Thermodynamic and Economic Analysis of 1.4 MWe Rice-Husk Fired Cogeneration in Thailand

Transcription

1 International Journal of Renewable Energy, Vol. 4, No., July 009 hermodynamic and Economic Analysis of.4 MWe Rice-Husk Fired Cogeneration hailand Prachuab Peerapong and Bundit Limmeechokchai,* he Jot Graduate School of Energy and Environment, Kg Mongkut s University of echnology honburi Sirdhorn International Institute of echnology, hammasat University, Pathumthani, hailand * Correspondg author: bundit@siit.tu.ac.th Abstract Rice husk is a residue from rice millg process. Rice husk can be used an economical way to meet the energy demand with the rice millg dustry by usg rice husk as fuel for heat and power production to supply the heat and electricity the processes, and to produce surplus electricity for sellg to the national grid. wo cases of the study: thermal-match and rice husk-match of different energy utilization are considered for economic evaluation of power plants to meet different demand categories. he capacity of the plant is 576 tons paddy/day. he total load of thermal energy consumption is,06 MJ/ton paddy and the electrical energy consumption of the rice mill is 6,58 MWh/year. he total capital cost of the thermal-match cogeneration plant is million US$ while the total capital cost of the rice husk-match cogeneration is.4 million US$. Results show that the rice husk-match cogeneration is more economically feasible than the thermal-match cogeneration. he capacity of back pressure steam-fired boiler is 8 tons/hour of steam at 5 bar (absolute) and 400 o C. he rice husk-match cogeneration can generate power of,4 kw while the thermal-match cogeneration can produce power only 9 kw. he economic analyses terms of the net present value (NPV), simple pay-back period (PBP), and ternal rate of return (IRR) are also evaluated. Results show that the rice husk-match cogeneration has NPV of 0.0 million US$/year, PBP of.7 years and IRR of 7%, while the thermal-match cogeneration has NPV of 0.8 million US$/year, PBP of 5.5 years and IRR of 7%. he two cases of the study are based on 80 days/year of operation of rice mill cogeneration. Results of the study also show that rice husk-match cogeneration is more profitable than the thermal-match cogeneration. Keywords Rice husk cogeneration, Rice mill, Economic analysis, Back-pressure steam turbe, Electricity generation.. Introduction hailand, an agricultural-based country, is one of the world s leadg producers of paddy rice. Rice husk generated as a by product of rice mill processes can be utilized as an energy source for husk-fuelled rice mills. he annual production of rice husk is estimated to be approximately 5 million tons or equivalent to GJ [, ]. aken to account as a widely available CO -neutral fuel source, contag Higher Heatg Value (HHV) of ab 5 MJ/kg [5,6], rice husk is a renewable energy resource with high potential. Over the last 0 years, rice husk has been utilized widely hailand for combed heat and power or cogeneration power plants []. he proposed combed heat and power (CHP) generation system this study usg rice husk as fuel coverts biomass to energy with the acceptable efficiencies with low costs. Also, hailand, the Mistry of Energy has strongly promoted the use of rice husk and other biomass fuels for electricity generation with an centive through feed- tariffs scheme for Small Power Producer (SPP) and Very Small Power Producer (VSPP).. Methodology Analysis of power generation system is of scientific terest and also essential for the efficient utilization of energy resources. he most commonly used basis for analysis of energy conversion process is the first law of thermodynamics. However, there are creasg terests of combed

2 International Journal of Renewable Energy, Vol. 4, No., July utilization of the first law and the second law of thermodynamics, usg such concepts as exergy and exergy destructions order to evaluate the efficiency with the available energy. Exergy analysis provides the tool for the clear distction between energy losses to the environment and ternal irreversibility the process. Exergy analysis is a methodology for evaluation of the performance of devices and processes, and volves examg the exergy at different pots a series of energy conversion steps. With this formation, efficiencies can be evaluated, and the process steps havg the largest losses can be identified. For these reasons, the modern method approach to process analysis uses the exergy analysis which provides a more realistic view of the process and a useful tool for engeerg evaluation. he objective of this study is to analyze the.4 MW rice husk power plant from energy and exergy perspective. Sites of primary energy loss and exergy destruction are determed. he effects of varyg the environment state or dead state on the exergy analysis are also discussed. Nomenclature ε o chemical exergy of fuel (kj/kg) exergy efficiency B boiler Ex ϕ ratio of chemical exergy to net calorific value DM de-meral water I B boiler irreversibility (kw) m steam flow rate (kg/s) I S turbe irreversibility (kw) I total irreversibility (kw) HP/H high pressure/high temperature I EXG exergy loss to exhaust gases (kw) W cogenerated power (kw) X exergy rate (kw) Q CG NCV ψ process heat (kw) net calorific value (kj/kg) specific exergy (kj/kg). Plant Description he rice husk power plant has a total stalled power capacity of,500 kw. he rice mill cogeneration has capacity of 576 tons paddy/day. he generated electricity is supplied to the heat processes the mill, and the surplus electricity is exported to the grid. he toppg cycle cogeneration with back-pressure turbe has been designed on the thermal match, which can produce electricity and thermal heat for the processes, and the rice husk match, which can produce surplus electricity as much as the rice husk fuel can be available. he capacity of boiler to produce the superheated steam to drive the turbe is 8 ton/h at let pressure of.5 MPa and let temperature of 400 o C. In thermal match case, the turbe can generate electricity of 94 kw or total electricity of 4,07.6 MWh/year. In rice husk match, it can generate electricity of,4 kw or total electricity of 6,87.9 MWh/year. Both cases are based on 80-workg day operation. Rice husk and coal properties based on proximate and ultimate analyses are compared able. he schematic diagram of the rice husk power plants is presented Fig. for the thermal match process and Fig. for the rice husk match process. he thermal match process defed by the energy consumption is based on heatg process requirement while the rice husk match process defed by the electricity can be produced as much as the rice husk is available. he operatg conditions of rice husk plant are shown able.

