Solid Fuel

Volatile substances of the fuel are called gaseous products of the decomposition taking place heating thermal unsteady molecules of solid fuel organic combinations. More volatile substances exude due to the less carbonizing extent of the fuel because more thermal unstable molecules exist in it. The most ones exude out of the wood – about 85 percent of its combustible mass, the least – of anthracite (2…9 percent). Remainder of solid fuel left after the isolation of volatile substances is called coke. The largest part of it consists of coal (about 95…98 percent).

Volatile ones burn down in the furnace, making up a flame. Their igniting temperature is lower than that of coke, therefore these substances ignite first and assist coke during burning. The main properties characterizing the fuel are: combustion heat, ash content, amount of exuded volatile substances.

Wood as fuel is used only in small boiler houses. Mostly the waste of the wood processing enterprises is burnt. Composition of different trees combustible mass is approximately the same. It consists of ash – to 1 percent and sulphur does not exist in it. Just now cut down wood consists of 50 percent or more moisture. Combustion heat of wood depends on the kind trees less and very much on their moisture. Combustion heat of wood is usually about 12500kJ/kg. Wood exudes a great amount of volatile substances (85 percent), so it is easy to ignite and burn it.

Peat is the youngest, according to geological point of view, fossil fuel. It is considered the first stage of vegetation remainders carbonization. According to the way of digging peat can be lumpy and crumby. The latter one is dug most effectively and economically. In the combustible mass of peat the amount of carbon is larger, the quantities of oxygen and volatile substances are less than in the wood (70 percent). Peat contains a comparatively small amount of ash (about 10 percent), however, it is very humid. The moisture of just dug peat is of 85-90 percent, so it is always dried up in the open air. Combustion heat of peak depends on its humidity and is not large (8500…12000 kJ/kg)    http://www.ecokatilai.lt/lt/katalogas/1/granuliniai-katilai

Coal is divided into brown, stone and anthracite. This division is approximate because distinct limits between these sorts of coal do not exist.

Brown coal is a product of the subsequent carbonization of vegetation remainders, that, compared to the peat, contains less hydrogen and oxygen, than coal. This sort of coal exudes rather many volatile substances (40…50 percent), much moisture (to 50 percent and more). Brown coal contains much ash (10…40 percent). Combustible heat is between 6300 and 17000 kJ/kg due to a large external ballast. Brown coal as other sorts of stone coal inclines to flame itself in the stacks. Brown coal is not valuable, so it is used near the extraction place.

Stone coal is the product of a larger carbonization extent. It contains much carbon (75…90 percent), less humidity (4…15 percent) due to small hygroscopic extent, volatile substances (10…45 percent). Combustion heat is large – 21000…29300 kJ/kg.

The product of the largest carbonization extent of initial material is called anthracite. Combustible mass contains 90…94 percent of carbon. It releases the least amount of volatile substances (2…9 percent), so it is difficult to ignite it. Anthracite contains less humidity (about 6 percent), ash content is to 20 percent, combustion heat is rather large – 25000…27200 kJ/kg (only some thin coal sorts containing more hydrogen have larger combustion heat).

Oil shale releases many volatile substances (to 80 percent). Characteristic feature is that it contains a great amount of mineral dash – to 70 percent of dry mass. Burning oil shale they decompose exuding CO2 and much ash. Combustion heat oh oil shale is small – 5800…10300 kJ/kg.