Coal is the principle source of energy in India and will continue to be so for a long-time to come. About 70% of coal production is used in the power sector and the remaining by steel, cement and other consumers.
The reported coal reserves in the country are as given in Table I & II:
|Table I: Coal Reserves (in Million Tonnes)|
|Type of Coal||Proved||Indicated||Inferred||Total|
|(A) Coking :-|
|(B) Non-Coking :-||79325||106316||35564||221205|
|Total (Coking & Non-Coking)||95866||119769||37666||253301|
|Table II: Coal Reserves (in Million Tonnes)|
|Other public sector||1.92||2.02||2.32||2.52||2.52|
Coal ReserveAs per the report of Working Group on Coal and Lignite for XIth Five Year Plan, the coal production is projected as under:
Coal is the basic fuel used in the Indian cement industry. While it mainly provides the requisite heat and temperature, the ash content in the coal also combines chemically with the limestone to help form cement clinker. In India, coal will, of course, continue to be the dominant fuel for a very-long time.
Coal is supplied to cement companies on the basis of what is called Fuel Supply Agreement (FSA), between coal companies and cement companies, generally at cheaper rates. Cement plants, besides, receiving the coal through FSA, are also procuring fuel such as imported coal, pet coke, lignite and coal through market purchase. This is because the amount of coal received through FSAs is invariably much less than their actual requirement.
Some of the important points regarding coal for the Cement Industry are given below:
- Coal is the basic fuel for manufacture of cement.
- Coal is in short supply. Approximately only 60% of the total requirement is supplied by coal companies through FSAs.
- Remaining quantity has to be procured from other sources such as open market, E-auction, imports and use of pet coke, at much higher cost.
- FSA coal price (notified) has increased several times in the last few years.
- Imported coal, pet coke and E-auction coal are procured at several times the notified price.
- Price of imported coal, pet coke and E-auction coal are sky rocketing. E-auction prices are generally double of Notified Prices.
- Due to high cost, the procurement of fuel on an economical basis is becoming very difficult.
One alternative fuel, already in use in cement kilns in several countries, is oil. However, due to its rising cost, a large number of existing cement plants have been converted from oil to coal firing. Further, almost no new cement plant is planning to use oil for heating its kilns. This option is therefore not being discussed in this article.
PetcokePetcoke is widely used as a supplementary fuel in the Cement Industries in many countries, including India.
Petcoke (full name Petroleum Coke) is a residual product of the crude oil refining process. It has a high calorific value, but low volatile content, thus leading to poor ignition characteristics. It is a black solid obtained as an end product from the distillation of heavier petroleum crudes.
Petcoke has higher levels of sulphur and nitrogen as compared to coal. A comparison of the composition and calorific value of typical petcoke and typical Indian coal is given in Table III:
|Items||Typical Petcoke (%)||Typical Indian Coal(%)|
|Net Calorific Value||33,700 (kJ/kg)||21,600 (kJ/kg)|
Use of petcoke presents certain problems due to its low volatility and high sulphur content. Due to its lack of volatiles, petcoke has to be ground very finely, in order to enable it to burn completely, so that full advantage of its higher calorific value is obtained. High sulphur content, creates operation problems in the kiln. This problem can normally be sorted out by using a bypass system.
Petcoke with a sulphur content of upto four percent, can be used 100% as a fuel. Petcoke with a sulphur content between four and six per centcan be used as 60% of the fuel. But to utilize petcoke with higher percentages of sulphur, a bypass system that withdraws around 15-20% of kiln gasses, is needed.
Another big advantage of petcoke, apart from its higher calorific value, is that, being almost a waste product, its price is generally much lower than that of coal.
Tyre ChipsTyre chips or Tyre Derived Fuel (TDF) was first used in cement kilns, in Germany, in the 1970s. Few years later, kilns in the USA began using TDF, which proved quite a popular step. At present, USA produces over 300 million used tyres annually, out of which around 150 million are converted into TDF, their cement industry using approximately 60 million tyres worth in its kilns.
Other countries soon started using TDF also, including the UK, which uses several million tyres worth annually.
TDF has several advantages. Firstly, it utilizes on a large scale, waste material which would otherwise require huge landfill sites for its disposal. Secondly, it has a high calorific value, generating around 25% more energy than good quality coal. Thirdly, when used in place of high surlphur coal (which type a good percentage of Indian coal is), it reduces Nitrous Oxide (NOx ) emissions. Fourthly, it can be used in the form of chips (5 cm x 5 cm being the most common) or also as whole tyres. Fifthly, if fed properly into the kiln, TDF produces a more even rate of burning, thus increasing the life of the refractory bricks used to line the kilns. Sixthly, as every kilo of TDF used reduces the use of coal by 1.25 kg. The wear and tear on coal roller mills is proportionately reduced. Seventhly, the steel in the tyres gets combined with the clinker material, giving a more consistent end product.
