- Dr Anjan K Chattejee
What have been the visible technological advancements in cement manufacturing during the last decade?
The cement industry in the world has grown phenomenally in the last decade and the production level of all varieties of Portland cements taken together has crossed four billion tonnes, which is the largest volume amongst all manmade materials. Such growth has been possible due to considerable advances made in the hardware and software of cement manufacture. The main drivers for these technological advances have so far been the ‘cost’ and ‘quality’ of products. The technological progress has been multi-dimensional as reflected in the following features:
1. The capacity of a single kiln for clinker making has reached 12,000-13,000 tonne (t) per day, although in the recent years there is a trend of installing kilns of lower capacity due to economic and logistics reasons.
2. With automation, instrumentation, computer-aided controls and integration of expert systems the man-hours per tonne of cement came down to one or even less, thereby reducing the application of human discretion and increasing the dependence on electronic gadgets.
3. The choice of grinding systems for raw material and clinker has been dependent on the better energy utilisation factor, which has led to more extensive adoption of vertical roller mills, high-pressure roll presses and horizontal roller mills.
4. The fourth generation clinker coolers are now available from several suppliers, operating with 75 per cent efficiency of the theoretical maximum.
5. Significant developments have taken place in multi-channel burners, which have been specifically designed for co-incineration of alternative fuels.
6. The efficiency of the thermal process inclusive of raw materials drying has now touched almost 80 per cent of the theoretical maximum.
7. Driven by the rising prices of power and fuel, experiencing concerns about grid reliability, and fulfilling the commitments to sustainable development, the cement industry has taken more interest in ‘waste heat recovery’. While the most common water-steam cycles operate at heat source temperatures as low as 3000C, for heat recovery from still lower temperatures, the Organic Rankin Cycle, utilising organic compounds as process flows or the Kalina Cycle, using a water-ammonia solution, are now available for implementation in cement plants.
8. For sustainable production, the AFR use has taken deep root in the operational philosophy. Depending on the social conditions, living habits, availability of AFR and its collection systems, the extent of use varies from country to country, although the objective is to maximise its use.
9. Process measures and secondary abatement technologies ensure low emissions of dust, NOx and SOx in all modern plants. Recently additional focus has been laid on emissions of mercury and carbon dioxide. In parallel, there has been significant progress in developing continuous emission monitoring systems.
10. There has been widespread application of computational fluid dynamics (CFD) and of physical simulation and modelling in solving process and design problems.
What are your observations on the progress achieved in reducing the energy consumption in manufacturing?
The global average thermal and electrical energy consumption levels are reportedly 800-850 kcal/kg of clinker and 100-110 kWh/t of cement. Compared to these levels the average specific energy consumption values in India are 725 kcal/kg clinker and 82 kWh/t cement and the corresponding best values obtained are 667 kcal/kg and 68 kWh/t. From these values it appears that globally there is still enough scope for better energy management, while in India the potential of energy conservation is rather limited.
In this context, it is important to note that more rigorous environmental norms will, of course, reduce the emission loads but at the cost of energy. Further, stricter specifications of products, more stringent control of particle size requirements, use of non-carbonate alternative materials, etc. are expected to integrate new or additional process measures, which might increase the energy consumption. Hence, the potential of further energy conservation in our country in particular will depend on the future course of product quality and environmental demands. In addition, the limitations of plant vintage, design and layout may act as obstacles in achieving further energy conservation.
Are you satisfied with the research done on low- and off-grade limestone?
While the use of low- or off-grade limestone is not a critical concern in many countries, it is certainly an issue that needs to be dealt with more seriously in our country, as it can create 25-30 per cent additional resource base for the rapidly expanding industry. Limestone having CaO content of less than 42 per cent and limestone containing impurities of high silica or high magnesia or high iron content fall in this category and viable technologies for their use will be of immense economic benefit. Researches in this field, however, are sporadic and academic. The current technologies are limited to ‘sweetening’, wobbling, belt sorting, and froth flotation.
