Coal

PK Nag POWER PLANT ENGINEERING 2nd Edition McGraw-Hill International Edition 2002.




According to geological order of formation, coal may be of the following types:
1. Peat
2. Lignite
3. Sub bituminous
4. Bituminous
5. Sub Anthracite
6. Anthracite, with increasing percentage of carbon. After anthracite, graphite is formed.
Anthracite contains more than 86% fixed carbon and less volatile matter. Volatile matter helps in the ignition of coal, so it is difficult to burn anthracite. Bituminous coal is the large group containing 46-86% of fixed carbon and 20-40% of volatile matter. It can be low, medium and high volatile. The lower of volatility is the higher the heating value. Lignite is the lowest grade of coal containing moisture as high as 30% and high volatile matter. According to ASTM (American society of testing and materials), peat not regarded as a rank of coal. Peat containing up to 90% moisture and is not attractive as a utility fuel.

Coal Analysis
There are two types of coal analysis, proximate and ultimate. Both are done on mass per cent basis. Both these types may be based on
1. As received basis useful for combustion calculation.
2. Dry or moisture free basis.
3. Dry mineral matter free or combustible basis.

Proximate Analysis.
The proximate analysis indicates the behavior of coal when it is heated. When 1 g sample of coal is subjected to a temperature of about 105C for a period of 1 hour, the loss in weight of the sample gives the moisture content of the coal.
When 1 g sample of coal is placed in a cover platinum crucible and heated to 950C and maintained at the temperature for about 7 min, there is a loss in weight due the elimination of moisture and volatile matter. The latter may now be determined since moisture has been calculated from the previous test. Volatile matter consists of hydrogen and certain hydrogen-carbon compounds which can be remove from the coal simply by heating it.
By subjecting 1 g sample of coal in an uncovered crucible to a temperature of about 720C until the coal is completely burned, a constant weight is reached, which indicates that there is only ash remaining in the crucible. Complete combustion of coal is determined by repeated weighing of the sample.
Fixed carbon is the difference between 100% and the sum of the percentages of moisture, ash and volatile matter. However, this difference does not represent all the carbon that was in the coal. Some of carbon may have been in the form of hydrocarbon which may have been distilled off while determining the volatile matter. It is also possible that some of this fixed carbon may include sulphur, nitrogen and oxygen. So, the proximate analysis of coal gives FC+VM+M+A=100% by mass.
The amount of VM indicates whether the coal will burn with a short or long flame and whether it will tend to produce smoke. The more volatile the coal, the more it will smoke.
The figure shows the trend in moisture, volatile matter and fixed carbon when expressed on a dry ash free basis. The general trend with increasing rank is an increase in the heating value and fixed carbon and corresponding decreasing in moisture and VM. This trend is pronounced that a classification system based on the fuel ratio (ratio of fixed carbon to volatile matter) has been used as rough indicator of a coal’s rank.
Lower Rank coals (lower fuel ratio) are characterized by a greater oxygen content, that aids ignition and enhances combustibility and flame stability. High combustibility improves carbon burnout (reduce carbon carryover) and hence boiler efficiency and for pulverized coal fire units, this allows the coal to be ground to a coarser size. Low rank coals (high moisture content) produce a “self pulverization” of the coal particles during combustion. As the inherent moisture in the pore structure of the coal is heated and expands rapidly, its volume increase (as water flashes to steam at atmospheric pressure, the volume expansion is 1600 to 1), thus fragmenting the coal particles. This exposed more surface area for combustion.

Ultimate Analysis
The ultimate analysis gives the chemical elements that comprise the coal substance, together with ash and moisture. The coal substance consists of organic compounds of carbon, hydrogen and oxygen derived from the original vegetable matter. The analysis shows the following components on mass basis: carbon, hydrogen, nitrogen, sulphur, moisture and ash, therefore C+H+O+N+S+M+A = 100% by mass. The dry and ash free analysis on combustible basis is obtained on dividing C,H,O,N and S by fraction [1-(M+A)/100)]

Coal Properties
There are certain properties of coal which are important in power plant application. They are swelling index, grindability, weatherability, sulphur content, heating value and ash softening temperature.