Seeing the advantages of TDF, the Indian Cement Industry requested the Ministry of Environment and Forests (MoEF) to allow its use in cement kilns. After some delay, MOEF has finally permitted the use of shredded tyres as a supplementary fuel in cement kilns. This decision has been conveyed to the Central and State Pollution Control Boards. (CPCB and SPCBs).
SludgeSeveral types of sludge can be used as alternative fuels in cement kilns. These include paint sludge, refinery (petroleum) sludge, Effluent Treatment Plant (ETP) sludge and Tar Waste.
During the last few years, several Indian Cement companies have undertaken successful plant trials using various types of sludges as alternative fuels, in their kilns. These trials were carried out in collaboration with the respective SPCBs and under the supervision of the CPCB.
The results of these trials were then submitted to the MoEF, and the latter has now granted permission for the use of paint sludge as a supplementary fuel in cement kilns. This permission has come along with certain conditions, including compulsory plant trials in each and every case.
BiomassThis includes agricultural waste, food industry waste and biodiesel waste.
Commonly used materials include rice husk, sawdust, animal waste and tapioca.
Rice HuskRice is the basic foodgrain consumed by billions of people around the globe. Rice husk is the inedible covering on grains of rice. It is removed during the dehusking or milling of rice. Around 600 million tonnes of paddy is produced worldwide every year. Paddy on an average, consists of 70-72% of rice, 5–8 percent of bran and 20-22% of husk. Hence approximately 120 million tonnes of rice husk are produced every year. Most of this is just destroyed by burning or is dumped somewhere. A small amount is used as fuel for generation of electricity, or as a bulking agent for composting of animal manure. The cement industry can easily use bulk rice husk from mills as kiln fuel, though transportation and storage may cause problems. Rice husk has one more advantage for the cement industry. Rice husk, after burning, produces about 20% ash. Rice husk ash has high pozzolanic activity, and so can easily be used to produce good quality blended cement. It is an excellent replacement for fly ash, slag or silica fume.
Animal WastesThese include both dung and fat. Dung, especially cowdung, has been used as a domestic fuel for centuries. However, it is not easy to procure in bulk quantities. Animal fat wastes however, are comparatively easier to procure. These can be got in bulk from the factories that do processing for the meat industry. In fact, several European cement factories have been saving around 20% of their kiln fuel, for many years, by using animal fat waste for combustion. These factories are located mainly in France and Germany.
Plastic BagsThe problem with the burning of omnipresent plastic bags, is that this is normally done at around 800 degrees Celsius. At this temperature, burning of polythene (out of which plastic bags are made) produces hazardous gasses. Disposing of plastic bags by burning them, has thereby been almost universally banned. They are therefore disposed off by burying in landfills, where they will take hundreds of years to decompose.
Cement kilns however, operate at 1200 to 1500 degrees Celsius, to produce clinker. At these high temperatures, polythene burns up completely, without producing any noxious gasses.
Some cement factories in Madhya Pradesh have received permission from environmental agencies, including the State Pollution Control Board, to carry out trials of burning used polythene bags, as a supplementary fuel, along with coal.
Problems Faced While Using Alternative FuelsThese include:
- Mass balance
- Kiln feed systems
- Requirement of extra oxygen
- Rules and regulations
AnalysisAlternative fuels often contain harmful elements such as mercury cadmium, arsenic, lead and nickel. Burning of material containing these toxic elements is subject to strict laws. Many countries do not allow incineration of products containing more than 0.5 parts per million (ppm) of mercury and cadmium. Arsenic and lead are similarly not allowed to exceed 10 ppm for safe combustion. Hence, alternative fuels have to be analysed, before being used, in order to prevent illegal emissions.
A further complication arising from the use of waste fuels is the considerable variation that can occur in the composition of fuel batches. This makes it difficult to define a method for monitoring different samples using only one type of reference material. Furthermore, Certified Reference Materials (CRMs) are generally not available for a majority of the materials contained in alternative fuels. While use of standardless methods may give good results in these situations, for extremely low detection limits (as required for Mercury and Cadmium) a quantitative analysis technique using certified calibration standards is necessary.
The method normally used for such analysis is energy dispersive X-ray fluorescence (XRF) spectrometry, details of which are beyond the scope of this article.
Mass BalanceThis is the process by which effect of different materials in the composition of the alterative fuel being used, on the finally produced clinker, is estimated. Corrective steps can then be taken to reduce or eliminate unwanted characteris- tics. A couple of actual problems faced by cement companies, who were going to use alternative fuels, will explain what balancing mass can entail.