The newer technological options of photometric sorting, electrostatic separation, bioleaching, or making products not conforming to the conventional types, continue to be exploratory in their development. On the contrary, utilisation of marginal grade limestone by the cement industry deserves a ‘mission’ status in our country. Since dry manufacturing systems can these days co-exist with wet preparation of raw materials, improved froth flotation and bioleaching techniques cannot be ignored.
More logical perhaps is to look at new products and new processes, High-belite cement and high-magnesia blended cement are examples of such possibilities. Use of dolomitic limestone for simultaneous manufacture of cement and magnesia is a technology worth re-examining. Broadly speaking, it is time to lay much greater emphasis on research on utilising low-grade limestone.
What is the status of research for enhancing the use of high-ash coal?
We all know that the cement industry has been a very effective user of high-ash coal. The kiln burners are designed suitably to combust high-ash coal and the plants make use of coal with ash content of 35-40 per cent in most cases. Mixing of coal with varying ash contents has also been in practice to facilitate the use of high-ash coal. Several attempts were made in the past to install small captive coal washing units in a few cement plants to upgrade the quality of coal for process consumption but not with success due to economic and operational reasons. A few pit-head coal washing plants are operating in the country to de-ash non-coking coal prior to supply to the cement units and other users. The aforesaid measures do not seem to be adequate to meet the future demand of clean coal. Hence, for enhancing the use of high-ash coal further it would be important to integrate the technology of coal gasification with the cement manufacturing process.
Technologies for coal gasification are decades old but their integration with the cement manufacturing process needs specific development with regard to the operational features and economic viability. It is pertinent to mention here that coal gasification is attractive from the economic and energy security perspectives but the overall carbon intensity is much higher than coal mining. The technology is also water-intensive. Nevertheless, the abundantly available resource of high-ash coal in the country needs to be considered an object of priority in meeting the energy demand by adopting such a technology. It is interesting to note that China has laid out plans to produce 50 billion cubic metres of gas from coal by 2020, enough to satisfy more than 20 per cent of total gas demand. Despite the stated environmental shortfalls, the technology has been introduced in order to exploit the stranded coal deposits sitting thousands of kilometres away from the main industrial consuming centres, as transportation of gas is deemed cheaper than transporting solid fuel. It might also be pertinent to mention here that in some countries the adverse environmental problems of gasification technology has led to considering the alternative ‘underground coal gasification’ process.
In brief, the process involves pumping oxygen and steam through a small borehole into the coal seam to cause local combustion. The synthetic gas product consisting of hydrogen, methane, carbon monoxide and carbon dioxide is siphoned off through a second borehole and is collected, transported, stored and used. It is reported that the underground coal gasification process substantially reduces the CO2 emission.
While on the subject, another widely known clean coal technology of ‘coal bed methane’ deserves a mention. The process is relevant for coal deposits that are too deep to mine. Water is sucked out of the seam and methane attached to the surface of the coal seam is freed and then collected. The CBM technology is said to have fundamentally changed the dynamics of the gas industry in Australia.
Considering the plethora of options for clean coal technology, it is important for the cement industry to be more involved in coal research but in a co-ordinated national strategy, as it cannot be handled at the individual company level.
What is the progress in real-time analysis for QC in cement plants?
Recent developments in the use of x-ray diffraction are changing the traditional methods of quality and process control, as they have the ability to measure mineral phases or compounds formed directly in real time. Cement and clinker production involves chemical reactions to produce precisely controlled blends of phases with specific properties. So far there has been overwhelming dependence on either off-line or on-line oxide or elemental analysis of raw or in-process materials for QC.
Methods and equipment are now available for continuous quantitative on-stream analysis of the mineral or phase composition of cement and clinker. The instrument is a stand-alone piece of equipment, which is installed at the sampling point. A sample for analysis is extracted from the process stream and after due preparation on-line the sample passes through the x-ray beam.
The diffracted x-rays are collected over 0 to 1200 by a detector. The Rietveld structural refinement technique is applied to analyse the resulting diffraction pattern. The analysis of the moving stream is done in close frequency of, say, once every minute. All analysis results are communicated directly to the plant PLC system. The real-time measurement of the mineral composition of cement and clinker for process control is a paradigm shift for the cement industry. The discernible benefits of using on-stream x-ray diffraction are the following: