Paperwhr Sb

Waste Heat Recovery- Practical Solutions for Cement Plants Gerald L Young, N. Jayaraman, Jayanta Saha and Dr. Suchismita Bhattacharya PENTA Engineering Corp., St. Louis, USA PENTA India Cement & Minerals Private Limited, Mumbai, India

SUMMARY

Due to process involved, cement industry requires very high electrical as well as thermal energy. Modern plants on an average take ~ 100 kWh of electric energy per ton of OPC production which is on higher side compared to old wet process plants. Advances in pyro technology have brought down the heat consumption of ~ 1400 kcal/kg clinker for wet process kilns to even < 700 kcal/ kg clinker for state of the art dry process kilns. Around 400 kcal/ kg clinker is required as heat of reaction for clinkerisation. The rest is needed to cover losses from radiation (~ 60 kcal/ kg clinker), cooler (~ 105 kcal/ kg clinker) and preheater exhaust (~ 160 kcal/ kg clinker). Around 30 kcal/ kg clinker comes from material and fuel as free heat. As a standard practice, part of heat from preheater/ cooler exhaust is utilised for raw material/ fuel drying. This paper will quantify, in detail, the waste heat that is actually available in cement plants and describe options for recuperating this waste heat for various location-based useful purposes. Among all applications, power generation from waste heat has become most popular. Cogeneration potential ranging from 3.0 to 20.0 MW (20- 30 KWh/ton clinker) exists in different plants depending upon the temperature, quantity of waste gases from preheater (PH) and cooler exhaust and raw materials/ fuel moisture content. The paper will also describe how waste heat can be utilised in multiple effect desalination (MED) plants to produce desalinated water. 1.0 INTRODUCTION The cement industry is highly energy intensive. Thermal energy required per ton of OPC is about 0.70 Mil.K.Cal and is generally supplied by fuels like coal, pet coke, gas or fuel oil. Electric power requirement of ~100 kWh/ tonne of OPC can be supplied from the electricity grid or through captive power plants depending on the facility available with the cement plant. Since the gaseous exhaust streams (from the preheater and the clinker cooler) are hot (200-400 °C), it makes sense to investigate the possiblility of using this waste energy. Waste heat produced in cement manufacturing facilities can be captured and put to use to produce steam for desalination or generating power (Bottom Cycle Cogeneration) or power can be generated using an onsite engine or turbine and its waste heat can be captured and used for some industrial process such as drying, preheating, and cooling (Top Cycle Cogeneration). This paper focuses on Bottom Cycle Cogeneration in cement plants. Based on annual global clinker production and an assumption of the technical potential of heat recovery per ton of clinker, a theoretical maximum electricity generation from the global cement industry can be estimated. In a large cement plant, it is technically feasible to generate 20-30 kWh/ t clinker via bottom cycle cogeneration. This leads to a saving of around 10 kg of coal per tone of clinker and corresponding reduction in CO 2 emissions. The technology for waste heat recovery is very mature in the Chinese cement industry as well as in Japan and Korea (20 years, over 70 installations). As per available reports, in India only 5 installations are in operation (refer Table 1). The intention of this paper is to show how waste heat 1

recovery (WHR) can be engineered into existing cement production lines as well as for greenfield projects. Table 1: WHR Systems in Indian Cement Plants Location India Cements Vishnupuram, AP, 1800 + 5200 TPD J.K cement Nimbahera, RJ 1200 + 1800 + 4800 TPD KCP cments; Mancherla, AP, 1800 TPD Shree Cement, Beawar, Unit 1, I million TPA Ultratech, Tadipatri, AP

1.1

MW Installed 7.7 steam turbine generator Rankine Cycle; 13.2 steam turbine generator 2.35 steam turbine generator Rankine Cycle; 5- 6 MW Steam generated

Vendor Kawasaki Heavy Industries Thermax boiler + Taiheiyo Engg. Corporation, Japan Transparent Energy Systems, Pune Transparent Energy Systems, Pune Transparent Energy Systems, Pune and ORMAT Systems

