Grate firing

The grate is a wood burning, wherein the fuel to a grate, an apertured supporting surface lying burns. The openings in the grid used to supply the necessary air for combustion ( " forced draft " ) and the discharge of the residual ashes. The movements necessary for circulating ( " stirring up " ) of the fire and the discharge of ash With simple manually grate with a poker, with larger grate automatically by proper motion of the grate.

History

Grate combustion is one of the oldest types of firing at all Represent provides an improved variant of the basic shape of the fireplace is as the man used since the Stone Age: With an open fire as the fire a solid fuel on a support surface is also burned lying but this edition had no openings; the air is supplied from the side and the ash remaining in the fire.

The main improvement over the basic shape has been to provide the supporting surface with openings through which the combustion air can flow from below and could fall through the ashes down. Thereby, the efficiency of the furnace was increased and the air supply was uniform, the amount of air could be controlled by flaps and it was easier to " stoke " the fire.

Until the 19th century the grate furnace ( " grating types " see section ) was with static grate the only available of firing. The heater had to stir up the task to feed the furnace with fuel, and to adapt to the heat demand. He could influence the fuel loading and distribution of the fuel on the grate and the combustion airflow by adjusting throttle. For steam locomotive boilers, there is still the possibility of a blow pipe to increase the suction. The heater had to constantly monitor the firing, reloading fuel, distribute the fuel on the grate and to dissipate Poking the ashes.

The grate was further developed in the 20th century again: fuel supply, forward movement, stoking and ash removal are automated and take place continuously in modern grate furnaces in industrial and power plants, as well as the control of the combustion air.

Design and operation of an industrial grate

In a modern, larger grate, as today is used in industry and Heiz-/Kraftwerken, the fuel supply to the grid automatically and continuously, for example by means of screw conveyors, mechanical pushers or double flaps. Through these conveyors a conclusion is made to the fuel storage tank and prevents burn-back time.

The fuel is by a moving mechanism ( to which different types see " rust types") funded by the continuous entry for ash removal and automatically circulated ( " stoked "). In the first region of the grating is followed by drying and degasification of the fuel. It joins the main combustion zone, and in the last section of the grate burnout occurs.

The openings in the grate are called the free surface rust. By these support openings, the primary combustion air passes from the bottom of the fuel ( " forced draft "). Large frames are divided into zones, each having a separate bottom wind supply so that the amount of air can be adjusted by the throttle valve in the individual zones independently. In addition, secondary air is fed above the grate. The air-fuel ratio λ is 1.4 to 1.8. It is important to distribute the fuel on the grate evenly to avoid an uneven burn with locally heavy air passage. Excessive rust loading must be avoided because they would impede the air supply and would occur an incomplete combustion with the formation of carbon monoxide. In the extreme case, there is a risk of explosion.

The drying of moist fuels (biomass, municipal waste, ...) can be supported by pre-heated primary air, or it will be to the primary air exhaust added from the exhaust path. This admixture is called flue gas recirculation and also serves as a nitric oxide - reducing measure in the high peak flame temperatures (especially for fuels with high calorific values ​​). For larger plants in addition a part of the exhaust gases is added to or above the grate. Similar to the so-called secondary air nozzles Rezirkulationsdüsenebenen be used.

The flue gas outlet is disposed above the degassing zone may vary depending Feuerungstyp ( Gegenstromfeuerung ), in the middle of the grate ( Mittelstromfeuerung ) or at the end of the grate ( Gleichstromfeuerung ). In particularly wet fuels, the hot flue gases are preferably passed in countercurrent to the movement of the fuel on the grate and thus an intensive drying and degasification of the fuel is achieved. The air is supplied as primary air beneath the grate, stair and as secondary air above the grid.

The waste from the combustion ash falls mostly as coarse, sintered slag at the end of the grill by the so-called " ash fall " and is collected in the ash pan or continuously discharged via a ash dischargers. Finer ash constituents already covered road through the free surface rust also in the ash dischargers / ash pan. To cool the ash and extinguishing the embers of this is usually carried out as a water bath, which is also the air statements for the under negative pressure combustion chamber.

The grate is exposed to extreme thermal loads, the operating temperatures on the surface can be as high as 800 ° C (see fire casting). The passing through the combustion air cools the grate. At very high heating values ​​water-cooled grates can be used. The combustion chamber is always refractory lined because it prevail in the fire temperatures around 1000 ° C.

Pros and Cons

Compared with dust firing (combustion in flight ) have grate some important advantages and disadvantages:

Advantages:

• Simple, clear structure
• Cheap partial-load and load change process
• Wide range of permissible fuel range and fuel combinations
• Low self - energy demand

Cons:

• Power limitation through the grate construction
• Slow controllability, because of the large amount of fuel on the grate
• High excess air, thereby deteriorating efficiency

Rust types

The different types of rust differ fundamentally in the nature of the mechanism that causes the fuel - motion:

Flat grate

In this simplest and oldest form of the grate (see " History ") of the rust is immobile and flat. The ash falls into the ash pan below the grate. There is no automatic lacing, but this must be - where necessary - manually by means of Poker, jogging, etc. carried out. Also, the ash removal must be done manually.

In larger industrial furnaces of this type is no longer used today, but probably in smaller furnaces in domestic and small commercial sector: Fireplace / Stove / oven room, barbecue, wood heating, etc.

Traveling grate

The traveling grate consists of an endless stainless belt having movable members, like a conveyor belt. The endless belt is guided over two rollers, and moves with a gear wheel drive.

The speed of the feed grate is adjustable and is used for power control. The grate bars ( links ) are movable. A support surface holding the rods of the upper belt in the horizontal position. On return of the grate to make the bars in the vertical position so that the ash can fall through (folding frame). The fuel feed is limited by a layer height control. The air chambers in the bottom blown are arranged between the upper and lower band. The grid size is limited due to the force acting on the grate bars bending forces. For lower wind zone traveling grates the maximum grate area is 70 sqm.

The traveling grate was a type often used for burning particulate fuel with low fines content. It is suitable for both gas-rich and dry coal and wood as wood chips. The traveling grate was in the 30s to the 60s of the 20th century in power plants of medium size (up to 150 t / h steam) widespread. The design used nowadays nurmehr for biomass combustion. Coal is burned today mainly in dust firing and fluidised bed.

Stepped grate / bulk rust ( Feed / reciprocating grate )

A step grate / bulk rust looks similar to a flat staircase with a gradient 8-15 degrees. The fuel is thereby moved over the grate that some steps ( every second or two out of three ) oscillating back and forth and then push the fuel forward. Depending on the direction of the grate bar movement of the step grate and grate or reciprocating grate is called (in both cases, however, produces a forward movement for the fuel ).

Stepped grates are used for coarse and high-ash fuels, which require an improved lacing, such as wood, recycled waste, or less commonly today lignite.

Roller grate

For a roller grate, the grate surface is made large, back -side, rotating roller drums that are partially equipped with drivers and entrain the fuel through its rotation. A particularly strong circulation and stoking the fire is achieved. By special patented roller grate seals between the rollers, the furnace is energy-saving optimized.

This grate type is mainly used in waste incinerators for very problematic, unrefined, non-homogeneous fuel mixtures that tend massively for bonding or slagging.

Underfeed

In a underfeeding an auger pushes the fuel ( eg wood pellets) from the bottom of an annular burner plate. On the burner plate burning the material, the ash is pushed out and falling into the ash pit of the burner plate later.