Coal combustion atmospheric pressure

Examples of the need for multimedia models are found in contemporary problem areas. Polynuclear aromatic hydrocarbons and metals are emitted into the atmosphere as trace impurities with the products of coal combustion. The organics have low vapor pressure and partially condense on emitted particulates in a stack plume. The particulates are transferred to the soil by dry deposition, rainout or washout. The metals manifest... 

Stoic Also called Foster Wheeler-Stoic and FW-Stoic. A two-stage, nonslagging coal gasification process, operated under atmospheric pressure and using air as the oxidant. Initially developed by Stoic Combustion Limited Pty, South Africa licensed and further developed by Foster Wheeler Corporation, United States. First used in South Africa in 1950 now widely used in Europe, the United States, and South Africa. 

Pulverized coal combustion systems are most commonly used in power plants. In pulverized coal combustion, temperatures typically reach around 1480 °C at atmospheric pressure. In the past couple of decades, fluidized bed combustion (FBC) technologies have been commercialized. These combustors often use limestone bed materials to capture sulfur gases. They operate at about 880 °C and usually at atmospheric pressure (Smoot and Smith, 1985), 38. 

Carbon dioxide is usually nonreactive and nontoxic. At normal atmospheric pressure and temperature, it is colorless and odorless. Carbon dioxide will not bum, nor will it support combustion. Unrefined carbon dioxide gas is typically obtained from the combustion reaction of coal, coke, natural gas, and other carboniferous fuels. Other major sources are... 

Illite -H2O-H2 System. Vaporization of potassium from the highly acidic illite system, in neutral atmospheres, is expected to provide a relatively insignificant source of alkali in most coal combustion systems. However, in the presence of reactive combustion gases, such as H2O and H2, thermodynamic considerations predict a significant KOH partial pressure. In addition, an increase in the K-pressure should result from a reduction in the O2 pressure, in the presence of H2. However, KMS experiments did not indicate formation of KOH or additional K in the presence of H2 gas. Thus, thermodynamic equilibrium does not appear to have been established in this heterogeneous system, even though the temperatures were sufficiently high to have normally ensured a rapid approach to equilibrium. 

Postcombustion capture In this case, the CO2 is separated from the flue gas emitted after the combustion of fossil fuels (from a standard gas turbine combined cycle or a coal-fired steam power plant). CO2 separation is realized at relatively low temperature, from a gaseous stream at atmospheric pressure, and with low CO2 concentration (ca. 5%-25% when air is used during combustion). SO2, NO2, and O2 may also be present in small amounts. 

The process (Figure 21.21) is based on an air-blown, atmospheric pressure, entrained-bed slagging gasifier and in the process part of the coal char is charged to the combustion section of the gasifier to supply the heat necessary for the endothermic gasification reaction (Patterson, 1976). 

Fluidized-bed combustion It accomplishes coal combustion by mixing the coal with a sorbent such as limestone or other bed material. The fuel and bed material mixture is fluidized during the combustion process to allow complete combustion and removal of sulfur gases. Atmospheric FBC (AFBC) systems may be bubbling (BFBC) or circulating (CFBC). Pressurized FBC (PFBC) is an emerging coal combustion technology. 

Fluidized bed combustion of coal is used to reduce the emission of sulfur dioxide and NO into the enviromnent. In this technology the pulverized fuel is burnt in a fluidized bed combustor after being blended with limestone or dolomite, which act as soibents for SO2. The burning temperature is significantly lower than is common in the production of fly ashes in suspension-fired furnace chambers, at around 850°C. The process may take place at atmospheric pressure (atmospheric fluidized bed combustion, AFBC) or elevated pressure (pressurized fluidized bed combustion, PFBC). 

Figure 5-6 is a simplified block flow diagram of the KoBra project. Raw brown coal is dried to 12% moisture by the newly developed WTA process which is a fluidized bed with steam and integrated waste heat recovery. Air for the gasification is extracted from the combustion turbine. The gasifier is sized at about 3,600 t/d of dry brown coal to match the 200 MW Siemens V 94.3 combustion turbine. The bottom ash and fly ash from the gasifier is combusted in a separate atmospheric pressure circulating fluidized-bed combustion (CFBC) boiler. 

Combustion Engineering Process. Combustion Engineering (C.E.) began development of a two-stage, entrained-flow coal gasification process in the early 1970s. This development work led to construction of a 120 t/d pilot plant at Windsor, Connecticut which operated at limited success in the late 1970s (20). This pilot plant was air-blown and operated at atmospheric pressure. 

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