Direct Combustion Technology

The essence of the combustion of coal is the presence of air in intimate contact with the coal and 

There are three main designs of combustion systems in which coal and air can be reacted as the start to power generation and they are (1) the fixed-bed combustor, (2) an entrained-bed combustor, also called a suspended-bed combustor, and (3) a fluidized-bed combustor. 

In a fixed-bed combustor, the air passes upward through the pulverized coal at a low velocity. The coal is held on a grate, and the bed of hot coal may be several inches thick. The coal remains in a fixed bed since the air velocity is not sufficient to lift the coal particles upward. Ash removal is continuous or semicontinuous by mechanical means. This type of bed, however, does not afford very efficient gas-solid contact. High combustion rates are not possible with this system. 

In the entrained-bed combustor, the feed coal must be introduced to the combustor as small. The particles (generally 200 mesh) are carried by the gas into the furnace and travel in a suspended state through a hot zone where they are consumed. After combustion, approximately 20% of the ash falls to the bottom of the furnace and is removed there. Combustion gases, which contain about 80% of the ash, pass out of the furnace and are treated to remove the remaining particulates (usually in an electrostatic precipitator) and sulfur compounds (usually in a stack gas scrubber). This type of combustion system (PCC) is in common use for large-scale utility boilers. Higher combustion rates can be attained in pulverized fuel combustors than in fixed-bed combustors. 

The third type of combustion system is the fiuidized-bed combustor in which the velocity of the oxidizing gas is sufficiently high to support orfioat the particles but not carry them out of the top of the reactor. In this combustor, the coal particles are much more concentrated than in a pulverized coal combustor, giving a high surface area of solids with which to react. Heat and mass exchange 

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Direct Combustion Systems. The direct combustion of biomass feedstocks is already widely practiced by several industries, especially the forest products industry. Many types of direct combustion equipment are commercially available for this purpose. New developments in direct combustion technology are expected to have a near term impact on energy supplies through the utilization... 

Two projects are currently being funded by the Biomass Energy Systems Program in the area of direct combustion technology. 

Fuel-bound NO. is formed at low as well as high temperatures. However, part of the fuel nitrogen is directly reacted to N2. Moreover, N2O and N2O4 are also formed in various reactions and add to the complexity of the formation. It is virtually impossible to calculate a precise value for the NO, emitted by a real combustion device. NO, emissions depend not only on the type of combustion technology but also on its size and the type of fuel used. 

The majority of these operations use boiler technology, which involves the direct combustion of biomass materials... 

Today s biopower capacity is based on mature, direct-combustion boiler/steam turbine technology. The average size of biopower plants is 20 MW (the largest approach 100 MW) and the average efficiency from steam-turbine generators is 17 to 25 percent. The small plant sizes lead to higher capital cost per kilowatt-hour of power produced and the low electrical conversion efficiencies increase sensitivity to fluctuations in feedstock price.658... 

The different pyrolysis and gasihcation technologies, consisting of the production of solid, liquid and/or gaseous fuels, have achieved a signihcant industrial throughput and could be a clean alternative to the direct combustion of plastics. 

It is noteworthy that of all the processes that can be used to convert biomass to energy or fuels, combustion is still the dominant technology. More than 95% of all biomass enei] utilized today is obtained by direct combustion. 

Flameless Distributed Combustion technology, unlike the Lummus/UOP Smart SM technology, does not directly combust hydrogen from the reaction mixture hence it does not obtain the benefit of a favorable shift in equilibrium. 

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