How a Toxecon 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
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) Particulate Control (Electrostatic Precipitator): The TOXECON configuration is specifically intended to preserve fly ash in the event that powdered activated carbon (PAC) must be used for mercury control. TOXECON adds a ‘polishing’ baghouse downstream of the Electrostatic Precipitator, or ESP.
ESPs use parallel, vertical plates to produce an electrical field that captures fly ash particles. As the gasses containing fly ash pass through the ESP, fly ash collects on these plates.
The intent of TOXECON is to capture over 95% of the fly ash in the ESP. Mercury ‘sorbent’ is then injected downstream of the ESP, thereby avoiding activated carbon contamination of the ESP fly ash.
8) 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 attracts the mercury, converting it to an insoluble form.
With a TOXECON configuration, mercury is captured by injecting higher concentrations of mercury sorbent downstream of the ESP and upstream of the polishing baghouse. Assuming the use of PAC for mercury control, the result is that ESP fly ash is not contaminated with activated carbon and remains useable as a replacement for portland cement. Only fly ash captured in the polishing baghouse is contaminated with activated carbon and must be landfilled.
Use of Amended Silicates sorbent in a TOXECON system allows for the blending of fly ash from the polishing baghouse with fly ash from the ESP, thereby preserving the beneficial use value of all fly ash produced and avoiding the need to landfill any fly ash.
10) TOXECON (polishing baghouse): Before exiting the exhaust stack, flue gases pass through the polishing ‘baghouse’ that contains long, cylindrical fabric filters. The remaining fly ash (not already captured by the ESP) and mercury sorbent are captured by these filters, and drop into hoppers where the fly ash mixture is collected.
When activated carbon is used, the high PAC concentrations require the fly ash/activated carbon mixture to be landfilled. Adoption of Amended Silicates sorbent in a TOXECON configuration allows for the blending of baghouse fly ash with ESP fly ash, thereby preserving the beneficial use value of all fly ash produced and avoiding the need to landfill any fly ash.