ENERGIZE® multistage grate incinerator

The ENERGIZE® multistage grate is the core of an energy-from-waste plant. This installation treats large quantities (typically in the range of 100,000–1,000,000 tonnes per annum) of residual municipal solid waste (MSW) or industrial waste similar to MSW. This waste is usually not pre-treated or shredded and is characterised by large variations in size, shape and composition.

Compared to HELIOSOLIDS® and TURNOVER®, the ENERGIZE® multistage grate is designed for mass burning large amounts of non-toxic waste. 

Waste is collected and transported by truck to the installation where it is dumped in the waste bunker. An overhead crane is used to mix the waste and feed it to the furnaces. For reasons of availability there are usually at least 2 treatment lines per installation.

The waste is fed in the hopper and slides through the feeding chute (that serves to seal the furnace from the outside air) to the feeding grate. The feeding grate consists of a pusher to dose the waste onto the combustion grate, described below. The combustion takes place on the grate and the residues of the incineration (bottom ash) drop off the end of the grate in the ash extractor. Particles that fall through the grate (siftings) are collected and transported to the ash extractor. Both the ash extractor and the grate siftings conveyors have waterlocks to prevent entry of outside air. Bottom ash is usually post-treated (metal removal, stabilisation) before disposal off site.

The combustible matter in the waste is transformed into hot gas by the incineration (oxidation) process. The necessary combustion air is supplied under the grate (this is called primary air) and above the grate (secondary air). For waste with a low heating value, primary air is preheated using steam, tapped from the steam turbine.

The hot gases are cooled in a steam boiler and cleaned to the appropriate standards in the flue gas cleaning system. The flue gases are evacuated via the stack to the atmosphere by the induced draft fan (ID fan) that also maintains a slight negative pressure in furnace and boiler. MSW to Energy plant, Beijing, China The steam boiler is usually designed as a so-called horizontal boiler with three empty passes where heat exchange is mainly radiation (vertical chambers built from membrane walls, but without heat exchanger bundles) followed by a horizontal section with blocks of convective heat exchanger bundles). Specific features are necessary to prevent corrosion of the metal heat exchanging surfaces, including special equipment for cleaning at regular intervals.

The steam is typically transferred to a steam turbine-generator block and condensed in an air or water condenser. This steam cycle is comparable to a fossil fuel fired power plant, although the steam pressure and temperature are limited in order to avoid corrosion problems in the boiler. The net electrical efficiency is typically 20 to 24%. Other energy recovery options (e.g. hot water production) are also possible.

Small start-up burners are used to preheat the furnace during start-up from cold condition. Auxiliary burners are installed in the first empty pass of the boiler to heat up the boiler during start-up and to help guarantee the legally required combustion temperature (usually 850°C). The plant is operated from the central control room, usually positioned with a view on the bunker. CCTV monitors allow the operators to observe the fire in the furnaces.

The ENERGIZE® multistage grate is the bottom surface of the furnace. It carries and transports the burning solid matter from the feeding section to the ash extractor.

The grate consists of identical elements grouped in 3 zones, named after the combustion stage taking place in each zone: drying zone, combustion zone, burnout zone. In the drying zone, moisture in the waste is evaporated by the heat in the furnace and the radiation from the first empty pass which is positioned just above this zone. In the combustion zone, the actual combustion takes place. The burnout zone is a buffer to guarantee the burnout quality and to cool the ashes as much as possible. Between each zone there is a step so that waste drops from one zone to the other, thus creating the necessary mixing of the burning matter.

Applications

Municipal solid waste (MSW)

Industrial waste (non-toxic) equivalent to MSW

Solid biomass (wood, demolition wood, ...)

+ limited amounts of sludge (max. 10%)

Large throughputs (5–30 TPH per line)

Advantages

Reduced wear (individually supported tiles)

Simple construction (free expansion)

Less expensive in maintenance (one single casting mode for grate bars)

Easy access for maintenance to hydraulics

Easily accessible furnace (horizontal steps)

Short erection time on site (modular construction)

High quality construction (completely constructed in workshop)

Excellent operation (even air distribution, high pressure drop over grate)

High quality burnout, limited amounts of siftings.