Moving grate incinerator

The moving grate incinerator is used for non-homegenous and low calorific waste. The domestic waste incinerator is offen called a "Waste-to-Energy" plant, WtE plant or "Energy from Waste" plant. The gate fee price of the household waste is app.120 - 125 euro/ton and a household waste incinerator of app. 14 ton/h has an investment price of +/- 90.milj.euro.

The biggest grate is capable of incinerating the design throughput of waste (31,25 tons/h) with the highest calorific value (2 200 kcal/kg). In normal practice, a household waste incinerator is capable of operating 8000 hours per year (in full continuous regime). The installation must comply with the regulations regarding:

• Environmental protection requirements on the base of the current EU legislation (emissions, odours, noise,...)
• Fire fighting standards 
• Safety at work and labour protection rules

The grate need to be cooled because the air is added from the bottom and high temperatures can damage the grate. Two different types of grate cooling systems exist: the air cooled grate and the water cooled grate.

Generaly, depends on domestic waste collection and recycling system, if the waste has:

• 6 - 10 MJkg --> Air cooled grate
• 8 - 15 MJ/kg --> Partly watercooled grate
• 12 MJ/kg and more --> Watercooled grate

The conversion of waste to energy requires an effective control of a complex series of physic - chemical processes, commonly known as “combustion”. We can distinguish two groups of processes, a “horizontal” group and a “vertical” group.

1. Horizontal: the conversion of the waste (a heterogeneous mixture of all kinds of solids and liquids) to gases.
2. Vertical: de oxidation of these gases resulting in complete combustion.

Each of these groups can be further sub-divided into sub-processes

Horizontal

1. Drying: this is the evaporation of the water present in the waste;
2. Outgassing: here volatile compounds in the waste escape as a result of heating up the waste;
3. Degassing/pyrolysis: in this sub-process certain compounds in the waste are transformed to combustible gases;
4. Burnout: what is left from the previous processes contains mainly combustible solids (fixed carbon) that are now burned in the presence of oxygen and transformed to gases.
5. Cooling of the ashes: the solid non-combustible material (inert) present in the waste have been heated up in the previous processes and are now cooled as much as possible in order to recuperate this heat.

Vertical

1. Gas production: at the lowest level the series of horizontal sub-processes takes place
2. Gas combustion: by adding air (or rather: by adding the oxygen in the air) the gases are fully burned
3. Gas burnout: in this zone the combustion is completed to ensure that the legal requirements are met: for a minimum of two seconds the gases remain at a temperature of at least 850°C in the presence of at least 6% (by volume) oxygen after the last addition of oxygen.

A domestic waste incinerator has not only a combustion chamber, there is a whole installation which creates a complete "waste-to-energy" plant. Here is a technical description of the main companents.

Weightbridge

Weighing of incoming and outgoing waste flows is performed by means of weighbridges. The vehicle weighbridges are offen from the electronic type. On this system also the invoice is prepaired, and the incinerator plant operator knows how much waste is in the waste bunker.

Waste bunker with overhead cranes and scredder

Waste is left in the bunker and overhead cranes manage the waste of different types in the bunker. Big size waste is scredded and the cranes put the waste in the waste hopper of the furnace.

Furnace

Support burners (and optional start-up burners) are installed to maintain a temperature of more than 850°C during at least 2 seconds also during start-up and shutdown. The combustion air is supplied in stages (in different phases) via the air supply system that consists of several fans and air pre-heaters. The combustion residues are evacuated by the ash extractor and are treated by the slag treatment system.

Burners

The burners installed can be removed from the furnace (whilst the incinerator is in operation). When this is done a screen is placed in front of the opening. This enables the burner heads to be cleaned easily.

Ash extractor

The ash extractor is installed at the outlet of the grate. Bottom ashes and siftings are cooled down below +/- 60°C before evacuation to the ash transport, a vibrating conveyor.

Slag treatement

After the ash extractor, the slag is transferred into a bunker by means of a vibrating conveyor. The ashes are transferred into a truck by means of cranes.

Slag treatement can be don on the incinerator plant or on a special recycle plant.

For example: A band magnet is positioned above the slag ash conveyor belt to separate out the iron in the slag. Also non-ferrous metals can be separated from the ashes (using an eddy current overband system).

Steam boiler

The thermal energy produced in a waste-to-energy installation is recovered by means of the production of steam in a heat recovery boiler (one per line). Downstream the energy of the steam is transformed into electrical energy in the steam turbine.

The boiler type wich is well known in municipal solid waste incinerators is the "TAIL-END" type, with several vertical radiant passes and an horizontal convective section.

The waste heat boiler is of the water tube type, with natural circulation.

Boiler Cleaning

Dust and fly-ash, carried by the flue gas stream, causes erosion and fouling of the boiler pipes, resulting in less efficient heat transfer, flue gas temperature increase through-out the boiler and considerable increase of pressure drop. This is why an efficient cleaning system is of great importance in a waste to energy installation.