A cement company once wanted to evaluate the implications of burning spent pot liners, from the aluminium industry, in its kilns. The liners were composed of carbon and were originally used to line the electrolytic cells used in the production of aluminium. The electrolyte used in these cells was cryolite and some of this was absorbed into the liners, which meant that they had high sodium and fluoride content. The carbon liners themselves had a good calorific value and the anticipated price to burn the liners was attractive; but what would be the impact of the sodium and the fluoride on the clinker? And would a bypass need to be installed. If so, how big would that bypass need to be?
Answering such questions requires the development of mass and energy balance. The mass and energy balance is required because the total energy input to the kiln has to be kept constant, as the fuel mixture is varied. Would replacement of coal with spent pot liners or other alternative fuels give a direct one-for-one replacement in terms of the energy input to the kiln? Probably not, as differences in the particle size and volatile content affect the heat release characteristics in the kiln.
Another company, a European one, was being paid to burn animal bones on a large scale, in its kiln. Burning of the bones produced significant quantities of phosphorous pentoxide. Impact of this gas on clinker mineralogy was totally unknown at that time. So it took several trials and subsequent kiln modification, before actual large scale usage of bones could begin.
WeighingOnce an alternative fuel has been selected, the rate of which it is to be fed into the kiln will have to be worked out. Obviously, once this rate is decided, weighing options will have to be considered. These will mainly depend on the type of alternative fuel and design of the kiln delivery system. Generally speaking, weighing is done electronically, using load cells, which are suitably located.
Kiln Feed SystemsFrom the storage silos, the alternative fuels are sent via conveyor belt, into the kiln. The feeding system must be such that it can operate successfully at kiln temperature. It should also ensure that there is no leakage of flue gasses from the kiln. And, of course, it should be able to feed fuel in the right quantities and at the required intervals. Normally, these systems consist of a number of flap valves operated by pneumatic cylinders. They can also contain anti-blockage air guns.
Requirement of Extra OxygenOxygen is required for any combustion process. Although air is the most common source of oxygen, it is not the most effective, since it also contains about 79% nitrogen. Nitrogen in air takes up volume, absorbs heat, and lowers flame temperature. Adding pure oxygen (oxygen enrichment) improves the overall combustion process by increasing the flame temperature and the amount of available heat. Inside the kiln, oxygen enhances burning zone control and improves kiln stability.
The use of oxygen to improve the combustion of alternative fuels has been shown to reduce or eliminate emission excursions and increase flame temperature. The resulting improved burning zone control enables operators to maintain feed and burn at a more consistent rate. Over time, improved kiln stability translates into more production, increased fuel substitution rates, and lower operating costs. Depending on the value of the alternative fuels, increased substitution can lead to the net negative fuel cost (including the cost of oxygen).
The extra oxygen must be suitably injected, depending on required quantity, timing and kiln design.
Rules and RegulationsMention has already been made, of various laws that may create problems for potential users of alternative fuels. In India, these include clearances required from local authorities, Central and State Pollution Control Boards, and the Ministry of Environment and Forests.
Cement manufacturers in other countries also face similar problems, especially as pollution control norms and laws vary from country to country and sometimes, from state to state. This often adversely affects the plans of cement companies that are trying to extend their operations overseas.
MethaneAlmost the entire world is seeing a tremendous expansion in the number of landfill sites. Generally located on the outskirts of towns and cities, these sites they generally cater to the household and municipal waste produced therein.
Landfill sites are notorious for generating large quantities of methane gas. This gas generally goes waste and only helps to increase atmospheric pollution. There is considerable scope to use this methane as an alternative fuel in cement kilns.
In fact, a Lafarge plant located just outside Kansas City (USA), is already using methane gas from two nearby landfill sites, using an underground pipeline system. The plant is thereby saving around 8,000 tonnes of coal every year. It has made plans to tap a third landfill shortly, and eventually save 20,000 tones of coal every year.
|Country||Replacement Level ( %)|
In conclusion, the author is happy to note that the use of alternative fuels is on the rise in the Indian Cement Industry. The latest on this front, is the construction of a modern waste processing plant, to produce alternative fuel for use in cement kilns. Located at Ajmer in Rajasthan, this plant was built by Chennai based Tecpro Systems Pvt. Ltd as a BOOT (Build, Own, Operate, Transfer) project. Technical help is being provided by MVW Lechtenberg of Germany. The site for the plant was given by the city authorities of Ajmer, who also provide 250 tonnes of household waste per day, for processing into alternative fuels. This waste is being given free of charge.
The author is grateful to New Building Materials & Construction World, the International Cement Review, ZKG International, World Cement, KHD Humboldt Wedag AG, the Institution of Engineers (India) and the Indian Express, for some of the facts and figures in the this article.