Proceedings of the 3rd Applied Science for Technology Innovation, ASTECHNOVA 2014 International Energy Conference Yogyakarta, Indonesia, 13-14 August 2014
Conceptual Design of Centralized Biogas Power Plant (Case Study in Bantul, Yogyakarta, Indonesia) Dintani Y N Na’imah, Rachmawan Budiarto Department of Engineering Physics, Universitas Gadjah Mada Jalan Grafika No. 2. Yogyakarta, Indonesia
[email protected];
[email protected] Andang Widi Harto Department of Engineering Physics, Universitas Gadjah Mada Jalan Grafika No. 2. Yogyakarta, Indonesia
[email protected]
ABSTRACT Since found in 1821, electricity has become one of most important needs to do daily activities. As the increasing of population and economy activities, electricity demand will grow as well. Indonesia is a development country which still depend its electricity supply on oil and gas sources. Since these sources can be diminished in any time, government should more invest on build renewable energy power plant so that the national energy dependence wouldn‟t be at stake. This research is designed approaching to Linko Biogas Plants, Denmark. Parameters which are used to design this plant are feedstock type, hydraulic retention time, and temperature operation of digester. This plant utilizes gas turbine to generate electricity. To get the optimum electricity production, gas turbine pressure ratio is varied, starts from 1.5 until 60.0. Centralized Biogas Power Plant in Bantul will process 25,000 kg of cow manure each day. All of feedstock will be mixed with water inside buffer tank. Then, digestion of this feedstock-water mixture takes place inside two stages digester, both are continuous flow type which is operates in mesophilic temperature (35oC). Total biogas yields from both digesters are 18,270 m3 per day. The raw biogas is being purified before it enters power generation stage. In power generation stage, gas turbine uses air as working fluid. The optimum pressure ratio of air compressor is 10.50, whereas, pressure ratio for biogas compressor is higher by 1.1. Capacity factor of gas turbine is assumed to be 25% so that the electricity generation of centralized biogas power plant is 2.109 GWh/year. KEYWORDS: Conceptual Design, Centralized Biogas Power Plant, Manure
1
INTRODUCTION
Indonesia is a development country. In 2011, Indonesia population reach 237,641,000 people (Ministry of Energy and Mineral Resources, 2011a). With the growth rate at 1.4%, this population will be as much as 273 million in 2020 (Ministry of Energy and Mineral Resources, 2011a). As the population grows, national electricity demands will also increases. Indonesia electricity consumption reach 90.348 billion barrel oil equivalent (BOE) in 2010 or increased by 7.781 billion BOE. By 2010, this rising of electricity consumption is the highest than any previous year. Figure 1.1 shows the electricity consumption from 2005 until 2010. This electricity consumption is predicted to growth 6.6% annually (Ministry of Energy and Mineral Resources, 2011a).
Perusahaan Listrik Negara (PLN), as the only electricity supplier in Indonesia, produces 25,670.51 MW of electricity. More than 30% of this electricity generates from steam power plant. While the other 28% electricity produce from combine cycle power plant. The rest electricity produced from hydro power plant, diesel power plant, geothermal power plant and renewable energy power plant (Ministry of Energy and Mineral Resources, 2011b). As the increasing of electricity generation in Indonesia is unstoppable, the main electricity source which is oil, coal and gas reserves slowly decline. None of oil and gas reserve resources will be last for more than 200 years (Sekretariat Panitia Teknis Sumber Energi.). Thus, PLN cannot only depend on these fossil resources as these resources are non-renewable ones. PLN must be looking for its replacement, such as renewable energy resources. Indonesia is blessed for having plenty of renewable energy resources. That Indonesia is an agriculture country, it has 49.81 GW biomass potential energy. But until 2005, only 0.3 GW has been used to supply the energy needs (Sekretariat Panitia Teknis Sumber Energi). Biomass energy potential will increase as the growth of integrated farming in Indonesia. Bantul was chosen because it has the most biogas technology potential than any area in Yogyakarta. Bantul has 12 potential farming group which spread through 9 subdistrict (Salbuyun, 2011). Pandan Mulyo, Bantul‟s biggest farmer group located in Srandakan, has 348 m3 biogas potential per day. Unfortunately, this potential has not been utilized as electricity source. 2
