THERMAL ENERGY AUDIT

A DIGNOSTIC TOOL TO ACHIEVE PAT TARGETS IN INDIAN CEMENT INDUSTRY

O P Grover, Sd. Rayees Ahmed, Prateek Sharma and Rabindra Singh National Council for Cement and Building Materials ABSTRACT Cement manufacturing is an energy-intensive process in which the cost of thermal energy accounts for about 50% of the total energy cost. Increasing cost of fuel over the years has stimulated Cement Industry for reducing heat consumption. Apart from this, energy and process related carbon dioxide (CO2) emissions from cement manufacturing are estimated to be about 7% of global CO2 emissions, nearly 40% of which are due to the combustion of fossil fuels. The 83 designated cement plants in India has an energy saving potential of 8,26,853 tonne of oil equivalent/year in the last year of PAT cycle (2012-2015). Hence, reducing fuel consumption which results in cutting down the CO2 emissions is top priority for majority of cement plants in India to achieve BEE PAT targets. Energy Audit Studies by NCB in a number of cement plants reveal the fact that cement kilns are still operated with heat consumption as high as 850 kcal/kg clinker and above indicating huge potential for thermal energy reduction. In Thermal Energy Audit (TEA) studies by NCB, various components of heat consumption / losses are estimated based on process parameters measurements and remedial measures indicating thermal energy saving potentials are suggested. The paper highlights case studies of TEA indicating excess heat loss on account of suboptimal performance of cooler and false air entry in preheater system etc. 1.0

INTRODUCTION

The Indian Cement Industry has accorded highest priority to energy conservation in its efforts towards overall cost reduction in cement manufacture, essential for survival in prevailing competitive domestic and international market scenario. It is a fact that energy constitutes about 35-40% of total cost of cement production and thermal energy constitutes about 50% of the total energy cost of cement production. The 83 designated cement plants in India has an energy saving potential of 8,26,853 tonne of oil equivalent/year in the last year of Perform Achieve and Trade (PAT) cycle (2012-2015). Hence, reducing fuel consumption which results in cutting down the CO2 emissions is top priority for majority of cement plants in India to achieve Bureau of Energy Efficiency (BEE) PAT targets. The significance of Thermal Energy Audit (TEA) for energy and cost saving in cement plants is highlighted in this paper through three case studies out of the large number of studies carried out by NCB in the recent past. 2.0

NECESSITY OF THERMAL ENERGY AUDIT

The Indian cement industry is quite responsive in adopting various technological developments. The significant fact is that the best operating level of thermal energy consumption in cement plants in India is 675-685 kcal/kg clinker, which compares well with world best figure of 650 kcal/kg clinker. Some of the cement plants by their pioneering efforts have reduced energy consumption by 10 – 20 % by retrofitting energy efficient systems during the last 10 years giving them competitive advantage over others. National Council for Cement and Building Materials (NCB)’s Center for Mining and Environment Plant Engineering and Operation (CME) divulges the fact that a number of cement plants are still not operating their kilns to their full potentials. These kilns are operated with heat consumption varying widely from 680 to 850 kcal/kg clinker. The high heat consumption is mainly due to inadvertent high heat losses through the various exit points of kiln during its continuous operation. Hence, huge potential exists for reducing thermal

1

energy consumption in these kilns. It, therefore, becomes a necessity for the cement plants to conduct TEA of such kilns and implement corrective measures to achieve PAT target and cost savings. TEA has emerged as a potential tool for identification and analysis of excessive heat consumption areas and implementation of energy conservation measures. Energy diagnostic studies through audit have found that there is potential for reducing specific heat consumption to the tune of 40 kcal/kg clinker in some Indian cement plants. Estimates reveal that a reduction of just 5 kcal/kg clinker heat in kiln confers cost advantage of Rs 45-50 lakh per annum for a million tonne cement plant. 3.0

SCOPE OF TEA AND NCB’S APPROACH

TEA involving detailed process measurements and examination of operational practices in kiln system is found an effective tool to diagnose the causes for high heat losses. In these studies, various components of heat consumption / losses are estimated and remedial measures suggested to accrue potential thermal energy saving. NCB is fully equipped with latest portable instruments required for precise assessment of thermal performance of kiln, preheater, precalciner & cooler. The scope of NCB’s thermal energy audit is quite comprehensive and covers the following specific activities with emphasis on process diagnostic:  Assessment of kiln operation through study of prevailing performance data.  Field measurements of gas/air flow rates, temperature, draught, O2 & CO levels in kiln circuit with portable instruments  Study of the profile of process parameters in preheater  Compartment-wise distribution of cooling air in the grate cooler  Establishing gas/air balance  Laboratory investigations at NCB on samples of selected in-process materials  Evaluation of kiln system including heat balance on kiln  Formulation of action plan for thermal energy conservation  Cost benefit analysis of energy efficient measures. Based on the studies, some of the major causes of thermal energy wastages identified are:       

