Biomass Power

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Although there has been considerable recent evaluation of the impacts of using biomass as a feedstock for the production of transportation fuels, biomass has long been considered as a fuel for electricity production. In general, the types of biomass feedstocks that have been considered for power production (such as waste woodchips and bagasse) are different than those most often proposed for transportation biofuels, which have focused on either sugars and starches (particularly sugar cane and corn) or biomass specifically grown as energy sources, such as switchgrass or poplar.

The key environmental issues associated with using biomass for electricity production are changes in air pollutant emissions and the impacts due to biomass production. Biomass used for power production can be used in conventional boilers, usually co-fired with coal, or alone in boilers specifically designed to burn biomass. In some cases, biomass is used in applications where the desired product is process heat rather than electricity, but even for these cases, biomass displaces fossil fuels and therefore can have an advantage relative to CO2 emissions.

Gasification and direct combustion of biomass are not without potential environmental problems. In direct combustion systems, particularly stoker-grate furnaces, the amount of carbon remaining in ash can be quite high. Bottom ash may have an unburned carbon content as high as 50%, which may make the bottom ash more difficult to reuse. In such cases, it is not uncommon for the ash to be fed back into the boiler with the biomass to achieve higher carbon burnout. Fly ash composition can also be quite different than coal ash, with the potential for arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), and Hg [61]. The high alkali metal content of many biomass feedstocks can also result in increased slagging and fouling of the boiler, which can reduce the thermal efficiency and therefore increase the total life-cycle GHG emissions associated with the production and use of biomass. Emissions from wood combustion, particularly from systems that are not well tuned to optimize the combustion process, can include organic compounds, including polycyclic aromatic hydrocarbons (PAHs), at higher levels per unit energy input than coal-fired systems. In units where carbon burnout is poor, direct biomass combustion systems can emit higher amounts of carbon monoxide (CO) and potentially even black carbon (BC), although BC emissions may not be as great a concern for industrial biomass use as for smaller, less well-controlled applications such as residential wood combustion.

Biomass feedstocks include dedicated energy crops, agricultural residues, forest management residues, and urban wood waste. Dedicated energy crops can result in displacement of other crops, eventually leading to significant changes in land use. It is likely that any significant biomass production specifically as an energy source will lead to more intensive agricultural or forest management practices. These more intensive practices will be more likely to have the types of adverse environmental impacts associated with modern intensive agricultural production, such as increased runoff of fertilizers, herbicides, and pesticides; higher irrigation demand; and potential soil degradation. Additional impacts associated with increased biomass demand can occur in situations in which existing forests or other growth is displaced to meet that demand, whether for fuel or other uses [62, 63]. A full life cycle analysis of the types of land changes and displaced growth is needed to fully evaluate the impact on net GHG mitigation, as well as impacts such as changes in wildlife habitat, local air quality, and water quality and quantity.

In addition to GHG emissions and reductions, land use can also impact climate change through changing the surface albedo, which can have significant global warming impacts [64]. The magnitude and even the direction of albedo changes associated with changes in land use for increased biomass production for energy are unclear, but should be recognized and incorporated into climate estimates as possible and appropriate.

Use of agricultural and forestry residues will not likely have significant land use impacts, but could result in other environmental impacts. Harvesting of agricultural residues could impact soil quality if those residues had previously been plowed back into the soil. Use of forest residues could also result in changes to forest soil quality through the removal of material that would otherwise decompose naturally, as well as change ecosystems in other ways by altering a component of the food chain. Disturbance of forest soils can also result in loss of CO2 in the soil to the atmosphere, reducing the GHG benefit of this source of biomass. In both cases, additional fuel is likely to be needed for collection and transport of the residues, resulting in additional emissions of CO2 and other air pollutants.

Urban wood waste - such as tree trimming residues, pallets, and construction materials - is typically disposed of in landfills, so collection and combustion of this feedstock could have significant benefits by reducing waste methane production, landfill size, and potential leaching of associated metals into ground and surface water. A potential drawback is the presence of treated wood or metals associated with wood waste, which could be emitted from the combustion process.

Because biomass requires a much greater land area to produce the same energy content as fossil fuels, there will be greater emissions from transportation from fuel production to plant site (or alternatively, a larger number of plants located closer to the fuel production). In addition, biomass is typically seasonal, which means that the fuel must be stored following harvest and used at a steady rate over the course of the year. Both the increased transportation and storage requirements can have adverse environmental impacts, whether through increased air emissions from trucks or potential runoff and leaching from storage facilities. Reduced plant sizes may be required to match available fuel supply, which could also have implications for the types of pollution controls (and subsequent effectiveness of those controls) installed at the plant.

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