ABSTRACT PRODUCTIVITY IMPROVEMENT AND ITS RELEVANCE IN INDIAN CEMENT INDUSTRY- CASE STUDIES Rabindra Singh *, Ramachandra Rao* AK Mishra * S Kumar**, Lokesh Bahety**, NKM Sudhakar*** * National Council for Cement and Building Materials ** OCL India Ltd, Rajgangpur *** Dalmia (Bharat) Cement Ltd, Kadapa Indian cement Industry has resorted to a number of measures like technology up gradation, process optimization, use of captive power plant, use of energy efficient equipment etc. to reach the present level of productivity. However, there still exists a significant Potential for improving productivity in a number of cement plants. Case studies have been cited to emphasize the role of kiln optimization leading to reduction in energy consumption. Capacity utilization affects energy consumption resulting in changes in productivity and profitability of cement plants. Pro-active approach of cement plant can result in timely action giving immense benefits in terms of increased production, improved quality with moderate to significant reduction in energy consumption thus improving the overall productivity of cement plant. This is demonstrated by NCB in a million tonne kiln in eastern part of India and a 1.5 Million tonne kiln in the Southern part of India. The paper highlights the efforts of NCB towards process optimization leading to improved production, quality and energy efficiency to prepare the plants achieve PAT targets of energy consumption besides abating the effect of global warming. 1.0 INTRODUCTION Cement kilns performance is dependent on a number of factors such as variations in quality of raw materials mined, fineness and particle size distribution of ground raw materials, various modulii values of kiln feed, coal quality and its variations. Ash content, volatile matter, Moisture content and fineness of fuel, besides process and operating parameters of kiln-preheater-calciner and cooler operation also influence the kiln productivity. A number of cement plants suffer from problems such as poor quality of clinker and cement, unstable kiln conditions, inadequate design capacity of primary air fan and burner, inappropriate flame, inadequate capacity of calciner, discrepancies in design and installation of preheater calciner-cooler system, poor heat recuperation of cooler etc. leading to sub-optimal productivity of plants. Inordinate delay on the part of plant in addressing the plant problems leads to reduced profitability besides increased energy consumption and deterioration of cement quality. NCB carried out kiln optimization studies one in the Eastern
part for a million tonne modern dry process plant and the other in the Southern part of India for a 1.5 million tonne modern dry process cement plant. The result of the optimization has been spectacular for both the kilns in terms of improvement in quality and productivity. For the million tonne kiln the production is reported to have increased from 2800tpd to 3200 tpd with significant reduction in heat consumption to the tune of 80 kcal/kg clinker besides improvement in clinker quality in terms of free lime. For 1.5 million tonne kiln the production has increased from the existing level of 5000tpd to 5500tpd and above with significant improvement in clinker quality. 2.0 CASE STUDY-1: 2.1 Problem Discussion The 3.8 m ID x 56 m long kiln (3000 TPD clinker) was having the problem of high free lime in clinker and coating formation in burning zone resulting in reduction of output besides high preheater exit gas temperature and high heat consumption of 913 kcal/kg clinker. 2.2 Observations- Based on the measurement of key operating parameters (Table 1) of kiln system (Fig.1), our observations are as under: Table 1: Operating Parameters of kiln PARAMETER Date 21/05/2012 Kiln Feed rate TPH 187-189 Kiln RPM 3.7 PH fan I/L Draft mmwg ( Old ) -682 PH fan outlet temp. deg. C ( Old ) 376 PH fan I/L Draft mmwg ( New ) -511 PH fan outlet temp. deg. C ( New ) 395 Tertiary air temp. deg. C 895-901 PC out let temp, deg. C 896 Kiln I/L- O 2, / CO % 0.51-.001/0.8-1.06 Precalciner outlet - O 2, %/ CO, % 0.5/0.08 i) Deficiency of combustion air -Low level of oxygen varying from 0.51% to 0.001% against normal value of 2% and high CO formation in the range of 0.8% to 1.06% against normal value below 0.1 % indicate deficiency of air in kiln, thus limiting the kiln coal firing to 25%and reducing the clinker output besides deteriorating clinker quality.
