PDF《1.6.1 General1-6》

2 mm and normally low moisture) is blown into the boiler and combusted by supporting it in air rather than on fixed grates. Rapid changes in combustion rate and, therefore, steam generation rate are possible because the finely divided fuel particles burn very quickly. A later innovation in wood firing is the fluidized bed combustion (FBC) boiler. A fluidized bed consists of inert particles through which air is blown so that the bed behaves as a fluid. Wood residue enters in the space above the bed and burns both in suspension and in the bed. Because of the large thermal mass represented by the hot inert bed particles, fluidized beds can handle fuels with moisture contents up to near

70 percent (total basis). Fluidized beds can also handle dirty fuels (up to

30 percent inert material). Wood fuel is pyrolyzed faster in a fluidized bed than on a grate due to its immediate contact with hot bed material. As a result, combustion is rapid and results in nearly complete combustion of the organic matter, thereby minimizing the emissions of unburned organic compounds. 1.6.3 Emissions And Controls7-12 The major emission of concern from wood boilers is particulate matter (PM). These emissions depend primarily on the composition of the residue fuel burned, and the particle control device. Oxides of nitrogen (NOx) may also be emitted in significant quantities when certain types of wood residue are combusted or when operating conditions are poor. 1.6.3.1 Criteria Pollutants The composition of wood residue and the characteristics of the resulting emissions depend largely on the industry from which the wood residue originates. Pulping operations, for example, produce great quantities of bark that may contain more than

70 weight percent moisture, sand, and other non-combustibles. As a result, bark boilers in pulp mills may emit considerable amounts of particulate matter to the atmosphere unless they are controlled. On the other hand, some operations, such as furniture manufacturing, generate a clean, dry wood residue (2 to

20 weight percent moisture) which produces relatively low particulate emission levels when properly burned. Still other operations, such as sawmills, burn a varying mixture of bark and wood residue that results in PM emissions somewhere between these two extremes. Additionally, NOx emissions from wet bark and wood boilers are typically lower (approximately one-half) in comparison to NOx emissions from dry wood-fired boilers. Furnace operating conditions are particularly important when firing wood residue. For example, because of the high moisture content that may be present in wood residue, a larger than usual area of refractory surface is often necessary to dry the fuel before combustion. In addition, sufficient secondary air must be supplied over the fuel bed to burn the volatiles that account for most of the combustible material in the residue. When proper drying conditions do not exist, or when secondary combustion is incomplete, the combustion temperature is lowered, and increased PM, CO, and organic compound emissions may result from any boiler type. Significant variations in fuel moisture content can cause short-term emissions to fluctuate. 1.6.3.2 Greenhouse Gases13-18 Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions are all produced during wood residue combustion. Nearly all of the fuel carbon (99 percent) in wood residue is converted to CO2 during the combustion process. This conversion is relatively independent of firing configuration. Although the formation of CO acts to reduce CO2 emissions, the amount of CO produced is insignificant compared to the amount of CO2 produced. The majority of the fuel carbon not converted to CO2, due to incomplete combustion, is entrained in the bottom ash. CO2 emitted from this source is generally not 9/03 External Combustion Sources 1.6-3 counted as greenhouse gas emissions because it is considered part of the short-term CO2 cycle of the biosphere. Formation of N2O during the combustion process is governed by a complex series of reactions and its formation is dependent upon many factors. Formation of N2O is minimized when combustion temperatures are kept high (above 1475o F) and excess air is kept to a minimum (less than