Waste to energy technology has been refined over the last 30 years in several hundred installations worldwide. Combustion and gasification are the two dominant WTE technologies available today. Combustion is a mature technology and accounts for most of the 600 WTE plants in operation worldwide. Gasification is a newer technology by contrast, but is increasingly common, with over 100 plants installed, primarily in Japan.
Combustion and gasification utilize similar processes. Waste is delivered by truck and is deposited into a large pit near the plant. The waste is loaded by crane into a hopper and then placed onto moving grates, which transport the waste into combustion or gasification chambers. The heat generated during the combustion or gasification process is used to boil water in a closed boiler system. The steam spins turbines, which in turn produce electricity. The exhaust gases that are generated through the combustion process contain s several air pollutants. These gases are transported through a flue, or a series of exhaust pipes and chimneys, and are cleaned using a variety of emissions control technologies before they are emitted from the plant. As a result, emissions from WTE plants fall well below EPA’s allowable thresholds for air pollutants. Aside from gaseous emissions, the other byproduct from WTE plants is the residual ash remaining after incineration. The ash byproduct is collected for metals separation or disposal.
Gasification decomposes MSW by heating it in the absence of oxygen in stages differentiated by temperature, to produce a gaseous, fuel rich product. This syn gas is then combusted, providing energy to the steam turbine. Incinerating the waste at increasing temperatures up to 1700 C allows for the separation of valuable materials in the MSW. Besides the fuel that is produced, slag, a solid byproduct, is also produced. Slag has a variety of uses in the construction and building industries.
The MSW to energy plant is designed to deliver a net electrical output of 30 MW. The plant consists of two parallel units that each include:
a MSW feed system, silo burner, oxidizer, HRSG heat recovery and stack, bag house, induced draft fan power train (turbine-generator), cooling tower, ash removal system, controls, and continuous emission monitoring system.
Each of the parallel systems produce adequate steam at 900 psig, 850 F to generate more than18 MW of energy. The steam from the two systems is accumulated in a steam drum and delivered to a 15 MW Condensing Turbine. All components are based on time proven designs, with the intent to create a simple, practical, and reliable MSW to energy plant.
The combustion chamber is a derivation of robust industrial waste incinerators first developed over 22 years ago. It is a vertical cylinder of HAC refractory of a diameter and height tailored to match the required capacity. It can withstand temperature excursions of up to 2,600 F. The processing of the waste begins with the delivery of solid waste to the site where it is weighed and deposited on the tipping floor. Large white goods and items which cannot burn, such as cinder blocks, are removed for recycling. The system is further enhanced with front-end treatment of the waste that shreds and palletized the fuel to pieces no larger than 3? to increase the surface area and, thereby, promote even greater combustion efficiency and energy recovery.
A crane places the waste into the feed system which begins with a walking floor conveyor, capable of handling even the bulkiest items. Depending on the local value of aluminum and other recyclables, picking stations will be located alongside the conveyor to facilitate the recovery of these goods, if desired.
Solid fuel is fed near the top of one side of a unique deep V-shaped hearth. The MSW falls on a sacrificial refractory cradle to cushion its impact as it falls from the load conveyor. Over time, the MSW moves down one side of the V-hearth. This represents as much as ten tons of fuel being dried, gasified and burned as it migrates down the V-hearth before its eventual removal as ash.
The primary oxygen-deprived combustion air (pyrolysis) is introduced along the lower portion of the feed half of the V-hearth through a distribution of ports, which can be externally cleaned. The secondary oxygen rich air (combustion) is injected through a number of ports approximately ten feet above the fuel entrance level equally spaced around the perimeter of the combustion chamber. The secondary air is injected at a high velocity and high pressure to penetrate and violently mix with the hot, viscous gases of the chamber. This is an important factor in insuring clean and efficient combustion.
During combustion of typical MSW, the volume is reduced over 90% and weight over 66%. Ash is removed from the incinerator to a water quench pit by a hydraulic ram. The quench pit cools the ash and controls fugitive emissions during final handling. When the ash house is filled, an inner door between the ash house and the combustion chamber is closed and the ash is removed by loader for transport to the landfill. Alternatively, an optional submerged mechanical ash convener for automatic removal of the ash from the ash house into the hopper or truck can be incorporated.
Flue gases are pulled from the combustion chamber through an oxidation chamber into the heat recovery steam generator (HRSG). This steam is used for power generation. The cooled gases are filtered in a bag-house. Prior to this step fine-spray water is injected into the gas stream to recover heat in the form of hot water.
Particulate generated by the combustion process is fly-ash, and drops out in the right-hand fall-out chamber and the oxidizer. An induced draft (ID) fan pulls gases through the complete system and maintains a slightly negative pressure in the combustion chamber to ensure virtually no odors escape the facility.