Processing Municipal Solid Waste into Energy Service
Most landfills operate as they have over the last 50 years where trucks arrive to the landfill, are weighed, and then the waste is dumped into the open landfill. The waste is compacted and covered with dirt or other type of fill material. The result is valuable fuel buried forever and methane gas (a major source of green house gases) spewing into the atmosphere for decades.
Hoskinson’s modern Waste-to-Energy (WtE) operations change this status quo dramatically where the waste can be used as fuel to generate clean and responsible power for thousands of homes and businesses for generations to come.
This modern and proven process begins after the garbage truck is weighed. The contents are dumped into the recycling operations section of the WtE Facility, where skid steers remove large or hazardous items for recycling and proper disposal.
The balance of the material is then shredded and processed through a combination of automated recycling machinery and manual labor stationed along a series of conveyor belts. This part of the process removes much of the metals, glass, and other non-combustible materials inappropriate for introduction into the combustion chamber of the WtE plant.
Recycled material is then packaged and prepared for shipment out of the Facility at regular intervals.
The remaining waste is moved by front-end loaders to a staging area inside the building where it is prepared for the WtE plant. Front-end loaders continuously load hydraulically operated ram feeder that introduces the waste into the main primary Pyrolytic Gasification chamber.
This proprietary Pyrolytic Gasification combustion process developed by Hoskinson nearly 40 years ago and perfected over the same time, provides a near complete level of combustion with virtually no fly ash.
The waste is gasified in the primary chamber into a synthetic gas or “syngas” that contains a highly combustible mixture of primarily CO, H2 and other hydrocarbons. The waste continuously moves to the rear of the primary chamber until it is consumed. The primary chamber operates in an oxygen-starved or substoichiometic environment that minimizes combustion of the syngas at this point.
The syngas then moves from the top of the primary chamber to the secondary chamber where an additional regulated amount of air is added to the syngas flow. While the syngas could be cleaned at this point and introduced into conventional combustion engines, the syngas is combusted in the secondary chamber more efficiently where temperatures may approach 2,300°F. This is where additional thermal oxidation reactions occur, now with trace amounts of super heated steam present in the waste. This is an extremely exothermic reaction that generates a tremendous amount of heat which is capture by a Heat Recovery Steam Generator (HRSG).
The HRSG generates superheated steam under moderate pressures and temperatures. This steam is regulated and introduced into the turbine / generator set which spins and creates the electric current.
The exhaust steam is condensed and reintroduced into the HRSG for reheating. The condenser, generator, and other parts of the plant can be air-cooled, which substantially reduces the amount of water needed by the Facility to operate.
The exhaust gas from the HRSG then enters the air pollution control system (APC system). This generally consists of a common powdered activated carbon (PAC) mercury control system, a dry-lime scrubber, a bag house, an induced draft (“ID”) fan, followed by a Selective Catalytic Reduction (SCR) system and integrated stack. The APC system is designed to remove mercury, acid gases, particulate matter, NOX and dioxin/furans. Most of the acid gas present is in the form HCl.
The reduction of mercury in the stack gases is accomplished by injecting powdered activated carbon (PAC) into a dry lime scrubber.
The entire APC system uses programmable logic controllers (PLC), which automatically regulates and adjusts each component as needed. The system is also equipped with alarms for all component failures.
Dry-lime (CaO) is fed into the scrubber and combines with the acid gases in the flue gas stream to form various compounds of calcium. These compounds, together with unreacted lime and small amounts particulate matter are mixed in the gas stream and pass to the bag house.
In the bag house the gas stream is filtered to remove the particulate matter. Any un-reacted lime present on the surface of the filter bags serves to further remove any residual HCl from the gas stream.
The bag houses are of the pulse-jet type, where the bags are cleaned on line by a pulse of air directed down the centre of the bags. Each bag house is sectionalized into compartments so that one compartment can be taken off line for bag replacement or other maintenance while the gases are passed through the remaining compartments.
Oxides of nitrogen (NOX) is controlled in the SCR by adding the reducing agent, ammonia, NH3, to the flue gases after they have been heated to the required operating temperature. Supplemental heat is added to flue gases prior to entry to the SCR and is provided by two small natural gas fired duct burners. The reducing reaction that converts NOX to N2 and H2O occurs in the presence of the catalyst housed in the reactor located downstream of the ID fans. The catalyst also serves to oxidize the dioxins and furans in the stack gases to form HCl, CO2 and water vapor. Both endothermic and exothermic reactions occur here, with a net heat reduction of about 15% in the air stream.
A full capacity bypass stack is provided around the entire air pollution control system, including the ID fan. This permits emergency shutdown of the plant in the event of a total power failure, or a serious malfunction.
The main stack (SCR stack) contains continuous monitoring equipment is the exit point for the cleaned air leaving the plant.
At the end of the gasification process, a small amount of vitrified and inert ash drops into a specially designed water-filled auger system below the primary chamber, which cools the ash and delivers it to containers for final disposition. The sand-like vitrified ash is safe and is suitable for use as landfill cover or potentially as an aggregate in embankments or asphalt products.
The entire Facility operates 24-7 with minimal downtime for maintenance. A rotational schedule of preventative maintenance keeps other portions of the Facility operating while each gasifier is taken offline once a month for very short periods of time.
If there is a need, industrial waste as well as waste presently in the landfill may be used as fuel. Methane and/or other gases that would otherwise spew into the atmosphere can also be piped into The Hoskinson WtE plant as used as auxiliary fuel.
Our power plants are designed to run using the least amount of natural resources, which includes water.
Our plants can also use air-to-air condensers instead of water cooling towers and condensers to conserve this very precious resource. We take other measures as well to reduce our carbon footprint while using garbage as clean fuel to generate power.