• Difficulties associated with low volatile coals– Ignition– Combustion stability (over load range)– Combustion efficiency• Causes– Lack of volatile material in the coal to support ignition– Rate of volatile matter release– High ignition temperature– Low reactivity of remaining char• Solutions– Finer pf grading– Longer furnace residence times– Careful air admission

– Furnace refractory cover where appropriate

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.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.

According to geological order of formation, coal may be of the following types:1. Peat2. Lignite3. Sub bituminous4. Bituminous5. Sub Anthracite6. 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 PropertiesThere 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.

GrindabilityGrindability 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 ValueThe 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 CombustionCombustion (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 following1. Rank of coal, low rank of coals are most susceptible because of their higher porosity.2. Amount of surface area exposed to air3. 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.

The reactivity of the coals correlated with the inverse of the fuel ratio (VM/FC).

Principal Research ResultsBackgroundAt present, bituminous coal that burns well with a high calorific value in the range of 6,500 to 7,000 kcal/kg and with a fuel ratio (weight ratio of fixed carbon to volatile matter) of 1.0 to 2.5 is used to pulverized coal fired power stations in Japan. As the demand for coal is likely to increase worldwide, particularly in developing countries, it is hoped that low grade coal that has not yet been used in thermal power stations can be utilized to reduce the power generation cost by reducing the fuel cost. The types of coal that should be considered for use are low grade coal with a low calorific value and a high moisture content or high ash content, and high fuel ratio coal which is difficult to ignite and which has a narrow stable combustion range.ObjectivesTo clarify the combustion characteristics of low grade coal that have a calorific value in the range of 3,000 to 5,000 kcal/kg and a high moisture content or high ash content and high fuel ratio coal that has a fuel ratio of 5 or greater with a pulverized coal combustion test furnace (combustion capacity of 0.1 t/h).

Principal Results1. Low Grade Coal(1) The NOx conversion ratios of lignite coal (weight ratio of moisture content to coal supply amount as dry ash free = 1.5) and subbituminous coal (same ratio = 0.3) were compared with that of low moisture coal under similar conditions to pulverized coal firedpower plants. It was found that the NOx conversion ratio of lignite coal was slightly lower than that of low moisture coal while thatof sub-bituminous coal was higher As lignite coal has a high moisture content, the latent heat of vaporisation is high. This reduces the flame temperature. As a result, the oxygen consumption near the burner is slowed and NOx production is suppressed.On the other hand, the oxygen consumption of sub-bituminous coal is fast near the burner and the formation of NOx is promoted.

As the reactivity of both types of coal after the evaporation of moisture is high, the uncombustion fraction of these types of coal are lower than those of low moisture coal.

(2) The greater the ash content, the higher the uncombustion fraction and NOx conversion ratio. As the reactive area decreases and theconsumption of oxygen near the burner is slowed with a higher ash content, the formation and decomposition of NOx are delayed

(3) As coal with a high moisture content has a higher reactivity than coal with a high ash content, the uncombustion fraction of coal with a high moisture content is lower. It is believed that coal with a high moisture content is easier to use for power generation purposes. Unfortunately, as this sub-bituminous coal tends to generate a large amount of NOx, it is necessary to develop acombustion technology that can suppress NOx emission.

2. High Fuel Ratio Coal(1) The higher the fuel ratio of coal, the harder it is to ignite. Studies show that by using a burner designed to create re-circulating currents to lengthen the time that coal particles stay in the high temperature region near the burner, ignition can be improved and the range of stable combustion can be widened .(2) Both the concentration of NOx and concentration of unburned carbon in fly ash are higher for high fuel ratio coal than for bituminous coal. The NOx conversion ratio of high fuel ratio coal, like that of bituminous coal, increases as the fuel ratio increases or as the nitrogen content decreases although the tendency is not as pronounced .On the other hand, the uncombustion fraction rises fairly sharply as the fuel ratio increases.(3) It is believed that among the latest pulverized coal fired power plants designed to meet environmental regulations, high fuel ratio coal with a fuel ratio of up to 5 can be used with slight modification of the burners.

Example Indonesia’s Coal Properties

Coal K needs the highest power of pulverizer to produce specified particle size. 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 specified particle size.

Coal B has the highest of 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.

Coal I has the highest of HHV. 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 highest HHV of coal means lower flow rate of coal to produce as same water vapor.

Coal K has the lowest of ash softening temperature. If the furnace temperature is higher than the ash softening temperature, the ash will melt and would come out of the furnace bottom continuously as molten slag. It would be much slag if use coal K.

Combustion ReactionOne of the most important items is that the correct amount oxygen must be supplied per unit weight of fuel burned to provide complete combustion. In addition to the correct “air-fuel” mixture, time must be allowed for complete mixing and burning, and the furnace temperature must be such as to support combustion.

Combustion in a furnace is best studied in connection with the ultimate analysis of the fuel used and certain simple chemical reaction. Combustion computation for coal assumed:Weight of air require for theoretical perfect combustion.Weight of gas formed per pound of coal fired with theoretical perfect combustion.Cubic feet of gas at boiler exit (600F) per pound of fuel fire with theoretically perfect combustion.Percentage CO2 in exit gas on both dry and wet basis.Air is mechanically mixture in various gases, but principally of oxygen and nitrogen being 23.3 percent oxygen and 76.8 percent nitrogen by weight.At 760 mmHg pressure (14.7psia) and 32F, 1 mole of any gas occupies 359 cu ft.