Waste to Energy
Waste-to-energy (WTE) plants burn municipal solid waste (MSW), which is trash, to produce steam in a boiler, and the steam is used to power an electric generator turbine. MSW is a mixture of energy-rich materials such as paper, plastics, yard waste, and products made from wood, etc.
WTE is a process that involves converting various types of solid waste materials into usable energy, typically in the form of electricity and heat. This approach helps address two key issues: the disposal of waste and the generation of renewable energy.
Mass burn facilities are the most common type of WTE combustion facility in the world. The waste used to fuel the mass burn facility may or may not be sorted before it enters the combustion chamber. Many advanced facilities separate the waste on the front end to save recyclable products, but this is not required and adds significant cost to the process without any significant environmental benefits.
Here's how waste to energy works:
1. Collection and Preparation
Municipal solid waste (MSW), which includes household and industrial waste, is collected and sorted to remove recyclable materials like paper, glass, and plastics. The remaining waste, often called "residual waste" or "non-recyclable waste," is used for the waste-to-energy process.
At a typical mass burn combustion facility, waste collection vehicles are weighed at a scale house and monitored for safety, prior to unloading on the tipping floor (the area within a facility where receiving activities, including unloading, loading, and limited sorting occur). Once unloaded on the tipping floor, the waste is inspected for non-processable materials prior to being placed in a trash storage bunker, also known as the pit. The waste delivery area incorporates numerous bays for unloading to receive multiple vehicle deliveries at a time. These bays can be designated for specific waste delivery streams, including MSW, vegetative debris and tires. The tipping floor and storage bunker is maintained at negative pressure during operation of the boilers, which mitigates odors outside of the facility as combustion air is drawn in through ducts above the waste pit and used in the combustion process.
The heat released from burning converts water to steam inside the boiler tubes. The steam is then passed through a superheater prior to being sent to a turbine generator which produces electricity. Bottom ash from the boiler grate is collected by conveyors and transported to metals separation equipment for removal of recyclable metals; and the bottom ash is stored where it can be sold as aggregate. Combustion gases pass through the boiler sections and are used to preheat boiler feedwater in the economizer.
An overhead crane with a grapple mixes the waste and then lifts it into a feed chute prior to entering the combustion chamber of the boiler to be burned. From the feed chute, a hydraulic ram feeds waste onto a stoker grate. A typical grate system uses automatic combustion controls to manage the ram feeders, grate activity, and the supply of under and over fire combustion air.
The residual waste is burned in a controlled environment. During combustion, the waste materials are heated to high temperatures, causing them to break down and release energy in the form of heat.
3. Energy Recovery
The heat generated from the combustion process is used to produce steam from water. The high-pressure steam is then used to turn a turbine connected to a generator, producing electricity. This electricity can be fed into the power grid for use by homes and businesses.
4. Emissions Control
The cooled gases exiting the boiler enter the Air Quality Control System. Powder activated carbon is injected into the flue gas exiting the economizer to remove mercury and organic compounds. Afterward, using lime slurry or dry lime hydrate, the dry scrubber neutralizes any acid forming gases, such as sulfur oxides and hydrogen chloride. A high-efficiency baghouse filtering system captures particulates. As the gas stream travels through these filters, more than 99% of particulate matter is removed. Waste-to-energy facilities are equipped with advanced air pollution control systems to minimize the release of harmful emissions, such as particulate matter and gases, which can be a concern when burning waste. These facilities must meet stringent environmental standards.
5. Residue Management
After combustion, there is typically a residue, often called "bottom ash" and "fly ash," that remains. Captured fly ash particles fall into hoppers and are either transported by an enclosed conveyor system to the fly ash silo or combined with the bottom ash. These residue materials are carefully managed and can be used in construction materials or disposed of safely in landfills.