
Biomass co-firing has additional benefits of particular interest to many developing countries. Co-firing forest product and agricultural residues adds economic values to these industries, which are commonly the backbone of rural economies in developing countries. This economic stimulus addresses a host of societal issues using markets rather than government intervention and involves rural societies with large-scale businesses such as utilities and chemical processing. Co-firing also provides significant environmental relief from field/forest burning of residues that represent the most common processing for residues. All of these benefits exist for both developed and developing countries, but the agriculture and forest product industries commonly represent larger fractions of developing countries' economies, and the incremental value added to the residues from such industries generally represents a more significant marginal increase in income for people in developing countries. Most developing countries are located in climatic regions where biomass yields are high and/or large amounts of residues are available. In countries that primarily import coal, increased use of biomass residues also represents a favorable shift in the trade balance
Co-firing of biomass and waste is now being actively considered. Blending biomass with non-toxic waste materials could regularize fuel supply and could enhance the prospects for co-firing. Certain combinations of biomass and waste have specific advantages for combustor performance, flue gas cleaning or ash behavior.
Source: Implementing Agreement for a Programme of Research, Development, and Demonstration on Bioenergy IEA Bioenergy Implementing Agreement and Implementing Agreement for the IEA Clean Coal Centre .
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.