Survival Fuels

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TECHNICAL FORUM

What affect do fuels have on product quality?

conomics are the driving force for a cement company to burn alternative fuels and therefore these are the first consideration. Will the gate fees be high enough? What will be the costs of handling, storing and delivering the material to the kiln? What equipment will be needed? How profitable will it be? How much of the material is available? How long into the future can the increased profits be anticipated? Next ones needs to know how difficult will it be to obtain a licence to burn the material? Can opposition be expected from the people living around the cement factory? Can opposition be expected from the workforce? How strict will the requirements be to comply with the licence? What monitoring and reporting will need to be put in place? What will be the penalties if licensing requirements are not met? There remain two further considerations. What will be the impact on the process? And the impact on cement quality? In most cases the economic imperatives mean that work-rounds will have to cope with process or product quality implications. Process and technicians predict and anticipate the likely implications and put in place the equipment and procedures to deal with them. Prediction and anticipation of the impact of alternative fuels rely on two methods. Experience of other cement companies when burning the particular materials, and the use of chemistry, process engineering and technical first principles. Usually both are used. Only in very rare cases has no cement company burnt the material before. For the multinational cement companies there is probably another group company that has paved the way. But how does an independent cement producer learn from the experiences of other cement companies? This is the role of the Cemtech conference, International Cement Review magazine, the

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Survival fuels by Dr Michael Clark Burning alternative fuels is very much on the agenda for many cement manufacturers. For many companies operating wet-process kilns it is an absolute imperative for survival. These kilns are generally of low capacity and inevitably fuel consumption is high due to the need to evaporate the water in the slurry feed. However, if the cement company is being paid a fee to burn alternative fuels, then high fuel consumption is an advantage, as it increases the quantity of these fuels that can be burnt and therefore the fees earned. CemNet Technical Forum, the Global Cement Network, and the individual consultants within that network.

What size of bypass is required? A cement company once contacted Whitehopleman in 2001 to assist in evaluating the implications of burning spent pot liners from the aluminium industry in its kilns. The liners are composed of carbon and are used to line the electrolytic cells used in the production of aluminium. The electrolyte used in these cells is cryolite and some of this is absorbed into the liners meaning that they have high sodium and fluoride content. The carbon liners themselves have 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 process? In particular what steps would be necessary to continue to meet the low alkali specification in the final cement? As the current cement was almost on the limit of this specification it was clear that a

bypass would need to be installed. How big would that bypass need to be? Would it be necessary to add calcium chloride to the feed to promote the alkali cycle? Answering these questions requires the development of a mass and energy balance, and alkali cycle model of the kiln. The mass and energy balance is required because the total energy input to the kiln will have to be kept constant as the fuel mixture is varied. Will 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. Differences in the particle size and volatile content will affect the heat release characteristics in the kiln. The model has to allow scenarios with varying energy replacement factors for different fuels. The mass balance is required because different fuels will yield different mixtures of combustion product gases. Carbon dioxide, CO2, and water vapour, H2O, occupy different volumes due to their different molecular weights. The size of the bypass will depend on the volume of the kiln exhaust gases at the kiln inlet as well as the proportion that must be bypassed to bring the alkali content of the clinker down to the level required to meet the low alkali cement specification. Temperature of the gases at the kiln inlet and the degree of calcinations of the feed entering the kiln from the preheater will also affect the volume at the kiln inlet.

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TECHNICAL FORUM Once the mass and energy balance has been developed the alkali cycle in the kiln has to be incorporated. Volatile species in the kiln feed pass down the preheater and into the rotary kiln. Some of the volatiles pass out in the clinker, but some evaporate in the burning zone and are carried away with the combustion product gases from the main burner. Any volatile material in the fuel joins the volatilised material from the feed, and together they are carried back to the preheater in the exhaust gases. In the preheater these volatile species condense on the cooler incoming feed and are carried back into the kiln, where they area again volatilised. The cycle builds up until conservation of mass demands that the amount of material exiting in the clinker equals the amount of material entering the feed and the fuel. The concentration of recirculating material in the kiln exit gases and the hot feed is much higher than in the feed, fuel or clinker. Modelling the cycle for a particular kiln requires knowledge of input levels of volatile species in the feed and the fuel, the output level in the clinker, and the recirculating level in the hot meal. This allows the volatility of the individual species in the particular kiln to be estimated. This is the key to predicting the effects of varying inputs of volatile species as the fuel mixture is changed. A bypass works because the concentration of recirculating material in the kiln exit gases is much higher than in the incoming feed or the fuel. Bleeding out a small proportion of the kiln exit gases takes out the equivalent mass of material that is entering the feed and the fuel, and the cycle is broken. As the model is tuned to the particular kiln the proportion of material that must be bled out through the bypass to achieve a given residual level in the clinker can be predicted. This proportion of volatile material can be translated into a volume of the gases at the kiln exit that must be bypassed. Volatility of different species varies from kiln to kiln, and also dependent on the relative proportions of the different species present. The order of volatility increases in the order Na2O < K2O = SO3 < Cl¯, with the volatility of K2O in particular depending on the amount of chloride, Cl¯, that is present in the kiln. This is why adding calcium chloride, CaCl2, to the feed can be useful to boost the alkali cycle and reduce the size of bypass to achieve a given level of residual K2O in the clinker. 54

