"10 per cent more efficiency can be achieved"
- PK Ghosh, Group Managing Director, Ercom Engineers Pvt Ltd
What is the unit per tonne of energy consumption consumption in a cement plant in India?
What is the unit per tonne of energy consumption consumption in a cement plant in India?
The average electrical energy consumption in India is around 90-95 kWh/tonne cement (OPC) whereas the best achievement is around 80 kWh/tonne cement (OPC). Similarly, average thermal energy consumption is around 760 kcal/kg clinker, whereas the best figures achieved are around 680 kcal/ kg clinker. There is scope for reduction in both thermal and electrical energy consumption in many of the cement plants in India.
The contribution of the various departments to power and heat consumption is given in the Table-1 (refer this table in the page 31).
The best achievable figures are generally observed for the larger scale plants in India using the state of the art technology when operating at or higher than design levels.
What are the factors affecting Energy efficiency? What steps/ operational measures you have initiated to achieve the target - for both thermal and electrical energy optimisation?
The energy performance of cement manufacturing units is dependent on the following factors:
a. The age and type of the manufacturing technology such as:
i. Number of preheater stages
ii. Presence and type of calciner
iii. Type of cooler
iv. Type of raw mill grinding system and the separator
v. Type of cement grinding system and presence of pregrinders
vi. Auxiliary material transport systems
b. Cement product type such as OPC, PSC or PPC
c. Capacity utilisation
The energy efficiency achieved is the result of optimization between capital expenditure and reducing operating expenses.
In case of specific heat consumption, approximately 20 per cent losses are through preheater exhaust gases, 12 per cent are through cooler exhaust gases and around 4-5 per cent are radiation losses (for 6 stage preheater - precalciner system with the state-of-the-art cooler).
These losses may be reduced by:
- Reduction of false air entry to reduce the amount of exhaust air
- Installation of VFD in cooler fresh air fans to reduce the inlet air efficiently in case the operation is at lower than design levels. This will ensure that excess air is not exhausted from the system.
- If PH gases exit at higher temperatures, it indicates that the heat exchange between raw meal and hot gas is not proper in the preheater cyclones. Root cause analysis will help indicate which of the cyclones are not functioning properly and the situation can then be rectified. Further temperature reduction of exhaust gas can be carried out by taking advantage of waste Heat recovery systems.
- In case cooler exhaust temperature and clinker temperatures are high, optimisation of cooler operations by adjusting cooler speed, bed height and balancing of the cooling air may be necessary.
- Radiation losses need to be taken care of by ensuring that refractory lining is intact in preheater, calciner, cooler and no hot zones are formed in the kiln shell.
The cement grinding department and the raw material grinding are the major consumers of electrical energy. The material transport systems have also to be looked in to ensure that the power consumption is lowered.
Some of the factors to reduce specific energy consumption (SEC) are as follows:
- The choice of grinding mills, the design of separators, and the layouts of these systems play a major role in determining have energy consumption.
- The process air fans are big electrical energy guzzlers. Since power consumption is a product of pressure drop and gas volumes, solutions can be worked out on both fronts.
- Installation of highly efficient screw compressors can help reduce the energy consumption.
Have you suggested any modifications/process improvement lately to help conserve energy?
Yes, once a detailed audit is carried out it becomes possible to carefully identify the improvement areas as well as classify them into:
- Opportunities requiring minimum investment - short term measures
- Opportunities requiring some investment - medium term measures
- Opportunities requiring major investment and down time - long term measures which result in considerable increase in productivity and reduction in specific heat consumption.
The projects for energy efficiency improvement will have to be prioritised based on possible benefits, funds available and required downtime. Some corrective measures are described as follows:
1. Short term measures
The short term measures are as follows:
- Replacement of worn-out parts of crusher and grinding machines
- Controlling the combustion air (10 per cent reduction in excess air can save 8-20 kcal/kg of clinker)
- Ensuring uninterrupted operation of the kiln
- Power factor improvement of electric motors and installation of VFDs / VSD.
- Depending on the specific site conditions, reduction in energy consumption of 10-15 per cent can be achieved by adopting these measures.
2. Medium term measures
i. Separate grinding of petcoke and coal: In many plants it is observed that petcoke and coal are ground together. This caused coal to be ground finer than required to meet the petcoke fineness requirement of 2 per cent on 90 microns. The energy consumption can be reduced by separately grinding coal and petcoke to their respective fineness. Expenses will be incurred in installation of separate bins and blenders for the mix fuel.
ii. Change in clinker grinding system: Vertical roller mills (VRM) to replace tube and ball mills: The power consumption of VRM is approximately 35 per cent less than the ball mill. Up to 25 per cent electricity may be saved by replacing ball mills with roller mills. Pregrinders can be installed before existing ball mills to reduce the power consumption in the totals circuit by 10-15 per cent. In new plants, roller mills or roller press/ ball mill combinations are recommended to be used instead of ball mills.
