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The purpose of this study was to optimize the process conditions i.e. location of Deaerator in 1MW Biomass Power Plant. Deaerator is an essential open feed water heater in the steam bottoming cycle to improve the efficiency and also to remove the dissolved gasses from the feed water. In the current work, steam has been generated in a boiler using biomass combustion. The steam is expanded in a turbine to generate the power. The fuel used is Rice husk the fuels were characterized by means of the higher heating value, proximate and ultimate analyses.
Keywords: Deaerator, Rankine cycle
Due to a continuous decrease in the amount and availability of conventional fossil fuels, it is necessary to seek alternative fuel sources, and the renewable nature of biomass makes it an attractive option. Compared to coal, biomass has a high volatile content. Biomass is generally deï¬ned as hydrocarbon fuel, as it mainly consists of carbon, hydrogen, oxygen and nitrogen. Direct energy recovery from biomass is by combustion, but other thermal processes such as pyrolysis or gasiï¬cation allow the conversion of biomass into more valuable combustible gases. Grieco and Baldi, (2011)
In general, biomass contains less sulphur than coal, which translates into lower sulphur emissions in higher blending ratios of biomass. Wood fuels generally contain very little ash (- 1% or less), so increasing the ratio of wood in biomass coal blends can reduce the amount of ash that must be disposed. A negative aspect of biomass is that it can contain more potassium and chlorine than coal.There are many different technologies for biomass combustion for power and heat. Rice is grown widely in tropical, temperate, and even in some cold zones. In the cleaning process, rice plant generates husks which are often burned a field or discarded. A huge amount of rice husks are utilized as a boiler fuel. Fluidized bed system, either bubbling or circulated, is employed for its combustion.
The performance of the Rankine cycle can be improved by increasing the mean temperature of heat addition, i.e. increasing the degree of superheat, steam pressure and by multistage expansion with reheating. The research so far was focused on improving the degree of superheat, increasing steam pressure and reheating. The efficiency of the steam power cycle can be improved to a large extent by the incorporation of feed water heaters (fwhs). A thermodynamic analysis of a steam power plant with fwhs involves a lot of mass, energy and exergy balance equations. In this work an attempt has been made to simplify this tedious task with the aid of generalization of the problem with 'n' fwhs. This work is useful for analyzing the steam power cycle with any number of heaters. Tsatsaronis and Winhold (1984) carried out detailed mass, energy, exergy and money balances for a reference steam power plant and investigated the effect of the most important process parameters on the exergic efficiency.
Capata and Sciubba (2006) described the feasibility of different power cycles from both thermodynamic and operative points of view. Dincer and Muslim (2001) conducted a thermodynamic analysis for a Rankine cycle reheat steam power plant. Rosen (2001) made a thermodynamic comparison of coal-fired and nuclear electrical generating stations using energy and exergy analyses. Rosen (2002) also explained his views regarding energy efficiency, loss and exergy based measures. Srinivas et al. (2006) presented thermodynamic analyses, i.e. both energy and exergy analyses for a coal based combined cycle power plant consisting of a pressurized circulating fluidized bed (PCFB) partial gasification unit and an atmospheric circulating fluidized bed (ACFB) char combustion unit.
The main aim of this work is to simplify the thermodynamic analysis of a regenerative steam power cycle with mathematical formulation for 'n' number of fwhs. This work is useful for thermodynamic modeling of a steam bottoming cycle in a combined cycle without repeating the mass and energy balance equations. In the present work, an attempt has been made to analyze the steam power cycle with fwhs from an exergy point of view.
Anil et al. solved the equations containing four atom balances C, O, H, and N and the equilibrium relations for gas compositions using MATLAB in atmospheric conditions.
2. Thermodynamic Analysis
The schematic flow diagram of a steam power cycle is shown in Figure 1. The proposed model consists of deaerator. The temperature-entropy diagram for the proposed model is shown in Figure 2. High pressure steam from the boiler enters the steam turbine to generate power. Some quantity of steam is extracted from the intermediate stage of the steam turbine to heat the feed water which is entering the boiler through the deaerator. A thermodynamic analysis is applied to the steam cycle having a single deaerator to find the state of steam extraction. The deaerator location is varied and the efficiency is obtained.