Building the ability to make these predictions into the model of the kiln requires that variation in the volatility of the K2O be taken into consideration.

Mineralisation Predicting the effects of the fluoride addition in the spent pot liners is a bigger challenge. The volatility of fluoride itself is fairly low and it does not build up into a significant recirculating load. However, increased stickiness of the feed in the lower sections of the preheater is reported. This is associated with low temperature eutectics and melts with mixtures of halide and sulphate salts in the feed in the hotter parts of the preheater. Within the kiln itself the fluoride has a mineralising effect reducing the temperature and therefore the energy required for clinker formation. Energy savings of between 20 and 50kJ/kg clinker have been achieved for each 0.1 per cent of fluoride addition. This energy saving can then be converted into additional output from the kiln by consideration of the reduced combustion product exhaust gas volume. But will the fluoride have this desirable effect when added in conjunction with increased sodium in the spent pot liners?

Clinker mineralogy A number of cement companies are adding calcium fluoride to produce mineralised clinker on their kilns, but the driving force is generally not reducing the fuel consumption or increasing the output from their kilns. Fluoride has two beneficial effects of the mineralogy of the clinker. The primary phase field of the alite, C3S, mineral is expanded, and the rhombohedral polymorph of the alite is stabilised. The first effect means the clinker can contain more C3S and therefore the lime saturation can be lifted. The second effect means the reactivity and strength development properties of the C3S are improved. Cement companies want to add more supplementary cementitious materials to blended cements and a more reactivity C3S mineral allows this without lowering quality. Modelling and predicting these effects is a real challenge. For certain the Bogue calculation of clinker mineralogy becomes more irrelevant. This is why the techniques of Dr Chromy become increasingly important. These can be combined with Whitehopleman raw mix proportioning software to provide a method for real clinker mineralogy control. Dr Chromy is a

INTERNATIONAL CEMENT REVIEW / OCTOBER 2003

specialist in clinker microscopy and has developed a unique approach to clinker mineralogy control. His approach is based on microscopic examination of the clinker, however the it would be equally valid with X-ray diffraction and Rietveld analysis. Samples of clinker from the kiln are analysed under the microscope and the quantities of alite, belite and interstitial material are estimated. Significant deviation from the targets for these mineral components triggers adjustment of the targets for kiln feed lime saturation and silica modulus in the Whitehopleman raw mill feed proportioning software. The estimate of the real clinker phase composition, the oxide analysis determined by X-ray fluorescence and the free lime content are also entered into the Whitehopleman software. Multiple regression between the true mineral composition and the oxide analysis is carried out providing coefficients for the multiple relationship between these dependent and independent variables. This relationship can then be used to predict the real mineral composition of clinker samples that have not been examined microscopically. Dr Chromy is an advocate of the visual inspection of clinker samples under the microscope, but skilled persons cannot be provided on every shift and inevitably the quality of analysis can vary. Whitehopleman advocates the use of Kvant pattern recognition software to automatically estimate the clinker mineral composition.

Techniques and other fuels These techniques can be applied to predict, anticipate and monitor the impact of any change in the cement kiln process. If you are thinking of burning meat and bonemeal you will be concerned about the impact of P2O5 on clinker mineralogy. If you are converting to petcoke you need to check the effect on ring formation in the kiln, possible formation of large balls of clinker, dusting of the clinker in the burning zone, recirculation of dust from the cooler, high free lime in clinker due to decomposition of C3S into C2S and CaO. Every new fuel or raw material is likely to have some impact on the process or clinker mineralogy. The way to predict and anticipate these is from experience gained elsewhere and chemistry, process engineering and technical first principles. When implemented the only way to monitor and correct for the effects is by controlling the real mineralogy of the clinker. ________❒