3. Long term measures
i. Replacement with LP cyclones: Depending on the efficiency of the fan, 0.66 to 0.77 kWh/t clinker can be saved for each 50 mm w.c. reduction in pressure drop. Installation/ modification of the inefficient cyclones often entail rebuilding or modification of preheater tower and implementation costs are site specific. An overall heat and mass balance would have to be carried out to ascertain the likely benefits which will be obtained after the modification.
ii. Addition for preheater cyclone stage: A 6-stage preheater calciner using coal shows exit gas temperatures of about 280°C, 5-stage preheater 310°C and 4-stage preheater, 350°C. Specific heat consumption would reduce by 20-25 kcal/kg clinker if a fifth stage is added. This may also result in higher pressure drop across the system and may warrant change of preheater i.d. fan. The implications on the cooler fans, the cooler i.d. fan and the preheater exhaust fan on addition of the Preheater stage will have to be evaluated in totality.
Addition of a cyclone stage is feasible if the original design was conservative. There may be efforts required for supporting of new cyclone if the original preheater building cannot bear the additional loads.
What is the scope of using flyash and other cementitious materials in cement manufacturing for reducing energy consumption?
The manufacture of PPC and PSC have gained momentum over the last few years. PPC is obtained by mixing clinker and gypsum with suitable Pozzolans such as flyash (FA). Composed typically of 60 per cent clinker, 5 per cent gypsum + approximately 35 per cent fly ash. As it prevents cracks, it is useful in the casting work of huge volumes of concrete.
Portland slag cement has a composition of 55-35 per cent clinker, 5 per cent gypsum + 40-60 per cent GGBS of fineness 3,000 to 4,000 blaine. It is thus generally used in water retaining structures or where structure is vulnerable to any form of chemical attack.
Consider the same plant described earlier, manufacturing PPC or PSC with suitable substitution proportions.
The reduction of heat consumption in PPC production (as against OPC) is directly correlated to the substitution percentage. In case of power consumption, depending on the fineness of flyash (FA) obtained, there may be the necessity for inter-grinding FA with the OPC. If the flyash obtained is of acceptable fineness, then the electrical energy consumption will be limited to blending and transportation of FA and overall reduction can be achieved per tonne of cement as shown in Table 2.
For PSC, some heat may be required for drying of slag. However the savings are achieved in overall heat consumption. The slag will have to be ground, preferably separately, before blending with the OPC. In spite of power consumption to the tune of 40 -45 kWh/ton of slag material ,when ground at a fineness of 4000-4200 blaine, as per Table 2, substantial gains are seen in SPC per ton of cement produced.
To what extent can IT be harnessed to save energy?
The use of IT and automation are essential to have in two major fronts.
1. Improvement of overall equipment efficiency
This has a direct influence on reducing the SEC as described below:
- Online monitoring of process and introducing Fuzzy Logic- and Neural Network-based control systems, helps in optimising operations and maintain product quality. Energy savings from process control systems may vary between 2.5 per cent and 10 per cent, and the typical savings are estimated at 2.5-5 per cent. The economics of advanced process control systems are very good and payback periods are quite attractive depending on the status of the plant operation.
- The move from breakdown maintenance to reliability-centered maintenance is possible by having remote monitoring of equipment health using IoT and cloud-based data handling systems and reducing unplanned shutdown.
- All these factors ensure that the availability of equipment is maximised and rejects are minimised and thereby increasing productivity.
- The implementation of MIS (management Information systems) like SAP helps in keeping track of key performance indicators. Management is able to monitor both plant operations and the productivity in a continuous manner.
2. Production planning and inventory management
- Plants having multiple cement products and optimise their production plans to meet the dispatch requirements with the implementation of relevant software.
- Similarly, the use of the correct mix of fuels, coal, pet coke, alternate fuels can also be managed properly vis-à-vis availability, cost and related process adjustments.
- The ordering of equipment spares, refractory and other consumables can be managed to ensure that there are no delays in plant start-up due to procurement related delays.
- Automation can be used to run equipment with heavy power consumption during low power tariff durations. This is applicable to the operations of clinker grinding mills and at times the raw mills. The power costs can be tracked in real time and operational modes can be made to adapt to the energy costs.
How cement plants can take advantage of the PAT initiative and RECS?
As per the last report BEE, in the first PAT cycle, the cement sector has saved 1.48 MTOE against a target of 0.815 million MTOE. This indicates that the cement sector has had the best performance amongst the designated consumers.
While measures are taken for reduction of energy consumption by improving operations and upgrading equipment, the use of renewable energy can also help in reducing the gate to gate energy consumption. This has been brought about in the following ways:
1. Increased use of alternate fuels - Currently best Indian plants score between 8-10% substitution whereas Europe has crossed more than 80 per cent, hence there is scope for more utilisation of renewables.
2. Harnessing solar energy - With around 25-30 MW total installations in the Cement Industry, scope exists to increase it considerably in the next 3-4 years. This should go up further since the Government of India's push for 100 GW of solar capacity by 2022.
3. Installation of Waste Heat Recovery systems - For the 420 MTPA installed capacity, WHR systems have been ordered/ installed of 350 MW capacity with a potential for going up to a total of 1000 MW.
ERCOM has carried out engineering for a 35.5 MW WHR at Gulf cement company Ras Al Khaimah, UAE, which is the largest WHR for a cement plant. Besides carrying out the feasibility and execution of solar power plants, ERCOM assists their clients in the designing of AFR handling and dosing/ firing systems in their existing facilities.
BS SRINIVASALU REDDY