METHODOLOGY
All vessels in this plant are designed as cylinder shape. Vessel is made with 10% oversizing volume for safety reason. Volume for each vessel is determined using Equation 1. (1) Height of digester is determined first according to height of Linko Gas digester. So that, Biogas Plant in Bantul has the same layer as Linko Gas Digester in Denmark. For another vessel, height diameter ratio (Eq 2) is determined first. Thus the vessel sizing can be determined using Equation 3 – Equation 5. (2) (3)
⁄ ( ⁄
⁄
√
)
⁄
(4) (5)
Air, along with biogas, will be burned in combustion chamber. Air is assumed to be ideal gas, so that
̅ (6) Compressor and turbine are work isentropically with specific pressure ratio. Pressure ratio defined as the comparison of fluid out from compressor/turbines with the inlet fluid to compressor/turbines, according to the following equations: (7) (8) : pressure ratio for compressor : pressure ratio for turbine The energy which is used by compressor is defined according to equation below: (9)
where : ideal work of compressor : Specific heat of working fluid (kJ/kmol.K) M : molecular weight of working fluid (kg/kmol) The temperature and pressure inside the compressor and turbine is correlated according to the following equations: ( )
⁄
(10) ⁄
( ) ( )
(
)
⁄
(11)
⁄
(12) ⁄
(
)
⁄
(13)
T1 : temperature of inlet gas of compressor T2‟ : the ideal temperature of gas leaving compressor to regenerator T4 : temperature of gas leaving combustion chamber to turbine T5‟ : the ideal temperature of gas leaving the turbine to high pressure regenerator k : specific heat ratio of the working fluid The difference between ideal and actual work of compressor and turbine are defined as isentropic efficiency of compressor and isentropic efficiency of the turbine . The actual temperature of gas leaving compressor and turbine are: (14) (15) Before entering to gas turbine, the heat of working fluid is added by using heat exchanger or furnace/combustion chamber. Combustion needs three requirement elements. It is fuel, oxygen and trigger. The heat which is added to the working fluid is correlated to the specific heat of working fluid, working fluid molecular weight and the temperature difference before and after combustion. It can be calculated using this following equation: (16) : Ideal heat added to working fluid : Specific heat of working fluid (kJ/kmol.K) M : molecular weight of working fluid (kg/kmol) T3 : temperature of gas leaving low pressure regenerator to furnace T4 : temperature of gas leaving furnace to turbine Due to caloric loss inside the furnace, the actual heat that has to be produce in furnace is: (17) : actual heat that must be added to the working fluid : furnace efficiency Then the exhaust gas from furnace will be expanded inside the turbine. This expansion produces energy whose amount can be calculated using the equation below: (18) Brayton cycle efficiency is defined as: (19)
: total efficiency of gas turbine (%) : mechanic efficiency due to friction on turbine shaft (%) : generator and conversion from mechanic to electricity efficiency (%) : net turbine energy which is produced by expansion inside turbine (kJ/kg) 3
MECHANISM OF COLLECTING FEEDSTOCK
Bantul is located at 110o12‟34” – 110o31‟08” longitude and 7o44„04” – 8o00‟27” latitude. The total area of Bantul is 506.86 km2. Bantul still has a lot of green areas, rice fields and gardens. Therefore, 25.56% people in Bantul work in agriculture sector (Bantul Government, 2013). In 2011, Bantul has 59,819 livestock which is 20.37% from the total livestock in Yogyakarta Province (Salbuyun, 2011). From the total livestock that lived in Bantul, 98% of it is population of cow. The other livestock are dairy cow, buffalo and horse. From 2010 until 2011, this number of livestock had increased 4.24% and this growth will increase annually. Total livestock in Bantul potentially will produce 1,707,588.10 kg manure per day. This manure production will yield 34,420.74 – 66,818.68 m3 of biogas per day. This biogas yield has the same value as 15,833.54 – 30,736.59 kg of Liquid Petroleum Gas (LPG), 17,898.78 – 34,745.94 liters of diesel fuel, 27,536.59 – 53,454.94 liters of gasoline, and 21,340.86 – 41,427.58 liters of kerosene (Salbuyun, 2011). The Centralized Biogas Power Plant is designed to supply electricity to city of Bantul. To minimizing loss at power distribution, three areas are chosen to be alternatives location of Centralized Biogas Power Plant. Feedstock will be collected from area nearby the plant location. Table 1 is shown the alternative locations for biogas feedstock. Table 1 Alternative Location of Manure Collecting Mechanism Alternatives 1