ii) High PH Exit Gas Temperature-The exit gas temperature for old PH string (PH1) and new PH string (PH 2) were high at 375 O C and 395 O C respectively against the normal value of 310-330 0 C for five stage Preheater and is a factor for high heat losses from preheater. Low Oxygen and high CO of 2.86-3.4% and 0.12-0.26 % for PH-1 and 1.93-2.39% & 0.12-0.39%for PH-2 against the normal 4%& 0.05 % respectively are the factors for high exit gas temperature in both the PH strings due to delayed combustion of coal on account of insufficient oxygen. iii) Low Flame Momentum- The prevailing air flows through kiln burner (pyrojet) shows that the flame momentum at 1275%,m/sec is low and it should be around 1500%,m/sec for efficient combustion of coal. iv) Kiln feed LSF variation LSF variation was found to be high in the range of 110-119 which could be one of the reasons for high free lime in clinker. v) Degree of calcinations - Degree of calcination of hot meal samples maintained at kiln inlet was very high at 98-99 % against the normal range 92-95 % due to high calciner outlet temperature of 886-896 0 C leading to high preheater exit gas temperature. vi) Thermal Performance The heat consumption for the kiln system is estimated to be 913 Kcal/ kg clinker at 120.5 tph clinker production. Factors behind the high heat consumption are high preheater losses of 243 Kcal/ kg clinker besides high heat losses of 192 Kcal/ kg clinker from cooler comprising of heat loss through cooler exhaust air (127.76 Kcal/ kg clinker), clinker (20.26 Kcal/ kg clinker), heat loss due to water spray in cooler (41.22Kcal/ kg clinker), convection and radiation loss of (3 Kcal/ kg cli nker) resulting in low heat recuperation efficiency of 52% for grate cooler at the existing cooler loading of 39.9TPD/M 2. The under grate pressure maintained in 1st grate was low at 420 mmwg as against the normal value of 750 mmwg, resulting in poor heat recuperation from clinker in cooler. It was also observed that fans A &B which are connected to 1 st grate were damper controlled with damper opening of 40% & 60% respectively resulting in low secondary air to kiln. 2.3 Suggestions & Recommendations a. CO content at kiln inlet should be less than 1000 ppm (0.1 %) whereas O 2 content should not be less than 2 % for efficient combustion of coal inside the kiln. b. Kiln inlet gas temperature should be maintained at around 1000 0 C by installing thermocouple and the gas temperature at calciner outlet should be reduced in steps of 10 0 C from 886-896 0 C so as to limit the degree of calcinations to 92-95%.
c. O 2 content at preheater outlet should be maintained at 4% with CO below 500 ppm (0.05%) ensuring minimum leakages across preheater without secondary combustion. d. The preheater exit gas temperature should preferably be maintained below 330 o C for 5 stage preheater. This will result in reduction of heat losses from PH exit gas to the tune of 43Kcal/kg clinker. e. The silo extraction mechanism needs to be changed from sequential to alternate segment besides reducing the extraction cycle time. The silo filling must be maintained above 75% to achieve higher blending efficiency for reducing the existing LSF variation of 110-119 for kiln feed to a suitable level so as to restrict the variation within ± 2. f. The kiln burner should be operated with a flame momentum of 1500 %m/sec which will improve fuel combustion in kiln thereby reducing delayed combustion in kiln besides reducing the free lime in the clinker. g. Strengthening of cooler with adequate fan capacities can reduce heat loss from the cooler from the existing level of 192 Kcal/Kg clinker to 120 Kcal/kg clinker resulting in a saving of 72 Kcal/Kg clinker. h. Cooling fans A&B which are damper controlled with damper opening of 40% & 60% respectively of grate-1 should be operated at 100 % damper opening. If required volume and pressure are not met, they should be replaced by low volume and high pressure fans operating at full rpm. These measures will help in increasing the secondary & tertiary air temperature & quantity and help in eliminating reducing conditions. 3.0 CASE STUDY-2 3.1 Problem Discussion: The kiln equipped with 6 stage preheater and precalciner supplied by FLS is 4.35 m ID x 67 m long with rated capacity of 4500 TPD clinker and produces around 5000 TPD clinker as against the maximum production achieved to the tune of 5800tpd. The plant has been experiencing the problem of high free lime in clinker, coating formation in burning zone, yellow core formation in the clinker and feed flush to kiln leading to fluctuating kiln feed rate and dusty kiln conditions. 3.2 Observation: Based on the measurements of Process & Operating Parameters of kiln system (Fig.3), given in Table 2, our observations are as under:
Table 2 : Operating Parameters of kiln Parameter Kiln Feed rate (TPH) 315-325 Kiln RPM 4.5-5 Kiln thermal loading (Gcal/hr/m 2 ) 4.31 Kiln filling (% ) 14.42 Volumetric loading (tpd/m 3) 6.09 PH fan inlet temperature (deg. C) 278 Tertiary air temperature ( deg. C) 1050 PC out let temperature (deg. C) 966 Cooler under grate (1 st grate ) pressure (mmwg) 787 Kiln inlet temperature (deg C) 1080 Kiln hood draft (mmwg) -6 Kiln Inlet O 2 (%)/CO (ppm) 3.6/200 Precalciner 3.75/200 Preheater outlet 4.36/85 i) O 2 and CO in kiln circuit: - O 2 and CO at kiln inlet 3.6% & 200ppm and at precalciner outlet 3.75% & 200 ppm respectively are within the acceptable limits. However, there is slightly more oxygen (4.36% against maximum of 4%) at preheater outlet. ii) Degree of Calcination: - Degree of calcinations has been calculated based on the LOI of kiln feed and hot meal sample collected from 6 th stage of cyclone meal pipe and found to be 98.08 % which is high and should preferably be maintained in the range of 92-95% for efficient kiln operation. iii) Material flushing: - It was observed from CCR that there are fluctuations in 6 th cyclone cone draft and Clinker deep pan conveyor tonnage (Fig. 4). It indicates the material accumulation from PC outlet to 6 th cyclone inlet duct resulting in flushing of material in kilncooler system. This may be one of the reasons for yellow core formation and dusty kiln condition. iv) Temperature, pressure and velocity profile:- To diagnose the plant problem measurements of temperature, drafts and estimation of velocity profile were carried out and it was found that the T & P between the PC exit and 6 th cyclone were low at 64 0 C and 40 mmwg
respectively which may be due to absence of diptube in the 6 th cyclone. Similarly there is no diptube in the 4 th cyclone which results in reduction of T & P. Based on the prevailing temperature and pressure and preheater gas flow of 611912 m 3 /hr i.e. 281796 Nm 3 /hr (1.35 Nm 3 / kg clinker) at 208.33 TPH clinker production, the gas velocity profile at different locations of the preheater cyclones were estimated at 5000TPD & 5800TPD and found to be in the range of 8.2-13.2 m/s at 5000 tpd and 9.5-15.3 m/s at 5800 tpd clinker production indicating suitability of Preheater for producing 5800 tpd clinker. v) Burner Operation: - The prevailing air flows through kiln burner (Duo-flex) gives the flame momentum of 1625% m/sec which is satisfactory. However, Swirl air and axial air opening maintained by the plant in the ratio of 68 % and 100 % in the burner. Opening of swirl air at 68 %is high for a fuel with high volatile matter to the tune of 40% and it should be restricted to 25-30%. vi) Blending Silo Operation: - Kiln feed LSF variation was found to be high in the range of 0.88-0.97 which could be one of the reasons for high free lime in clinker. Silo filling maintained by the plant was 50-60 % which is low and should be 75-80% and filling of silo was through two gates with silo filling period of 7 minutes from a single gate which is on the higher side. Feed extraction was from a pair of opposite gates with extraction from consecutive segments as against alternate segments. vii) Thermal Performance- The heat consumption for the kiln system is estimated to be 686 Kcal/ kg clinker. The heat loss through the cooler comprising of heat loss through cooler exhaust air (106.56 Kcal/ kg clinker), clinker (26.24 Kcal/ kg clinker), convection and radiation loss of (3 Kcal/kg clinker) is 135.8 Kcal/kg clinker. Based on the above, the heat recuperation efficiency of S.F. Crossbar cooler which is equipped with latest MFR technology works out to be around 65% against expected level of 75-80% at cooler loading of 42.8 TPD/M 2 indicating a further potential savings of 30-35 kcal/kg cl after MFR correction and making the kiln one of the best in terms of heat consumption at around 650 kcal/kg cl. 3.3 Recommendations & Suggestions 1. Material level in the silo should be maintained at 75-80% for improved homogenization. Reducing silo filling cycle time from 7 minutes to 3 minutes from a single gate and silo extraction pattern from consecutive to alternate gates with reduction in cycle time are likely to improve the homogenization in the silo and further reduce the fluctuations in kiln feed quality.