3 International Journal of Renewable Energy, Vol. 4, No., July able Analysis of rice husk and compares with coal. Parameters Rice husk Bitumous coal Proximate analysis (% as received) Fixed carbon Volatile matter Moisture Ash Ultimate analysis (% as received) C H O.4.08 N S Higher heatg value (MJ/kg) Lower heatg value (MJ/kg).40 - Source: herdyoth A. and Wibulswas P.[4]. Fuel Boiler 4 Desuperheater urbe make-up water Generator Process heat Process heat 8 Pump 0 Preheat water Process heat 9 Make-up water Fig. Schematic diagram of rice husk power plant based on the thermal match.

4 International Journal of Renewable Energy, Vol. 4, No., July able Operatg conditions of the rice husk power plant. Operatg condition hermal match Rice husk match Mass flow rate of fuel (kg/s) Steam flow rate (kg/s) Steam temperature ( o C) Steam pressure (MPa).5.5 Max. urbe capacity ( MW) MW.5 MW Electrical power put (MW) 0.94 MW.4 MW Source: Sanit Athasart [5]. Fuel Boiler 7 Desuperheater 6 5 make-up water urbe 4 Generator Process heat Condenser Process heat Process heat Mixg water 0 Preheat water 9 Make-up water Pump Fig. Schematic diagram of rice husk power plant based on rice husk match. 4. Energetic and Exergetic Performance Analysis Energy performance analysis is based on the first law of thermodynamics; the ma performance criteria are commonly power put and thermal efficiency [4,7].(Note: he reference citation is not proper order!) he parameters are also decisive performance criteria economic analysis of power plants [,]. Exergy performance analysis is based on the second law of thermodynamics. Exergy destruction is the measure of irreversibility that is the source of performance loss. herefore, an exergy analysis assessg the magnitude of exergy destruction identifies the location, the magnitude and the source of thermodynamic efficiencies a thermal system [6]. Mass, energy, and exergy balance for any control volume at steady state with negligible potential and ketic energy changes can be expressed [7-].

5 International Journal of Renewable Energy, Vol. 4, No., July Energetic Performance Analysis he power put of steam turbe is calculated as follows: m i m e. () Q W m ehe m ihi. () W m h h ) + ( m m )( h h ) + ( m m... m )( h h ). () ( n n Where, the subscripts of,,, n represent the number of steam extractions steam turbe. As ternal power consumption the plants, the calculation of pump power can be simply given as followg: P. (4) P W m h h ) ( Where, P is the pump efficiency. Net electrical power put is given by, W W. (5) Net WP he total required heat energy the boiler can be determed from, msh hsh hsh mrh hrh hrh Q [ (,, ) + (,, )] B. (6) B Where, the subscripts sh and rh dicate superheated and reheat condition, respectively. Also, B is the boiler efficiency and boiler let enthalpy h sh, Eq.(6) is calculated from energy balance at feed water heater. ( m shs ) + ( m fwhfw) ( m shs ) + ( m fwhfw). (7) Where, s and fw of the subscripts represent steam and feed water, respectively and thermal efficiency of the power plants can be calculated as follows: W Net th. (8) m fuellhv Where, LHV is lower heatg value of fuel and m fuel is the mass flow rate of fuel (kg./s) and it can be calculated from Eq.(9): Q B m fuel. (9) LHV 4. Exergetic Performance Analysis For control volume of any plant component at steady-state conditions, a general equation of exergy destruction rate derived from the exergy balance can be given as, E x Ex Ex Q o o Q D ( ) ( ) + [ W ( ( ) ( ( ) ] ±. (0) Where, the first two terms of right hand side represent exergy of steam enterg and leavg the control volume. he third and the fourth terms are the exergy related to heat transfer, 0 is the ambient temperature of the systems and Q represents heat transfer rate across the boundary of the system at a constant temperature, and the last term is the work transfer to or from the control volume. he exergy transfer by heat ( X heat ) at temperature is given by,

