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Blending of Coals in Power StationsDr V T Sathyanathan

(Former General Manager BHEL, Trichy)Technical Advisor, CVL, Trichy

Coal blending in power station is mainly adopted to reduce the cost of generation andincrease availability of coal. The low-grade coals can be mixed with better grade coalwithout deterioration in thermal performance of the boiler thus reducing the cost of generation. Many nations, blending of coal were being adopted for a very long periodmainly for increasing the availability of coal for power generation. To improve theavailability of coal and also to improve the calorific value of coal being fired, some of thepower stations look at the possibility of mixing high grade imported coal with the lowgrade high ash coals.

There are many methods adopted for blending which can be at Coal Mines, PreparationPlants, Transshipment point and Power Stations. The method to be chosen will dependupon the site conditions, level of blending required, quantity to be stored and blended,accuracy required and end use of blended coal. Normally in large power stations,handling very large quantity of coal, the stacking method with fully mechanized system isfollowed.

To decide blend or not, it is very important to understand the composition of coals thatare to be blended. This means one will have to understand the origin of coal, thechemistry of inorganic, chemistry of organic and the combustion properties & behaviour of the coals in question. It has been established that coals which are formed by DriftTheory of coal formation and the coals due to swamp theory of coal formation have to beblended with caution. The main difference between coal formed due to drift theory andswamp theory is that the coal formed by drift theory exhibits pronounced regionalvariation in thickness and quality of seams. They also enormously have very high ash

content with varying inorganic chemistry. The organics of the drift origin coal alsopossess problem mainly because the vegetation that lead to forming of coal are driftedfrom different places having different kind of vegetation. However the coals formed byswamp theory have more uniform organic property and much less ash content withconsistent chemistry of inorganic.

During combustion, it is really necessary to understand the physical conditions and coalproperties during heating of the particles, devolatalisation, ignition and combustion of thevolatile matter and ignition and combustion of the char. It is also equally important toknow the phase changes in mineral matter and other inorganics present in coal. Thecombustion efficiency and carbon loss will have to be also addressed during blending of coals. It is also necessary to look into the aspects of slagging, fouling and emissioncharacteristics like NOx, Sox and particulate.

It has been found from various literatures, discussions and conferences that blending of coals is expected to grow over the next decade as electric utilities attempt to reducecost, meet SO 2 emission limits and improve combustion performance of their coals. Allaspects of a blends behaviour and its effect on all components of power stations, fromthe stockpile to the stack, should be considered before the most appropriate blendcomposition is chosen.

Because of the complexity of the combustion process and the number of variablesinvolved (which are still not fully understood), it is difficult to extrapolate small scaleresults to full scale plant. Thus, power station operational experience in a wide range

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of plant configurations with a variety of coal feedstock is essential for determining thepractical significance of results from bench and pilot scale tests. More publisheddata on how the behavior of the coals / blends utilized in these tests differs from their actual performance in power station boilers are required.

Predicting the risk of spontaneous combustion of coal stocks is another aspect of currentfuel quality research. In addition to the inherent dangers, uncontrolled burning can leadto the release of pollutants; while the economic issues associated with the loss of avaluable energy resource is also a concern.

The presence of trace elements in coal combustion has also received increasedattention throughout the world during the last few years, with elements such as mercuryof particular concern. One way to reduce trace element emissions is cleaning the coalprior to combustion. The use of cleaner coals those with lower ash and sulphur contents can have the added advantage of substantially reducing operating costs.Again, however, some effects may be detrimental (ash deposition may be exacerbated,and the effects on corrosion and precipitator performance are uncertain) which makestesting vital.

It has been found from field data that even if the blended coal closely resembles thedesign coal for the boiler, the blend need not perform the same way. This is mainly dueto the transformation of inorganic particles during combustion and the way in which theorganics are dispersed in coal. A limitation to blending coals is the compatibility of thecoals themselves and problems are more likely when blending petrographically differentcoals or coals with different ash chemistry. Non-additive properties make blendevaluation for power generation inherently complex. More work is required onunderstanding how the inorganic components of coals in the blend interact and how itaffects ash behaviour including its emissivity, reflectivity and thermal conductivity.

Blending decisions should be based on the knowledge of the specific behavior of a given

pair of coals, rather than an assumption of linear variation of properties with blendtraction. The ever more stringent constraints like environmental regulations, maximumefficiency at reduced cost of power generation, improved availability and reliability etc.,placed on coal-fired power stations worldwide and the continuing development of newtechnologies means that the issue of fuel quality improvement will remain a primaryfactor.

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Basis for recommendation Mainly based on site trials

Flame stabilityCarbon lossHeat flux reduction due to slagging and foulingHeat flux regain ability on wall blowing

SH and RH spray levelsEmissionAcid dew pointAsh resistivity

Prime Concern

Boiler furnace SlaggingLead to outages in many casesLead to load reductionPerformance parameters change

Arriving at the right proportionMaintaining the right mix

Practical approach

How power stations to viewLook at proximate and ash analysisLook for FC/VM ratio idea on reactivityAsh composition ratios slagging

Trial with low mixture ratio 95% + 5%95% of regular usage coal

Watch parameters for about a week likeSH spraySteam tempExit gas tempUnburnt carbonLoad on bottom ash collection after maintaining loadSteam pressureExcess air Mixture ratio

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