2. Reducing Swirl air from existing 68% to 30% after observing the clinker quality and CO level at kiln inlet by the plant resulted in stable kiln operation with improved granulometry of clinker with increased kiln feed from 325 to 345 tph. 3. Plant was requested to reduce 6 th cyclone outlet temperature in steps of 10 0 C to reduce the degree of calcinations to around 95% for stable kiln operation. 4. Air blasters should be installed at PC outlet to 6 th cyclone inlet duct to avoid accumulation of material. Alternately, the slope of PC outlet to 6 th cyclone inlet duct should be increased to reduce time of stay for raw meal in the duct. These measures are likely to result in reducing the instances of material flushing in the kiln and cooler system with stable kiln condition and reduced yellow core formation. 5. Plant can avoid recirculation of material in the 6 th cyclone by incorporation of dip tube and the temperature of the calciner can be maintained at around 850 0 C to get the degree of calcination to the tune of 92-95 %. Variations in fine coal residues should be reduced for stable kiln operation. This is likely to reduce incidence of yellow core/free lime formation besides reducing heat consumption due to lowering of PH exit gas temperature. 6. There are wide fluctuations in the fine coal residue on 90 microns in the range of 16.3 to 26.5% for US coal. Variations in fine coal residues should be reduced for stable kiln operation. 7. AM and SM maintained by the plant are in the range of 0.72-0.81 and 1.73-2.1 respectively which are low. Increasing the alumina content at the cost of iron is expected to improve the clinker quality and productivity. 8. The heat recuperation efficiency of SF crossbar cooler having MFR technology is determined to be 65 % which is low as against 70-75 % for latest generation cooler. Cooler MFR s to be checked and corrected in consultation with plant supplier for improving heat recuperation efficiency of the cooler. This will also help the plant in eliminating water spray in cooler besides saving 30-35 kcal/kg cl in heat consumption amounting to annual savings of around Rs 4.2 crores considering 5000 tpd clinker production and Rs 6000/t coal as the cost of fuel on account of improved cooler operation.
4.0 CONCLUSION Kiln optimization is a potential tool and can be exploited effectively by Indian cement Industry in achieving its PAT targets. This is amply demonstrated through the above case studies. M/s OCL (India) unit-1 could achieve increase in production from 2850 tpd to above 3200 tpd with saving in thermal energy more than 70 Kcal/kg clinker besides improving the clinker quality after implementing NCB recommendations without any capital expenditure. Similarly, implementation of NCB recommendations by M/s Dalmia Cement (Bharat) at Kadappa in A.P. resulted in increased clinker production to the tune of 5500 TPD from the existing level of 5000 TPD with simultaneous improvement in clinker quality besides reduced heat consumption without any capital expenditure. Technical expertise of NCB is available for cement plants interested in meeting the PAT targets and improving the plant productivity. Acknowledgement The authors have freely drawn the information/data from published literature and NCB reports. The paper is being published with the permission of the Director General, NCB, Ballabgarh. Special thanks to the management of M/s OCL (India), Rajgangpur, Odisha and M/s Dalmia Cement (Bharat) Ltd, Kadapa, Andhra Pradesh for reposing faith in the Capability of NCB Team for solving the plant problem. We are also thankful to Dr.S.C Ahluwalia, Mr. S kumar of M/s OCL India ltd and Mr. Anil bajaj, Mr. Karunakar Rao, Mr. NKM Sudhakar of M/s Dalima Cement (Bharat) limited for extending their full support in implementation of NCB recommendations for successful completion of project
Fig. 1: Operating Parameters of Kiln System Fig. 2: Operating Parameters and Airbalance of Cooler System
Fig. 3: Operating Parameters of Kiln System at M/s DCBL, Kadapa Release of Material Release of Material Release of Material Release of Material Fig. 4: Fluctuations in 6 th cyclone Inlet draught and DPC Tonnage on release of material buildup