6 International Journal of Renewable Energy, Vol. 4, No., July X heat ( 0 /) Q. () And the specific exergy is given by heat required, ψ h h 0 ( s 0 ). () 0 s hen the total exergy rate associated with a fluid stream becomes, E x X m ψ m ( h h0 ) 0 ( s s0 ). () Where, h and s represent specific enthalpy and entropy, respectively. otal exergy destruction rate the plant can be determed as sum of exergy rate of components: +. (4) E xd, total ExD, i ExD, B + ExD, + ExD, C + ExD, P ExD, H For the whole power plant, the exergy efficiency can be given as: W Net Ex. (5) m fuelex fuel he exergy performance equations for ma components of the thermal power plant are presented able. able. Exergy performance equations for ma components of a thermal power plant. Component name Component figure Exergy destruction rate Exergy efficiency. Boiler Flue gas Fuel Air Ma steam water feedg E x D, B X fuel X X EX, B X X X fuel. urbe W E x Ex Ex Ex W W EX, D, Ex Ex Ex. Condenser 4 E x D, C Ex + Ex Ex Ex4 EX, C Ex 4 Ex Ex Ex W 4. Pump E x Ex Ex + W D, P EX, P Ex Ex W 5. Heat Exchanger 4 E x D, H Ex + Ex Ex Ex4 EX, H Ex Ex Ex Ex 4

7 International Journal of Renewable Energy, Vol. 4, No., July Results and Discussion Exergy analysis of rice husk cogeneration both for thermal match and rice husk match are shown able 4 and able 5, respectively. able 4 Exergy analysis of the rice husk cogeneration power plant for thermal match at o 0.5 K, P o 0.5 kpa. Pot ( o C) P (MPa) Flow rate (kg/s) h (kj/kg) s (kj/kg. o C) Energy (kw) Exergy (kw) able 5 Exergy analysis of the rice husk cogeneration power plant for rice husk match at o 0.5 K, P o 0.5 kpa. Pot ( o C) P (MPa) Flow rate (kg/s) h (kj/kg) s (kj/kg. o C) Energy (kw) Exergy (kw) he rice husk cogeneration was analyzed at ambient condition of 0 C and 0.0 MPa. he mass flow rate of rice husk fuel of the power plant shown Fig. is 0.87 kg/s, while the mass flow rate of rice husk fuel of the power plant shown Fig. is.405 kg/s. he energy balance both thermal match and rice husk match cogeneration are shown able 6 and able 7.

8 International Journal of Renewable Energy, Vol. 4, No., July able 6. Energy balance of rice husk cogeneration on thermal match. Components Heat loss (kw) Percentage (%) Boiler, Net power put Heat process, Heat process 4, Heat process, Desuperheater Preheat water Pump (make up water ) Pump (water feed) urbe otal 0, able 7. Energy balance of rice husk cogeneration on rice husk match. Components Heat loss (kw) Percentage (%) Boiler Net power put,857.9, Heat process, Heat process Heat process 4, Condenser Desuperheater Preheat water Pump (make up water ) Pump (water feed) urbe otal 5, able 8. Exergy destruction and exergy efficiency of rice husk cogeneration thermal match. Components Exergy destruction (kw) Exergy destruction (%) Exergy efficiency (%) Boiler 7, urbe Heat process Heat process, Heat process Desuperheater Preheat water Pump (make up water ) Pump (water feed) otal / exergy efficiency,

9 International Journal of Renewable Energy, Vol. 4, No., July able 9. Exergy destruction and exergy efficiency of rice husk cogeneration rice husk match. Components Exergy destruction (kw) Exergy destruction (%) Exergy efficiency(% ) Boiler, Heat process Heat process Heat process, Condenser Desuperheater Preheat water Pump (make up) Pump (water feed) urbe otal / Cycle 4, urbe, 5.70 kw Others, 7.6 kw Process heat, kw Boiler Process heat urbe Others Boiler, kw Fig. Exergy diagram of thermal match cogeneration. he exergy losses are also shown Fig. and Fig. 4 of thermal match and rice husk match, respectively. It can be concluded that the process heat consumes most of energy the rice husk cogeneration. It means that the cogeneration needs much thermal heat the processes. Exergy and percentage of exergy destruction along with the exergy efficiencies are summarized ables 8 and 9. For all components presented the power plant of both cases of rice husk cogeneration, it was found that the exergy destruction the boiler is domant over all other irreversibilities the cycle. he exergy destruction boiler is accounted for 7.4% of total system exergy destruction thermal match and 77.90% rice husk match. he exergy destruction the condenser is only 0.86%. he real loss is primarily back to the boiler where entropy was produced. Contrary to the first law analysis, the second law analysis demonstrates that significant improvements exist the boiler system rather than the condenser.

