Perfect combustion of biomass and most other fuels would help minimize the emissions of particulate matter, and gaseous and toxic compounds. In practice, this can be difficult to attain, even with natural gas, the dominant component of which is methane, the simplest of all organic fuels (Chisholm and Klass, 1966). The carbon monoxide content of the flue gas is a good indicator of the completeness of combustion. The less carbon monoxide, the more complete the combustion process and the lower the emissions of organic compounds. As already indicated, a wide range of variables and independent parameters affect how closely perfect combustion can be approached for solid biomass fuels: particle size range and distribution, moisture content, composition, and heating value; air-fuel ratios, mixing, and reaction time; temperature and sometimes pressure range in the combustion chamber; and furnace design and heat-transfer methods. A compromise is often made between the operating parameters that promote complete combustion and those that minimize the emissions of inorganic derivatives. Under the best of conditions, a solid biomass fuel should consist of small, uniform particles, be low in moisture and ash contents, and have zero to very low chlorine, nitrogen, and sulfur contents. With the proper processing, virgin biomass such as wood can approximate these characteristics, whereas municipal solid waste is representative of biomass fuels that might be the furthest removed from them.

Emissions from biomass-fueled boilers can be controlled by a variety of methods. The control systems needed depend mainly on the composition of the feedstock. First, good combustion control is essential to maximize combustion and to minimize emissions of unburned hydrocarbons and carbon monoxide. Efficient removal of particulate matter in the flue gases can be achieved by various combinations of cyclonic separation, electrostatic precipi­tation, agglomeration, and filtration. Removal of acid gas emissions can be achieved by flue gas scrubbing and treatment with lime. There are several approaches to the control of NOx emissions (Clearwater and Hill, 1991). Combustion control techniques include use of staged combustion, low excess air, and flue gas recirculation. Staged combustion involves reduction of the maximum attainable flame temperature and the control of residence time. In the primary stage of combustion, the maximum flame temperature and thermal NOx formation are reduced by the transfer of heat, which is not returned, or by combustion with substoichiometric amounts of air. The formation of NO from chemically bound nitrogen is also largely avoided in the primary stage under these conditions if the residence time is sufficient to permit nitrogen to form. Combustion is then completed in the second stage with excess second­ary air at short residence times to minimize NO formation. Potential add-on controls include selective noncatalytic and catalytic reduction and natural gas reburning techniques. Selective noncatalytic reduction, such as ammonia injection, is one of the preferred methods because it has been effectively employed in several MSW combustion plants and has been shown to afford NOx emissions in the range of 120 to 200 ppmv. As discussed at the end of this section, staged combustion can be carried out without actually using hardware designed for staged combustion. The technique is quite effective for minimizing NOx in the Burlington, Vermont power plant.

Toxic polychlorinated dibenzo-p-dioxins (“dioxins” or PCDDs) and poly­chlorinated dibenzofurans (“furans” or PCDFs) can form on combustion of chlorine-containing biomass and be emitted in the flue gases and possi­bly in adsorbed form on flyash or in particulate matter. The isomer 2,3,7,8- tetrachlorodibenzo-p-dioxin, a strong carcinogen and a contaminant found in the defoliant Agent Orange used in the Vietnam War, is claimed to be the most lethally toxic of man-made chemicals when administered to guinea pigs (Esposito, Tiernan, and Dryden, 1980). The compound is destroyed when the temperature is 1000°C for as little as a millisecond or at lower temperatures for longer periods (Barnes, 1983). At stack gas temperatures below 200°C., the PCDDs and PCDFs are predominantly found on the particulate matter. These data suggest that control of combustion and downstream temperatures coupled with particulate matter reduction measures can reduce or eliminate PCDD and PCDF emissions.

MSW-fueled boiler systems present greater air pollution problems than most other biomass-fueled plants. Some of the advanced emission control systems used with a 350-t/day plant that meets California’s stringent South Coast Air Quality Requirements are described here to illustrate how effective the controls are. This particular plant is believed to have the lowest emissions of any refuse — to-energy plant in the world (Moore and Cooper, 1990). It is designed to generate 52,200 kg/h of steam at 435°C and 4483 kPa to supply a turbine generator developing 11.4 MW of electrical power. Combustion of MSW results in the formation of acid gases derived from chemically bound chlorine, fluorine, and sulfur in the refuse. These gases and particulate carryover from the boiler

must be removed before the flue gases are exhausted to the atmosphere. The flue gases enter the bottom of a dry scrubber through a cyclonic section designed to remove flyash particles larger than 150 /cm. From the cyclone, the gases flow upward through a spray section where atomized lime slurry is introduced. The lime reacts with the acid gases to produce nonacidic salts. Water in the slurry is completely evaporated by the flue gas, which lowers the temperature of the gas leaving the unit. The resulting dry reaction product falls out into a scrubber hopper. The remaining particulate matter in the flue gas is conditioned by a material (Tesisorb) that promotes particle agglomeration for subsequent removal by glass fabric filters. The removal efficiencies of the acid gases and particulates are in the high 90s on a percentage basis. Nitrogen oxides are reduced by ammonia injection above the combustion zone, and careful control of combustion conditions minimizes carbon monoxide, dioxin, and furan emissions. The carbon monoxide and nitrogen oxide emissions ranged from 18 to 25 ppmv and 48 to 69 ppmv, respectively. The dioxin emissions meet California’s requirements, assuming the lower analytical detec­tion limit is the actual emission.

It cannot be emphasized enough that the combustion process in biomass — fueled power plants should always be controlled with the objective of maximiz­ing boiler efficiency and minimizing stack gas emissions. These goals might be considered to be contradictory, since high-efficiency combustion generally means higher flame temperatures, which can result in higher NOx emissions. However, in a power steam generator firing whole tree chips, it is quite possible to achieve rated boiler efficiency and low NOx formation at the same time. Operation of the 50-MW plant in Burlington, Vermont under the proper conditions with 100% green, whole tree chips containing 40 wt % moisture afforded NOx emissions as low as 0.062 kg/GJ while still achieving a boiler efficiency in the range 68 to 73%, which is in the high end of the design range, without the use of postcombustion treatment or flue gas recirculation (Tewksbury, 1991). At full load, this plant is designed to burn 90.7 t/h of green wood fuel; the nominal steam capacity is 217,687 kg/h at 8828 kPa and 510°C. This performance was achieved at full load by careful control of the fuel distribution on the grates and the air-fuel ratio, and by balancing the overfire air and underfire air. Substoichiometric firing of the wood on the grates kept flame temperature and NOx formation low, but generated a high level of CO. A second level of combustion higher in the fire box occurs when additional air is added to complete the combustion process at a temperature where little or no further NOx is created. This operating mode simulated a two-stage combustion system. When the plant must be operated at minimum load, about 33% of normal load, a similar operating mode provided the best results, although the NOx emissions were slightly higher than before.