04-07-2012, 01:03 PM
Seminar on Solid Fuels
Solid Fuels.ppt (Size: 199 KB / Downloads: 168)
Introduction
Solid fuel embraces a wide variety of combustibles, ranging from wood, peat and lignite, through refuse and other low calorific value substances, to coal and other solid fuels derived from it.
Coal represents by far the largest component of the world’s fossil fuel reserves.
In thermal terms 90% of the known hydrocarbon fuel deposits are formed by coal.
The carbon: hydrogen ratio of coal is the highest of the fossil fuels, hence the calorific values of coals are principally determined by the carbon in the fuel.
It is usual to consider coals in terms of their rank: in general, a high ranking coal will have a high carbon content.
The other major coal constituent element, hydrogen, is present in hydrocarbons which are released as volatile matter when the coal is heated.
Coal Classification
As the rank of a coal increases, its carbon content increases from 75% to about 93% (by weight), the hydrogen content decreases from 6% to 3%, and the oxygen content decreases from 20% to 3%.
A useful method for analyzing a coal is the proximate process.
Proximate analyses of some common fuels are given in Table 9.1 (next slide).
The moisture in coal is made up of two components: surface moisture and inherent moisture.
The former is affected by the way in which the coal is stored, and is thus variable.
Coal Properties
There are a number of properties which are important in identifying the suitability of a coal for any given application:
Size Some common size groups, together with their rather picturesque names, are given in Table 9.3 (next slide).
Ash Fusion Temperature
The melting point of the ash left after combustion of the coal is of particular importance in terms of the combustion and ash disposal equipment.
If the ash fuses it produces a glassy, porous substance known as clinker (slag).
The combustion equipment will be designed to handle either clinker or unfused ash, and use of the wrong type of coal can have dire consequences.
Sulfur Content
Many deep-mined coals have a fairly high sulfur content, typically around 1.5% by weight.
The same consideration apply to coal-fired installations as to oil-fired combustion equipment namely that condensation inside the plant must be avoided and that the design of the flue must ensure that ground concentration of sulfur oxides are controlled within acceptable limits.
Coal Combustion
Coal combustion is a two-phase process and the objective of the burner is, as always, to achieve complete combustion of the fuel with maximum energy efficiency.
Three common ways of burning solid fuels are currently in use and are briefly reviewed below.
Pulverized Fuel
The coal is ground to a very fine size (about 0.08mm or more than 70% pass through #200 mesh) when it can be made to behave rather like a liquid if air is blown upwards through the powder.
The preparation and handling equipment is very expensive and pulverized fuel installations are generally only economically viable in very large scale applications, such as thermal power stations.
Coal Storage and Handling
Solid fuels are stored in bunkers-normally a quantity equivalent to 100 hours at peak firing rate is the target storage capacity, with a minimum amount of 20 tonnes.
Coal is usually conveyed into storage from the delivery vehicle by tipping or by pneumatic conveyance along pipes.