How an SDA Works
Electricity Generation
Coal is burned in order to boil water, producing high temperature, high pressure steam that can be fed into a turbine to generate electricity.1) Coal: Coal is pulverized and fed with air into the burners that heat the boiler. Steam coal is typically divided into three categories that vary by energy and moisture content. Those categories are Bituminous coal, Sub-bituminous coal and Lignite.
2) Boiler: Burning coal creates a fireball which boils and then super-heats water flowing through pipes in the boiler. Steam typically reaches a temperature of over 1000° F.
3) Turbines: Steam is expanded through turbines that spin a generator which, in turn, creates electricity. This electricity passes through a transformer station before entering ‘the grid’.
4) Condenser: After the turbine, the low-pressure steam passes through a condenser to convert it back to liquid water before being pumped back to the boiler.
Emissions Control
After the coal is burned to generate electricity the combustion gases are passed through a series of pollution control devices. Pollutants are sequentially removed from the flue gas before it exits through the exhaust stack.5) Low NOx Burners: Nitrogen Oxide, or NOx, is a regulated emission created during the coal combustion process. NOx contributes to smog and acid rain, and is regulated by US EPA as a ‘criteria pollutant’. ‘Low NOx’ burners are designed to reduce the amount of NOx produced.
6) NOx Control: Additional NOx emission control may be installed at some power plants. A fixed bed catalyst is inserted into the flue ducting. This catalyst promotes the reaction of a reagent with NOx to convert it into nitrogen and water.
7) Mercury Control: Mercury is harmful to humans and has been designated by US EPA as a ‘hazardous air pollutant’. Several states currently regulate mercury and US EPA is expected to establish a nationwide mercury regulation by late 2011. Current expectations are that EPA will require up to 90% removal of mercury emissions from coal boilers.
Mercury (Hg) is scrubbed by injecting a very fine powder, or ‘sorbent’, into the flue gas stream. The sorbent captures the mercury, converting it to an insoluble form. The sorbent is then collected together with fly ash in the particulate control system (10).
Use of Amended Silicates sorbent converts mercury into an inert and insoluble form.
8) Sulfur Control: Sulfur Oxides, or SOx, include SO2 and SO3 which are regulated emissions created during the coal combustion process. SOx is a component of acid rain and is regulated by US EPA as a ‘criteria pollutant’.
Sulfur content in Sub-bituminous coals is relatively low. However, some states require even low-sulfur coal boilers to scrub sulfur emissions. In these cases, sulfur is typically scrubbed by injecting a reagent of lime into a Spray Dryer Absorber (SDA). The lime reagent interacts with the sulfur creating calcium sulfite and sulfate particles which are collected together with fly ash in the particulate control system (10).
9) Particulate Control (baghouse): Before exiting the exhaust stack, flue gases pass through a ‘baghouse’ that contains long, cylindrical fabric filters. Fly ash, un-burnt carbon, mercury sorbent and calcium particles are all captured by these filters, dropping into hoppers where the fly ash mixture is collected for disposal or recycling.