Alternatives 3
Number Number of cows Sub-district of cows 2657 Pleret*) 1 3440 2 2255 Bantul 2657 3 2495 Jetis 2255 4 3295 Imogiri 2495 5 4739 Bantul 2657 6 3716 Kasihan 2985 7 4574 Piyungan 4347 8 2913 Banguntapan 1422 9 Pandak**) 3716 TOTAL 25231 26644 25974 *) Sub-district with mark *) is the location of Centralized Biogas Power Plant Location with mark **) is the farthest sub-district from the plant. Thus manure from this sub**) district will be collected to meet the feedstock requirements (25,000 cows) For alternatives 2 and 3, Biogas Plant will be built in Bantul and Pleret. Sub-district Panjangan*) Srandakan Pandak Bambanglipuro Jetis Bantul Pundong**)
Number of cows 4574 3995 3716 4739 2255 2657 3295
Alternatives 2 Sub-district Bantul*) Jetis Imogiri Pundong Bambanglipuro Pandak Panjangan Sanden**)
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DESIGN OF BIOGAS PRODUCTION PROCESS AND POWER GENERATION
After collected from all nearby places, feedstock enter buffer tank to get conditioning treatment. Inside the buffer tank, feedstock is separated from all impurities, like stone, and mixed with water. Centralized Biogas Power Plant in Bantul will utilize two stage digesters. Both digesters are continuous flow digesters. Digestate from first stage digester will enter the second stage digester to get more biogas production. The other purpose of multiple digestions is to minimize the probability of fresh manure being washed out before producing biogas. Biogas yield from both digesters is cleaned from contamination such as hydrogen sulphide, carbon dioxide and water vapour. Then biogas ready to process in power generation stage. Centralized Biogas Power Plant in Bantul will produce electricity at the same amount in every unit time. It means that the electricity production does not depend on its hourly demand. The electricity installation is on-grid type. If the electricity production from Biogas Power Plant doesn‟t meet the demand, it needs supplementary supply from other sources. Centralized Biogas Power Plant in Bantul utilizes regenerative gas turbine. This gas turbine uses exhaust gas which is mixture of air and biogas from combustion chamber as working fluid. The air is assumed to be gas ideal, so its specific heat stays constant at various temperatures. Besides, the air volume in 1 atm and 308 K is 25.28 m3/kmol. All process is summarized in Figure 1 and Figure 2. 4.1
Biogas Production
Centralized Biogas Power Plant in Bantul is using multiple digesters which can increase biogas production by 5% (Ellegard, 2013a). Feedstock of this plant is manure from cows, in specific, beef cattle that can produce 29 kg of manure per day. Using manure properties from Table 2, total feedstock collected is 725,000 kg/day. The first digestion produces 17,400 m3 of biogas, whereas the second digestion produces 870 m3 of biogas. Kawentar (2010) mentioned that low heating value of biogas is 33,496 kJ/liter. Thus, total of biogas yield in centralized Biogas Plant contains 7,083.01 kJ/s of heating value. Biogas-to-manure ratio of this plant is 8.98. It means 1 kg of manure produce 8.98 m3 of biogas. Table 2 Manure properties Parameter Manure production Total dry matter Volatile slurry Biogas yields from 1st digestion Biogas yields from 2nd digestion 4.2
Amount 29 12% 80% 25%
Unit Kg/day From total manure From total dry matter From total volatile slurry
5% From 1st digestion biogas
Conditioning Stage
Conditioning stage occurs inside buffer tank. Manure will be mix with water with manure-water ratio is 1:1. It means 1 kg of manure needs 1 liter of water (Hanif, 2010). The other purpose of this stage is to stabilize the input flow to digester. Buffer tank has two inlets at top of it, one for manure inlet and another is for water inlet. Manure is pumped into the buffer tank at 2.10 x 10-2 m3/s. Water is delivered into the buffer tank 8.39 x 10-3 m3/s. To avoid agglutination, buffer tank also equipped with two low speed agitators. Manure is set to stay inside the buffer tank for 30 minutes.
The height to diameter ratio of buffer tank is designed to be 1.25. In consequence, buffer tank‟s diameter and height is 5.72 m and 7.16 m respectively.