Swelling index.
Some types of coal during and after release of volatile matter become soft and pasty and form agglomerates. These are called caking coal. In a fixed bed, such as traveling grate stoker, the coal must not cake as it burns. The consequent agglomeration disturbs greatly the availability of air and so the coal does not completely burn yielding low combustion efficiency. Coal that does not cake is called free burning coal. It breaks s apart during combustion exposing large surface to the air, thus enhancing the combustion process efficiency. Caking coals are used to produce coke by heating in a coke oven in the absence of air, with the volatile matter driven off. Coal devoid of volatile mater is coal coke, which is largely needed in a steel plant. A qualitative evaluation method, called swelling index, has been devised to determine the extent of caking of a coal. A free burning coal has a high value of swelling index, which indicates what it some what expand in volume during combustion When modern pulverized coal burner are used, the swelling property of coal is, however, of less importance.


Grindability
Grindability is often an important criterion for selection a coal. This property of coal is measured by standard gridability index, which is inversely proportional to the power required to grind the coal to a specified particle size for burning. Grindability of standard coal is defined as 100. If the coal selected for use at power plant has grindability of 50, it would require twice grinding power of standard coal to produce e specified particle size.

Weatherability.
It is measure of how well coal can be stockpiled for long periods of time without crumbling to pieces. Modern power plants normally stockpile 60 to 90 days’ supply of coal in a large pile near the power plant. The coal unloaded from wagon is packed in a long trapezoidal pile. Excessive crumbling or weathering of coal due to climate conditions may result small particle of coal which can be dispersed by wing or rain.

Sulphur Content.
Sulphur content in a coal is combustible and generates some energy by its oxidation to SO2, sulphur dioxide is major source of atmospheric pollution. There is an environmental regulation on SO2 emission. The operating cost of SO2 removal equipment need be considered while selecting a coal with high sulphur content.

Heating Value
The heating value of caloric value of coal is property of fundamental importance. It may be determined on an as received, dry or dry and ash free basis. It is the heat transferred when the products of complete combustion of a sample of coal (or other fuel) are cooled to the initial temperature of air and fuel. It is normally determined in a standard test in a bomb calorimeter, where a coal sample of known mass is burnt with a pore oxygen supply completely in a stainless steel bomb or vessel surrounded by a known mass of water and the rise in water temperature is noted. Two different heating values are citied for coal. The higher heating value (HHV) assumed that the water vapor in the products condenses and thus includes the latent heat vaporization of the water vapor formed by combustion. The lower heating value (LHV) assumes that the water vapor formed by combustion leaves as vapor itself, therefore

LHV = HHV – mw.hfg

Where mw is the mass of water vapor formed given by

mw = M + 9H + γa.wa

Where M and H are the mass fractions of moisture and hydrogen in the coal. And γa is the specific humidity of atmospheric air and wa is the actual amount of air supplied per kg of coal. For energy balance and efficiency calculation of steam generators, HHV of fuel is considered in the USA, whereas LHV is the standard used in European practice.
If the ultimate analysis is known, the HHV of anthracite and bituminous coal can determined approximately by using Dulong and Petit formula as given below

HHV = 33.8C + 144.45[H-(O/8)] + 9.38S in MJ/kg


Where C, H, O and S are mass fractions of carbon, hydrogen, oxygen and sulphur in coal. Assuming the latent heat of vaporization hfg at the partial pressure ofwater vapor in the combustion product as 2.395 MJ/kg, the lower heating value of coal is given by

LHV = JJV – 2.395.mw

Softening Temperature.
The ash softening temperature is the temperature at which the ash softens and become plastic. This is somewhat below the melting point of ash. The design of steam generator greatly depends on the ash softening temperature (a.s.t) of of the coal. If the furnace temperature is higher than the a.s.t a;; the ash will melt and would come out of the furnace bottom continuously as molten slag. For furnace that would discharge ash in the solid form, as high as softening temperature would be required. A stoker furnace must use coal with a high a.s.t otherwise clinkers would be formed. Clinkers which are large masses of fused ash, cause troubles in discharge and also make combustion inefficient.

Spontaneous Combustion
Combustion (oxidation) of coal can take place rapidly as in a furnace or slowly on a stockpile. If it takes place slowly, there is a degradation or loss of energy content and hence in the value of fuel. The factors which influence spontaneous combustion and which can lead to a big fire, are the following
1. Rank of coal, low rank of coals are most susceptible because of their higher porosity.
2. Amount of surface area exposed to air
3. Ambient temperature, with high solar insulation aiding it.
4. Oxygen content of coal.
5. Free moisture in coal.
6. Configuration of coal stockpile, steep conical piles with coarse coal at the edges and fines near the top are more susceptible because they promote natural convection (chimney effect) and good air flow through the pile to support combustion as it develops.
To prevent spontaneous combustion, it is important to maintain a dry pile and compaction at regular intervals.