Dry sorbent injection (DSI) is a process used to control acid gases by injecting a powdered sorbent into the flue gas stream. The sorbent can be injected into the furnace, boiler area or the ductwork/reaction chamber prior to the air pollution control device. The injection point depends on the type of sorbent and required reaction time. For example, some sorbents need to be injected at elevated temperatures to undergo a decomposition reaction before they can effectively remove the acid gas. Figure 1 shows a schematic of a typical dry injection system.
Components of a dry injection system
The dry sorbent injection system is a very simple system that consists of a dry sorbent storage tank, a weight feeder to meter the required amount of sorbent, a blower and transfer line, and an injection device such as a venturi. The dry sorbent material is blown through a pneumatic line to the injection area where transfer through the pneumatic line provides fluidization of the sorbent material. Injection into the duct is generally done countercurrent to the gas flow to create added turbulence and promote mixing. An expansion/reaction chamber may be included to increase the residence time of the acid gases to react with sorbent.
The simple dry injector process described above is capable of achieving moderate control of acid gasesfor example 50% SO2 and 90% HCl removal on municipal and medical waste combustors. The acid gas removal efficiencies can be increased by cooling and/or humidifying the flue gas stream. Exhaust gases from industrial boilers or refuse combustors can range from 600oF to 400oF. The flue gases can be cooled (and the humidity increased) by using a heat exchanger or a dry quench chamber upstream of the injection point. Cooling the flue gas temperature increases the rate of reaction between the sorbent and acid gases. But, the temperature must be maintained high enough (300-350oF) to ensure that all the water droplets used to quench are evaporated.
Recycling a portion of the collected particles and unreacted sorbent is another method used to increase overall effectiveness of dry scrubbing systems. As stated previously, it is difficult to mix a dry solid and a gas stream; therefore, additional sorbent (above stoichiometric amount) must be injected. As a result, there is unreacted sorbent captured in the baghouse or electrostatic precipitator. In some instances a portion of this waste stream is recycled back to the injection point.
In order to achieve high removal efficiencies using relatively inexpensive calcium sorbents, most dry injection systems have to operate at higher stoichiometric ratios than a spray dryer would. For example, stoichiometric ratios of 2.0 to 4.0 are used on municipal waste combustors to achieve moderate acid gas control. This increased sorbent usage limits their application to smaller sources such as medical waste incinerators. Table 1 lists some facilities that have installed dry injection acid gas control systems.
Examples of dry injection systems on medical and municipal
waste incinerators
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