WATER AND ENERGY CONSERVATION IN A THERMAL POWER PLANT

Abstract

The energy and water consumptions in thermal power plants are enormous, which can be saved for efficient use of these. In India, approximately 57% of total power produced is generated through coal based thermal power plant (TPP). However, combustion of coal in power plant leads to lower flame temperature, higher exhaust losses, lower thermal efficiency and higher emissions of carbon dioxide, which are unfavorable factors. On the other hand, water consumption in coal based TPP is also huge. Therefore, energy and water conservations are important national policies in India especially in TPP. Many investigators conserve water and energy in TPP considering a particular area such as exhaust flue gas, low grade waste heat, boiler blowdown, circulating cooling water, substitute of freshwater such as treated municipal wastewater, etc. However, they did not carry out systematic analysis where all possibilities of water and energy conservations should be accounted. Thus, in view of these, in this work a systematic study is conducted for heat recovery in boiler cycle, steam cycle, combined cycle of TPP. Further, simultaneous water and energy (SWE) conservation study is also performed. Moreover, techno-economic analyses of retrofitting schemes are also carried out. For conservation of energy and water in TPP a typical plant is selected, which is located in state of Rajasthan, India. The overall power generation capacity of this plant is 500MW having two units of 250MW capacity each. Lignite coal is used in this plant as a fuel. The water and energy conservation study is carried out in different sections of TPP such as boiler cycle, steam cycle and combined cycle. Along with this, simultaneous water and energy (SWE) conservation study is also performed. For each section possible areas are identified where waste heat is available and where it can be utilized. In the boiler cycle of TPP, flue gas as well as boiler blowdown are considered as possible areas of study. To utilize heat of these areas six different retrofit designs are proposed using pinch analysis. These design options are compared based on different factors such as location of newly added heat exchanger, amount of flue gas and exit temperature, environmental aspect, percentage of energy savings and maximum temperature across additional heat exchangers to choose best retrofitting option. In the boiler cycle, option-6 is found as best retrofitted design. In this option, firstly boiler blowdown is flashed at temperature 293°C. Further, flashed steam is used to transfer latent heat to feedwater to increase its temperature from 283°C to 285.73°C. Along with this, temperature of flue gas decreases by 20°C considering dew point effect and consequently, 6.3MW heat is available for heating of secondary air. Economic analysis of i option-6 illustrates that Rs. 9,23,09,300 is required to install air preheater (APH), flash tank, heat exchanger and additional heat transfer area in the existing APH. Payback period, discounted payback period, net present value and internal rate of return of option-6 are computed as 1.7 years, 2 years, Rs. 23,81,87,302 and 43.3% respectively. Further, saving in energy is found as 1.3%, which is equivalent to saving of standard coal equivalent (SCE) as 8.1g/kWh. Saving of 7.3t/h of demineralized (DM) water is also observed through option-6. To apply pinch analysis on steam cycle, all extractions exiting the turbine are considered as hot utility and feedwater as cold utility. Pinch analysis shows the potential savings in hot and cold utilities as 9.7% and 0.8%, respectively. Considering heat transfer across the pinch in condenser, heat available in boiler blowdown and flowrate of three extractions of low pressure turbine (LPT), six different energy integration schemes are proposed. Results show that extraction of LPT entering to low pressure heater-1 (LPH-1) is eliminated, which saves amount of steam extracted from turbine for preheating the feedwater and thus, extra power is generated. It is also seen that flowrate from other two extractions of LPT decrease marginally. Amongst six energy integration schemes, the best one i.e. scheme-6 indicates that using pinch analysis, net generated power is increased by 0.6% and DM water demand is reduced by 57.6%. Further, economic analysis of Scheme-6 illustrates payback period, discounted payback period, net present value and internal rate of return as 8.3 months, 9.1 months, Rs. 17,22,60,350 and 122.7% respectively. In the present study, combined cycle of boiler and steam sections of 250MW TPP is targeted and designed using SuperTarget software. The minimum hot and cold utilities are calculated as 563.7MW and 316.9MW, respectively. However economic analysis of heat exchanger network (HEN) proposed by SuperTarget is not feasible and very expensive. So, pinch cum heuristic approach is used with the six different retrofitting cases to recovery of heat of flue gas and boiler blowdown of combined cycle. The proposed retrofitting cases are compared based on energy saving, enhancement in TPP efficiency and CO2 reduction. The novel heuristic approach shows saving of 16.8 to 18.1t/h of DM water for proposed six cases. Using heat of waste stream, steam extracted from LPT to preheat feedwater is saved. Consequently, power output of turbine increases, which in turn enhances total efficiency of TPP. The results indicate that the energy saving effects are significantly higher in case-6 as compared to other cases. The net additional power supplied in the proposed case-6 retrofitting is 3.5MW. It also results in decrease in the net coal consumption rate of 8.6g/kWh and CO2 emission reduction rate of 3.3t/h. The economic analyses results indicate that the payback period, discounted payback ii period, net present value and internal rate of return for case-6 are computed as 9.5 months, 10.4 months, Rs. 35,14,38,432 and 105.8%, respectively. It is observed through retrofitting cases that reduction in higher stage extractions i.e. extractions at higher temperature, results in greater power generation and eventually shows better TPP efficiency. Further, it is observed that case-6 results in maximum increase in efficiency amongst best energy integration scenarios in 3 sections i.e. option-6, scheme-6 and case-6. Finally, a modified methodology is proposed for SWE conservation in non-isothermal system. The method proposed consists of two major steps. In the first step, a procedure is developed to design the water allocation network with modified concentration potential of the demand (CPD) order as per the processes restrictions. Further, in second step, heat exchanger network is designed by calculating hot and cold utility requirements through pinch analysis and intuitively designing WAHEN (Water allocation heat exchanger network). It is observed that the minimum freshwater requirement and wastewater generation is independent of change in inlet and outlet temperatures of the processes. TPP is investigated considering total dissolved solids (TDS) and total suspended solids (TSS) as single contaminant as well as multi contaminants. It is observed that TSS is the deciding component for SWE conservation in TPP when multi-contaminant (TDS and TSS) case is considered. The minimum consumption of freshwater for multi-contaminants case is found as 744.1t/h. It shows saving in freshwater by 5.2% and reduction in wastewater by 21.6%. WAHENs of all the cases of TPP are investigated and modified based on actual process conditions. Results of all sections of TPP, obtained in the present study, are compared well with that of published literature.