In PAT-2 notification all sectors, including cement, have been given new sets of target SEC to be achieved at the end of three years cycle. The average percentage of reduction of target is 5.6% of the SEC values achieved in the assessment year of PAT-1 cycle (2014-15). We would like to debate here whether this reduction target is technically feasible and economically viable? Secondly, what are the energy saving options left with the cement plant (to achieve the target) in PAT-2 cycle?
To find the answers to these questions one need to look at the energy requirement of cement making process. It is to be noted here that a typical integrated cement plant (having both Clinkerization and Cement Grinding unit) is having Heat to Power ratio in the range of (7.5 : 1) to (9 : 1). This means- use of thermal energy (coal/pet-coke/ MSW etc) is 8 to 9 times more than the use of electricity. The entire thermal energy is used in the “Kiln Section” (consisting of pre-heater, calciner & rotary kiln) whereas electrical energy is distributed throughout the plant. Let us examine the energy distribution of a typical cement plant having clinker and cement production (average) of 5319 and 6290 MT per day. Total energy consumption of the plant is 4421 Million Kcal per day (all energy converted into Kcal) out of which 88.27% comes from fuel and 11.73% from electricity. Now look at these two different type of energy separately.
Electricity: Electricity consumption per hour is 25.11 MW of which 44.18% (11.096 MW) is consumed by the cement mill. Second largest electricity consumption comes from the Kiln area (mainly blowers & girth gears) - 35.50% (8.916 MW). This means almost 80% of the total electricity need comes from these two sections alone. Rest of the areas like Raw mill- 6.25% (1.571 MW), Coal mill- 6.11% (1.536 MW) & Utility- 7.93% (1.991 MW) consumes very insignificant part of electrical energy. Therefore it is clear that efficiency enhancement of major unit operations (crusher & conveyor) or utility (compressor & blower) will not lead to any substantial improvement of energy baseline. In fact all these areas have already been taken care of in PAT cycle-1 and they are operating at their peak efficiency. Given below the energy performance data of this plant section wise-
- Crushing, grinding and conveying of limestone: 452 kW/ton of limestone
- Crushing, grinding and conveying of pet-coke: 53 kW/ton of pet-coke
- Kiln rotation & air supply: 67 kW/ton of clinker
- Cement grinding process: 34 kW/ton of cement
I don’t think much elbow room is available on electricity front for further reduction of SEC of the above mentioned sections.
Thermal Energy: As mentioned earlier application of thermal energy is concentrated in one area i.e. Kiln section. Hence to analyze the distribution of thermal energy –energy balance of kiln section is very crucial. Let us see how the total thermal energy input (3902.73 Million Kcal/day) is distributed in the plant-
- 68% of the thermal energy is being consumed in the calcium carbonate decomposition reaction which is endothermic.
- 17 to 20% of the heat is taken by hot clinker (910 - 980 C) as sensible heat
- 6 to 9% of the heat is taken by hot flue gas (930 - 980C) as sensible heat
- 2 to 6% of the heat is lost to the atmosphere thru radiation & convection loss from Kiln surface (average surface temp 238 - 270C)
Now let us evaluate all the above four areas to find out energy saving potential-
- Area A: Nothing can be changed as activation energy and heat of reaction (endothermic) is fixed for a particular chemical reaction.
- Area B & C: Much of the sensible heat is used to preheat the feed material and combustion air. However degree of heat recovery largely varies on air flow dynamics and system design. Most of the plants in PAT cycle-1 had achieved 360 to 380 C of material temp after preheating by hot air. Raising feed material temp beyond this needs major change in design and air flow dynamics. The flue gas which is released to the atmosphere is having temp between 180 to 230 C leaving some scope of waste heat recovery. The most convenient waste heat recovery can be done by installing a VAM and generate chilled water for district cooling. However most of the cement plants are located in remote/ isolated areas where there is hardly any taker for chiller water.
- Area D: Lot of research had been conducted to reduce the kiln radiation loss by use of low emissivity coating but nothing has been commercially viable.
Under such circumstances, most of the cement plants shall find it difficult to achieve the target of PAT cycle-2. Time has come that they now look into the process and try to improve “Clinker to Cement ratio”. Presently this ratio varies from (1 : 1.52) to (1 : 1.58) for most of the plants. Higher ratio shall obviously reduce the SEC of final product. However this advantage can’t be taken by stand alone “Clinkerization” or “Cement Grinding Unit”.