10 International Journal of Renewable Energy, Vol. 4, No., July Condenser, 4.99 kw urbe, kw Process heat, kw Others, 09. kw Boiler Process heat urbe Condenser Others Boiler, kw Fig.4 Exergy diagram of rice husk match cogeneration. he calculated exergy efficiency of power cycle thermal match and rice husk match is.40% and 6.8 % respectively. It is relatively low the case of rice husk match because the exergy loss and the exergy destruction boiler are much more than the thermal match cogeneration. In order to quantify the exergy of the systems, both system and the surroundg must be specified. It is assumed that the environment does not significantly change properties of the process. he dead state is the state of a system which it is equilibrium with its surroundgs. When a system has the same temperature and pressure with the surroundgs, there exists no work potential such stances, as shown able 0. he reference environment state is irrelevant to calculation of a change thermodynamic properties by the first law analysis. However, it is expected that, the second law analysis, the dead state has some effects on the results of exergy. As shown able 0, by varyg the ambient temperatures, it was found that the differences ambient temperatures have little effect on the exergy efficiencies of major components the rice husk cogeneration for the rice husk match case. able 0 Exergy efficiency of major components at different ambient temperature. Components 6 o C 8 o C 0 o C o C 4 o C Boiler urbe Condenser Economic Evaluation he two cases of rice husk cogeneration are compared on the economic costs and the revenues from electricity sold to the grid based on 80-day plant operation (able ). It shows that the rice husk match cogeneration has higher electricity generation capacity of,4.4 kw and more total revenues with around 0.0 million US$/year, but lower payback period of.7 years.

11 International Journal of Renewable Energy, Vol. 4, No., July able Summary of economic evaluation of rice husk cogeneration. Items hermal match Rice husk match Plant capacity 94.5 kw,4.4 kw Capital vestment cost.567 million Baht 9.7 million Baht Variable cost Rice husk 0.6 million Baht.980 million Baht Matenance cost.678 million Baht.966 million Baht Man power million Baht million Baht Water cost and other 0.5 million Baht 0.65 million Baht otal cost (A).89 million Baht 4.85 million Baht Electricity sold and avoided 8.60 million Baht million Baht From sellg ash 0.40 million Baht million Baht otal revenue (B) million Baht 5.46 million Baht Net present value (B-A) 6.76 million Baht million Baht Payback period 5.5 years.7 years % IRR 7.% 6.7% (Exchange rate: US$ Baht 5) 7. Conclusion In this study, energy and exergy analyses as well as the effects of varyg the reference ambient temperatures on the exergy analysis of an actual rice husk cogeneration are presented. In the considered power plant, exergy analysis of the rice husk cogeneration power plant shows that the exergy destruction boiler is accounted for 7.4% of total system exergy destruction thermal match and 77.90% rice husk match. he exergy losses turbe of both cases are around %. It can be concluded that the boiler is the major source of irreversibility the rice husk cogeneration power plant. In thermal match case, the calculated exergy efficiency of cogeneration is.40% while the rice husk match case, the exergy efficiency is 6.8 %. Both are acceptable because this rice husk cogeneration plant not only generates electricity but also requires so much energy its thermal processes, and the calculation of put exergy efficiency is based on both electricity and heat process put. In case of rice husk match, the exergy efficiency is relatively low because of the exergy destruction boiler is much more than the thermal match case and it gives the guidele for engeers on process improvement. It also means that its power to heat ratio is too low. Although the exergy efficiency of each component the system changes with ambient temperature, the changes are found to be significant this study. Fally, considerg both cases of rice husk cogeneration based on economic evaluation, it can be concluded that the rice husk match is more economically feasible and is more profitable than the thermal match because the rice husk match cogeneration can generate power of,4.4 kw e, or it can generate electricity around 6,87,968 kwh/year, and it has NPV of 0.0 million US$/year, PBP of.7 years and IRR of around 7%. 8. Acknowledgement he authors would like to thank the Jot Graduate School of Energy and Environment (JGSEE), Kg Mongkut s University of echnology honburi (KMU) for fancial support on the study and the Sirdhorn International Institute of echnology (SII), hammasat University for supportg the facilities and utilities durg the study.

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