Inefficient operation of cooler Incomplete/ delayed burning of fuel in kiln/precalciner False air infiltration through preheater circuit Inconsistent kiln operation Lack of adequate monitoring and instrumentation Inherent deficiency in design of system Poor refractory practices

4.0 4.1

THERMAL ENERGY AUDIT – A FEW CASE STUDIES Methodology The methodology for carrying out TEA studies involves collection of relevant data through data collection format, field measurements of process parameters with the help of portable instruments and analysis of data for identifying the areas of thermal energy saving. 4.2

Case Studies Thermal energy audit study was carried out by NCB in three cement plants during 201213. Plant ‘A’ having Fuller Inline Calciner (ILC) kiln equipped with 4 stage preheater and grate cooler, Plant ‘B’ having FLS ILC kiln with 6 stage PH and grate cooler and Plant ‘C’ having

2

KHD ILC kiln with 4 stage preheater and grate cooler with IKN stationary grate at cooler inlet. These three kilns having rated capacity of 3000 tpd, 5250 tpd and 3000 tpd were producing 4390 tpd, 5182 tpd and 3024 tpd clinker respectively during the period of the thermal energy audit study. Plant data and some of the performance indicators of these kilns are given at Annexure I. 4.2.1

Heat Balance The flow diagrams of kiln circuits of all the three plants are shown in Figs. 1-3.

Fig 1: Measurements in kiln circuit (Plant A)

Fig 2: Measurements in kiln circuit (Plant B)

3

Fig 3: Measurements in kiln circuit (Plant C) The heat balance on these kilns was prepared indicating heat consumption for clinkerisation and heat losses through various exit points across kiln system as shown in Tables 1-3. Table – 1 : Heat Balance of Kiln (Plant A) – 4390 TPD Item Item kcal/kg

cl

Heat Input Sensible heat in coal Sensible heat in air Sensible heat in kiln feed

Combustion difference)

heat

through

Total

1.4 3.9 16.9

coal

(by

762.8

785.0

Heat Output Theoretical heat of reaction Heat in preheater exit gases Heat in preheater return dust Heat in cooler vent air Heat in clinker leaving cooler Heat for moisture removal Radiation losses

&

convection

Total

Table – 2 : Heat Balance of Kiln (Plant B) – 5182 TPD Item kcal/kg Item cl Heat Input Heat Output Sensible heat in coal 1.4 Theoretical heat of reaction Sensible heat in air 7.5 Heat in preheater exit gases Sensible heat in kiln feed 16.8 Heat in preheater return dust Heat from combustibles in raw meal 8.0 Heat in cooler vent air Heat in clinker leaving cooler

kcal/kg cl 413.7 164.9 11.1 112.4 21.2 10.0 51.7 785.0

kcal/kg cl 417.6 141.4 3.4 120.1 35

4

Combustion difference)

heat

through

coal

(by

Total

747.8

781.5

Heat for moisture removal

11.6

Radiation losses

52.4

&

convection

Total

781.5

Table – 3 : Heat Balance of Kiln (Plant C) – 3024 TPD Item kcal/kg Item cl Heat Input Heat Output Sensible heat in coal 2.0 Theoretical heat of reaction Sensible heat in air 6.2 Heat in preheater exit gases Sensible heat in kiln feed 14.0 Heat in preheater return dust Heat in cooler vent air Heat in hot air to 2 CVRMs Heat in clinker leaving cooler 806 Heat for moisture removal Combustion heat through coal (by difference) Radiation & convection losses Total 828.2 Total

kcal/kg cl 407.5 184.7 13.9 31.3 98.1 25.2 13.7 53.8 828.2

4.2.2

Factors for Excess Thermal Energy Consumption The factors contributing to excess Thermal Energy Consumption with their actual levels and normative values are shown in Table 4. Table 4: Factors contributing to High Heat Consumption Item Actual Recommended Plant A Plant B Plant C Cooler vent air temperature (0C)

430

330

380

65-75 for Retrofit cooler