ENERGY CONSERVATION IN COAL BASED SPONGE IRON CLUSTER USING TOTAL SITE INTEGRATION

Abstract

An enormous potential for saving energy is available in coal based sponge iron plants as these are associated with high percentage of energy wastage per unit sponge iron production. To deal with the current issue of energy crisis, various design and operational modifications are proposed to sponge iron plants. For this purpose, actual data of three sponge iron industries such as Process-1, Process-2 and Process-3 being operated with different capacities in an industrial cluster of India are considered. All these three processes are SL/RN based (Stelco-Lurgi / Republic Steel-National Lead). They consist of rotary kiln, rotary cooler, dust settling chamber (DSC), after burning chamber (ABC), evaporating cooler, electrostatic precipitator (ESP), wet scrapper and chimney as important equipment. In each process potential areas are identified where heat is lost and can be recovered and where this heat can be utilized. Considering these potential areas, two different design and operational modifications, Scheme-1 and Scheme-2, are proposed based on principles of process integration and applied in all three processes of coal based sponge iron cluster, individually. Further, two strategies, Strategy-1 and Strategy-2, are suggested for total site integration (TSI), considering all three processes as a single process to conserve energy in total site of plants of similar type where conventional methods are not applicable. These schemes and strategies vary in terms of modification, savings and investments. Scheme-1 accounts for preheating kiln inlet streams using waste gas exiting ESP and cooling kiln outlet using same ESP exit gas. However, for Scheme-2, initially kiln outlet is cooled using kiln air and further kiln inlet streams are preheated using waste gas exiting ABC. In Strategy-1 waste gas that exits from ABC of Process-1 is used for preheating kiln feed and slinger coal of all three processes, individually. Kiln outlet stream is cooled using kiln air of respective process. Further, kiln air is preheated using waste gas to maximum possible temperature such that ΔTmin equal to 50ᴼC is maintained. In addition to it, waste gas streams from ABC exits of Process-2 and Process-3 are combined and used for power generation. On the other hand, for Strategy-2 waste gas streams exiting ESPs of Process-1, Process-2 and Process-3 are mixed and used to preheat kiln feed and slinger coal of all three processes at Process-1. Further, kiln outlet stream is cooled using kiln air ii of respective process. Along with this, Strategy-2 proposes waste heat recovery boiler, for power generation, in Process-1 and Process-2 using waste gas exiting from ABC of respective processes. In the present work, the modified approach is developed to propose and solve energy conservation schemes as well as strategies. In this approach following steps are proposed: Step-1: Define a strategy for energy conservation, Step-2: Data extraction for all processes together in total site according to the strategy, Step-3: Preparation of stream table considering all processes together and selection of ΔTmin, Step-4: Utility targeting of total site, Step-5: Designing HEN of total site and Step-6: Modification of total site PFD. Guidelines to perform all these steps are also proposed. This method considers only six steps for TSI, which are far less as compared to conventional method where ten steps are involved. Pinch analysis is used to compute hot and cold utility requirements for schemes and strategies. In sponge iron processes, considered in the present work, utility and process streams are same i.e. coal and air where coal is burnt in the presence of air to provide necessary heat in the kiln. Therefore, once amount of hot utility changes coal and air requirements also vary. Due to this fact, flow rates of process streams (mainly coal and air) vary during solution of the problem. This requires trial and error computation technique to solve the problem. Thus, the present approach includes iterative method, if utility and process streams are same, as well as non-iterative method, if these are different. Further, as coal is used as utility and process stream in sponge iron processes, a revised model to compute its consumption is also proposed based on hot utility, heat of reactions, kiln feed and air preheating, radiation losses, dolomite decomposition, heat required to vaporize the coal volatiles, etc. For each process of total site, at the existing conditions, coal utility factor, a fractional use of total energy released from coal combustion inside the rotary kiln, is computed from modified model developed for coal combustion. Further, this utility factor is used for computation of coal consumption in all design modifications proposed. Apart from this, constant properties at average temperature in each iteration are considered in the present work. For each scheme and strategy economic as well as operability analyses are presented, which include capital investment, coal consumption, water requirement, energy consumption, savings, net profit value (NPV), internal rate of return (IRR) and discounted payback period (DPP). Further, based on these factors along with waste gas generation and %heat recovery, selections of best scheme for heat integration and strategy for TSI are carried out. Capital investment is done to iii install new gas-gas and gas-solid heat exchangers, gas carrying ducts with or without insulation, conveyor belts to carry hot solid streams and forced draft fans. Results of Scheme-1 and Scheme-2, applied for individual processes, indicate that Scheme-2 offers more coal and water savings over Scheme-1 in all three processes. Consequently reduction in waste gas generation is found more in Scheme-2 than Scheme-1. Energy ratio (ER), defined as ratio between actual energy consumed in the kiln to theoretical energy required for the reduction reactions to continue, is found less in Scheme-2 than Scheme-1. Thus, actual energy consumption, in Scheme-2, is more close to theoretical energy required than in Scheme-1. Based on these factors Scheme-2 is selected as best heat integration option for all three processes. On the other hand, results of Strategy-1 and Strategy-2, proposed for TSI of sponge iron cluster, indicate that both strategies offer better results over the existing system. However, savings in coal and water and hence, reduction in waste gas generation offered by Strategy-1 are found significantly higher as compared to Strategy-2. Excess energy consumption, which is 76%, prior to TSI, is reduced to 8% and 37% through Strategy-1 and Strategy-2, respectively. After final design Strategy-1 recovers 99.8% of waste heat available in the modified site whereas, Strategy-2 recovers 86%. Total annual cost (TAC) is 11.8% less for Strategy-1 as compared to Strategy-2. The discounted payback period for Strategy-1 is 11.65 months and that of Strategy-2 is 10.46 months. Thus, Strategy-1 is considered better heat recovery option over Strategy-2. Further, the results obtained for Strategy-1, are found significantly greater than Scheme-2 and various design modifications reported, based on single site integration, in literature. Hence, Strategy-1 is selected as the best design for cluster of any sponge iron plants and integrating as a total site is more profitable than integrating individually.