Modular Waste-to-Energy Plants

There are four main steps: waste incineration, heat evaporation, air pollution control and power generation.

Waste incineration

The wasteWOIMA® W2E power plant technology is based on the well-proven grate incineration technology. The fed-in waste fraction moves forward on the reciprocating grate through the combustion phases: drying, pyrolysis and char combustion. Eventually the burn residue, or ‘bottom ash’, falls off into a cooling pool. Primary air is fed through the grate to support full combustion. It also acts as a cooling media for the grate, reducing the need for maintenance work. Depending on the composition, and so the calorific value of the waste, the incineration capacity varies between five and seven tons an hour.

A system of conveyor belts automatically transports the cooled bottom ash to an ash processing system, which removes excess water and returns it to the cooling pond. The remaining ash is then compacted for landfilling or used in infrastructure construction or cement production, for example.

Heat evaporation

The furnace opens to the post-combustion chamber, where the gasified fractions of the incinerated waste burn. Secondary air is injected to help ignite the gases and ensure full combustion. The heated flue gas flows into the flue gas duct which is comprised of evaporator surfaces. At the same time, feed water is supplied to the membrane pipes of the evaporation surfaces which constitute the walls of 1st, 2ndand 3rd empty passes and the convection area. The lengthy channel guarantees the EU-standard residence time (2 sec @ 850°C) for the flue gas, required to fully burn out the highly toxic elements (furans and dioxins).

Waste heat from the radiation channel is collected in the recovery boiler, which includes the superheater, boiler banks, economizer, and air preheater. They are a series of piping arrays designed to collect the remaining heat in the flue gas through convection. They convert the steam generated in the membrane walls of the radiation channel into superheated steam for the turbine.

The fly ash within the flue gas accumulates on the wall and piping surfaces in the boiler and reduces its heat transfer efficiency over time. So, an efficient soot removal process is needed to keep the boiler functioning properly.

Air pollution control

The air pollution control (APC) is based on a dry system. The wasteWOIMA® power plant is equipped with Hydrated Lime, Ca(OH)2 and Activated Carbon dosing systems and reactor, combined with a fabric filtration system. This system satisfies the World Bank Air Pollution Guidelines and EU Emission Standards. Both systems produce dry ash residue, so there’s no sludge to treat.

Bottom and fly ashes are ‘clean’ enough to be used in road construction or cement manufacturing, for example. The APC ash, on the other hand, contains a high proportion of heavy metals and other toxic residues and so has to be placed in a landfill or processed further, possibly in a metal extraction plant. The bottom and fly ash quantity is some 15% and the APC ash some 3% of the total incinerated waste amount.

 Power generation

The saturated and superheated steam (400°C @ 40 bar) is fed to a steam turbine-generator set, which transforms the heat and pressure of the steam through a rotating turbine into electricity. A standard back-pressure or condensing turbine is used, depending on the customer’s requirements. The used steam then moves to the condensing system, where the steam vapor turns back to water. This closed circuit ensures efficient operation by keeping all the plant-generated heat within the system. The power plant can generate steam, electricity, thermal power, potable water, or a combination of the above.

One standard feature of the plant is an auxiliary diesel generator set, used during the start-up and shutdown of the plant to run the conveyor belts and air blowers. During the maintenance period, it can also power local operations, reducing dependence on outside power.