Solid fuel

from Wikipedia, the free encyclopedia
Firewood - the classic solid fuel in the home

A solid fuel is a fuel that before the burning in the solid aggregate state , ie as a solid state , is present.

Solid fuels have some special features during combustion that must be taken into account in the construction, design and operation of a solid fuel furnace and the upstream fuel processing.

Preparation and loading

Since solid fuels cannot be stored in tanks or conveyed via pipelines (with the exception of pneumatic dust conveyance, which in turn is associated with other problems), complex methods are required for processing, storage and transport:

Combustion behavior

When it comes to combustion behavior, a distinction must be made between an industrial process or a conventional process such as domestic fires .

Mixing and burning behavior

View into an industrial solid fuel furnace ( grate furnace )

Due to the low degree of fragmentation and the resulting low specific surface area , the burn-off takes place at a low speed; This means that the fuel needs to remain in the furnace for a long time to ensure good burnout .

The slow combustion is usually a hindrance in large-scale furnaces because the long dwell time at a given throughput means that the combustion chamber must have a very large volume and react very slowly. In smaller and manually loaded furnaces, inertia can also be a desired effect: The amount of fuel in the furnace acts as an energy store , fluctuations in the furnace control (fuel supply and burn-off) are compensated, the heater is relieved and does not have to be constantly supplied with fuel to care. For example, wood stoves and chimneys in the home are usually fired with logs so that wood only has to be refilled every now and then.

If rapid combustion is desired, the specific surface of the solid fuel must be increased by comminution and the mixture of fuel and air must be promoted by good stoking and a uniform air supply. In extreme cases, the fuel may rise to dust particle size are ground and in a pulverized combustion in entrained flow burn.

Combustion air supply and stoking

Because of the low specific surface area of the fuel is at Feststofffeuerungen the combustion air particularly to a sufficient, uniform and well-distributed supply look for, otherwise, the combustion in addition to the low supply of reactive surface on the side of the fuel addition is also by local lack of air brakes, to be carbon monoxide - emissions (CO) leads. Solid fuel burners are therefore operated with a significantly higher combustion air ratio (lambda) than oil or gas burners.

In the case of industrial grate firing , the air is supplied from below ("underwind") through openings in the grate. By moving the grate, the fire is stoked and the fuel and ash are transported forward.

With fluidized bed and dust firing , the solid fuel burns in suspension or in flight, so that if the furnace as a whole receives enough air, the individual fuel grains are also adequately supplied with air.

ash

Embers and ashes from a charcoal fire

Another special feature of solid fuels is the fact that, in contrast to gaseous or liquid fuels, they contain a considerable amount of mineral, non-combustible components that remain in the furnace as ash after combustion . This ash must be removed from the furnace so that it does not "clog" the furnace, impede the air supply and fuel transport.

The ash is normally removed by stoking , i.e. mechanical stirring / circulation of the fuel-ash mixture, which detaches the ash from the still unburned fuel and either falls down ("grate ash") or flies away with the combustion exhaust gas (" fly ash ") ) . In industrial furnaces, the bottom ash is normally removed from the furnace by a suitable technical device (e.g. a wet ash collector ). The fly ash is separated using a flue gas dedusting system .

A particular problem arises when the ash has a low softening and melting temperature. The ash then becomes doughy / pasty, at even higher temperatures even thin. The soft ash, also called " slag " , sticks the furnace and thus hinders the fuel transport, the air supply and the stoking. Dangerous caking (“ guarding ”) can occur on the walls of the combustion chamber in the area of ​​flow dead spaces , which, if they break off, can cause damage by falling. In the heating surfaces downstream of the furnace, the slag cools down, solidifies and sintered , forms incrustations that hinder the passage of heat , thus worsening efficiency , promoting local overheating and also increasing the pressure loss of the furnace on the flue gas side .

Industrial furnaces have automatic or semi-automatic cleaning devices such as knockers , sootblowers or water lances to clean the furnace and heating surfaces from caking ash and slag .

Common fossil fuels have ash softening points of 1000 to 1200 ° C, which is significantly lower than the adiabatic combustion temperature with stoichiometric combustion, which can be more than 2000 ° C with a high calorific value. Melting chamber firings are deliberately operated above the ash melting point; the ash is drawn off in liquid form. Melting the ashes is undesirable in all other furnaces; To prevent this, the furnace must be cooled - for example by flue gas recirculation - in order to keep the temperature below the ash softening point.

Some fuel constituents lower the ash softening point. The exact processes are very complex and can hardly be theoretically predicted. Under appropriate conditions, even the smallest traces of a substance can have a significant effect on the melting point if, for example, it forms low-melting eutectics with the ash . For example, some alkaline earths and fertilizer residues , such as those present in increased concentrations in straw and stalk-like biomass , are known to reduce the ash melting point to well below 1000 ° C. Furnaces for such biomass therefore have a strong tendency towards slagging and require special countermeasures.

Solid furnaces

The type of combustion technology used depends primarily on the grain size of the fuel:

Grain size: dust medium Chunky / pellets Bale
Firing: Dust firing Fluidized bed combustion Grate firing Cigar burner

Systematics of solid fuels

Coal in lump form
Wood pellets
Status Fossil fuels Biogenic fuels Residual and substitute fuels
coal peat Wood Other biomass
Lumpy Lump coal Sod peat Firewood , sticks , wood chips , ... Minor grains , coffee grounds , straw ( bagasse , rice straw , rapeseed straw , ...), shells, pods and kernels from various plants, ... Waste ( household waste , waste wood , ...), sewage sludge , rejects , ...
ground Coal dust , coal grit Peat dust Wood dust / sawdust
pressed Egg charcoal , briquette Peat briquette Wood briquette , wood pellet Straw bales , straw pellets , residual value pellets
smoldered Coke , Grude Peat coke / peat coal Charcoal Biochar , Cococha

Ecological aspects

Solid fuels are considered to be particularly polluting. In particular with regard to the sulfur content, which is responsible, among other things, for acid rain , and some other pollutants, some states have issued legal regulations that restrict the operation of combustion systems accordingly. In Germany, this includes the regulation on small and medium-sized combustion systems as well as the regulation on large combustion, gas turbine and combustion engine systems .

Individual evidence

  1. ↑ Solid fuel. RP Energy Lexicon, accessed April 19, 2020 .