Gas Composition Using Equilibrium Constants
The biomass is defined by a general formula as Ca1Ha2Oa3Na4. For every single atom of carbon in fuel, the coefficient a1 becomes one. The coefficients a2, a3, a4 are the H/C, O/C, and N/C mole ratios, respectively; the moisture content in the biomass fuel is neglected. The reactions are solved at thermodynamic equilibrium. The biomass products contain CO2, H2, H2O, and N2. The following is the chemical reaction in biomass combustion
Ca1Ha2Oa3Na4+ a5 (O2 + 3.76N2) + a6H2Oâ‡¾b1CO2 + b2 H2O+ b3O2 + b4N2 (1)
The coefficient a5 can be obtained from the ratio of actual air fuel ratio to the stoichiometric air fuel ratio. The stoichiometric air fuel ratio for a typical rice husk is around 5:1. Similarly, the coefficient a6 can be determined from mass ratio of steam injection to rice husk.
Using the results of the fuel analyses for the biomass materials the air fuel ratio, chemical equilibrium calculations were performed for representative combustion over a range of reaction temperature.
The value of b, d, e, f, g, and x can be determined from the energy balance of above equation. The High Calorific Value of rice husk has been found as 14842.76 KJ/Kg and the Low Calorific Value of rice husk has been found as 13802.18 KJ/Kg.
Ca1Ha2Oa3Na4+ a5 (O2 + 3.76N2) + a6H2Oâ‡¾b1CO2 + b2 H2O+ b3O2 + b4N2 (3)
The coefficients a2, a3, a4 are the H/C, O/C, and N/C mole ratios. For every single atom of carbon in fuel, the coefficient a1 becomes one.The value of b1, b2, b3, b4 can be determined from the exergy balance of above equation.
Hf,fuel+a5 (hT,o2+ 3.76 hT,N2)+ a6 hf,h2o(l) = b1(hf,co2+hT,co2)+ b2(hf,H2O+hT,H2O)+b3(hT,O2)+ b4(hT,N2) (4)
The Actual Air supplied per kg of fuel has been found as 5.55kg.
Theoretical work done of the turbine has been calculated by the formula
WT= (h1-h2) + (1-m) (h2-h3) (5)
And it has been calculated as 899.62KJ
Work done of the pump has been calculated by the formula
Wp1= vw* (p1-p2) (6)
And it was been calculated as 5.382 KJ/Kg and the work done of the second pump was been calculated as 5.54 KJ/Kg.
Cycle Efficiency= (WT -Wp1 - Wp2)/(m1*(h1-h7)) (5)
Integration of combustion to Boiler feed:
The temperature of boiler is 1200°C and T13 is 300°C.
h10=b1hT, co2+ b2hT, h2o+ b3hT, o2+ b4hT, N2 (8)
h13=b1hT, co2+ b2hT, h2o+ b3hT, o2+ b4hT, N2 (9)
(h10 - h13) = ms mol* (h1-h7) (10)
Power has been calculated by the formula P= ms mol* Wnet (kJ/kg.mole) (11)
The standard fuel ratio is found as 3.5 and calorific value of fuel is 14842.76KJ/kg and molecular weight of the fuel is calculated as 0.318 for the rice husk which is used over.
ms = 1000kw / wnet (kg/sec for 1MW) (12)
The Efficiency of the plant is found by (P/mf.cv)*100 (13)
Results and Discussion
A thermodynamic analysis has been carried out in deaerator in order to investigate the performance of a steam cycle, plant efficiency, Power output. The net work output of cycle, in the percent of lower heating value as well as exergy of biomass, is expressed as energy efficiencies respectively to evaluate the cycle.
The efficiency of cycle and plant varies with respect to change in deaerator location. The variation of plant efficiency with respect to location of deaerator is shown in graph,
Plot for Deaerator vs. plant efficiency
Plot for Deaerator vs. Cycle efficiency
Plot of Deaerator vs. Power in watts
In this work, the process of Optimization of deaerator location in a biomass power plant is analyzed with the help of energy cycle. Using MAT lab, the deaerator location is iterated and a graph is plotted with respect to cycle and plant efficiency and also power for 1MW. The efficiency of cycle and plant varies with respect to change in deaerator location. And the paper analyzes the perspective of improving the efficiency of biomass for power generation.