4 MW Organic Rankine Cycle,

Heat Consumption in Cement Manufacturing Process

The cement kiln consumes heat < 700 kcal/ kg clinker in modern state of the art plants to about 800 kcal/ kg clinker in older dry process plants. A typical heat balance (Reference 0 ° C) for 6-stage ILC preheater with modern clinker cooler is given in Table 2. Table 2: Heat balance (relative to C) Parameter

1.2

kcal/ kg cli

Heat of reaction

400

Heat Loss from PH Gases

160

Radiation loss from PH and kiln

60

Cooler Loss

105

Heat in with raw meal, air and fuel

-30

Total

695

Availability of Waste Heat

The PH gases exit the system at 275-350 °C, while the excess air from the clinker cooler exits the system at 250-300 °C. Increase in number of PH stages reduces the heat in the PH gases and consequently the fuel requirement. Clinker coolers with higher efficiency (73 to 78 %) also recuperate more heat from the hot clinker and consequently the amount/ temperature of cooler excess air is minimised. Table 3: Raw Material Drying Capacity of Waste Heat 100% Kiln Gases to Raw Mill Kiln Heat Consumption PH Exhaust Gas Temperature Exhaust gas Quantity Drying Capacity in a VRM running 20 hrs /day (@15 °C ambient)

-

5 stage ILC

6 stage ILC

kcal/kg cli

705-715

690-700

°C

312

280

kg/kg cli

2.18

2.16

%H2O

8.6

7.3

2

Waste heat from the preheater and clinker cooler is usually used for drying of raw materials and fuel. Depending on raw materials/ fuel moisture content, number of cyclone stages is selected. However, due to increase in electrical power consumption, number of cyclone stages is restricted to maximum six. Table 3 shows the maximum moisture that can be dried from raw material if 100 % PH gases are sent to raw mill. The remaining available heat is then may be considered for recovery. Steam or hot water production makes sense if there are industrial consumers or district heating in proximity to the cement plant. Preheating of raw material is usually more energy efficient than the cogeneration of electricity. Cogeneration systems convert thermal energy to electrical energy at about 30% efficiency (typically about 2504 kcal are required to produce 1 kWh (860 kcal)). 2.0

WHR TECHNOLOGY

Waste heat is used to produce steam in a boiler which can be used to generate electricity, desalinate sea water and for other purposes like domestic heating etc. Power generation requires a heat recovery boiler and a turbine system and can be based on Steam Rankine cycle, Organic Rankine Cycle (ORC) or Kalina process. Based on the chosen WHR process, 10-12 kWh/ t clinker can be produced from the cooler waste air and 11-15 kWh/ t clinker from the PH gases. Hence up to 25% of the power consumption of a cement plant can be produced by these technologies without changes in kiln operation. With suitable modification in kiln operation it may be possible to produce up to 30 kWh/t clinker. Power generation can be further increased by additional co-firing into the boiler. Some systems report generation of up to 45 kWh /t clinker. 2.1

Steam Rankine Cycle

The steam turbine is the technology best known from power plants. In the Rankine cycle waste heat is supplied to vaporize the working fluid (water) in the boiler to a high pressure steam which expands in the turbine to produce electricity by rotating it. The expanded water vapour is condensed to low pressure liquid in the condenser and is recycled back to the boiler for continuing the cycle. A typical flow sheet is shown in Figure 1. In modern power plants an efficiency of 40-45% can be achieved, but the relatively low temperature of the waste heat in cement plants (200 -400 °C) limits the efficiency to a maximum of 20-25%. Technology suppliers are modifying the tubes of the boiler (vertical versus horizontal) to prevent dust accumulation on the tubes. Some heat recovery boilers are provided with dust rapping systems to periodically remove any accumulated dust – especially for the PH boiler. Figure 1: Flow sheet of Typical Steam Turbine WHR System

3

2.2

Organic Rankine Cycle / Kalina Cycle

The ORC and the Kalina technologies use organic substances (pentane) or NH 3 as cycling media which evaporate at lower temperatures and can therefore produce electricity at a lower temperature level than steam turbines. The efficiency however is on lower side. The ORC uses a high molecular mass organic fluid to recover heat from low temperature (