Figure 1: Biogas Production and Purification Stage
Figure 2: Power Generation Stage
4.3
First and Second Stage Digester
Digester which is used in Centralized Biogas Power Plant in Bantul is continuous flow type. Continuous flow digester is compatible to be used for large-scale biogas plant. That it requires less volume than the batch type one is one of continuous flow digester advantage. Besides, the gas production from continuous flow digester is more stable to be delivered to gas turbine system. Furthermore, operation and maintenance cost of continuous flow digester is less than batch type digester. Centralized Biogas Power Plant in Bantul is designed to have multiple stage digesters. The purpose of multiple stage digesters is to degrade the feedstock again, in case that a few of fresh manure had been washed out. The utilizing of multiple stage digesters will increase biogas production by 5% (Ellegard, 2013b). Centralized biogas power plant in Bantul processes 265,000 ton of manure per year (725,000 kg/day). This number is close with feedstock of Linko Gas plant in Denmark, which is 260,000 ton/year (712,328.77 kg/day). Therefore, Centralized Biogas Power Plant in Bantul adapts Linko Gas biogas plant. Like Linko Gas, Centralized biogas power plant will have digesters whose height is 8.7 meter, with safety margin by 10%. So, the effective volume is 90% from the actual volume. Having huge number of feedstock, Centralized biogas plant in Bantul has 15 first stage digesters and 10 second stage digesters. The first digester retention time is 18 days. Then the manure will be transferred to second stage digester and stay there for 6 days. The height of second stage digester is still the same with the height of first stage digester, which is 8.7 m. Height of first stage digester is 22.26 m, while the diameter of second stage digester is 6.48 m. Both digesters will have continuous low speed top mounted agitator/mixer. Agitator applies in digester to homogenize the substrates. It prevents floating and sinking layers and encourages fast discharge of gas from substrate. Both first and second digesters have three agitators inside the digester. The agitator is submersible top mounted assembly that works at different level of fluid. Agitators must work thoroughly to prevent dead space. The agitator and its assembly can be seen in Figure 3.
Figure 3: Left: slow speed agitator; Right: Agitator assembly in digester (Source: Optimum energy balance thanks to Amaprop submersible mixers. Accessed from http://www.ksb.com/ksbuk/Products_and_Services/waste_water/Biogas/630864/biogas_low-speed_amaprop_art.html, 27 July 2013) 4.4
Purification Stage
From digester, the first impurity to be removed is hydrogen sulphide (H2S). This compound has to be removed to increase low heating value of biogas. In this plant, hydrogen sulphide is removed using adsorbtion technique which utilizing activated carbon as adsorbent. Because activated carbon absorbs best at 50oC, before raw biogas enters adsorption column, it is compressed until its temperature reach 50 oC (Vinci, et al. 2013). The activated carbon is impregnated with potassium iodine (KI) or sulphuric acid (H2SO4) to fasten the reaction inside adsorption tank. The activated carbon needs to be replaced when it is saturated. In this process, H2S is converted to elemental sulphur (S).
After hydrogen sulphide had been removed, biogas is delivered to CO2 absorption column (see Figure 1 and Figure 4 for more details). In this column, CO2 is washed out using water scrubbing technique. Biogas is fed from the bottom of vessel, where it meets a counter flow of water which is fed from the top of vessel. Since carbon dioxide is more soluble than methane, it is absorbed by water. Biogas which exits from the absorption vessel is saturated by water. Hence, it needs to be dried to remove water vapour.
Figure 4: Flowchart of Biogas Purification Stage Not only carbon dioxide that is absorbed by water, but also some methane and hydrogen sulphide. Because of that, water from absorption column is led to flash tank. Water will be heated up until it reach dissolved methane boiling point. The dissolved methane will become vapour and then fed back to the absorption column. While the methane is fed back, water is sent to desorption vessel. Carbon dioxide is separated from water by air scrubbing. As the water is being exposed to atmospheric air, the tendency to approach equilibrium atmospheric partial pressure will transfer oxygen into the water and carbon dioxide out of water (Vinci, et al., 2013)Error! Reference source not found.. The exiting water is then cooled down before the water is returned to absorption vessel. The vent gas from desorption vessel consists of air, carbon dioxide and small amount of methane. From desorption vessel, the vent gas is delivered to gas treatment to be filtered before it is exiled to environment. After purification, biogas only consists of methane (CH4), hydrogen (H2) and a little bit nitrogen (N2). Then, it will be transferred to gas holder. Although biogas impurities had already been removed, methane and hydrogen are assumed to be loss by 2%. 4.5
Power Generation
After filtering unit, biogas will be placed temporary in gas holder. The other benefit of gas holder is to stabilize flow rate to power generation. Centralized biogas power plant in Bantul needs 5 gas holders which each capacity is 676.67 m3. Each gas holder height is 11.04 m and the diameter is 8.83 m. Air compressor ratio is varied from 1.5 until 60. The result is shown in Figure 5. The highest efficiency is reached at 23.46%, when the pressure ratio is 10.50 atm. At that pressure, power that can be generated is 1,425.42 kW.
TOTAL EFFICIENCY 25.00% 20.00% 15.00% 10.00% 5.00% 1.50 5.00 8.50 12.00 15.50 19.00 22.50 26.00 29.50 33.00 36.50 40.00 43.50 47.00 50.50 54.00 57.50
0.00%
Figure 5: Total Efficiency of Power Generation From total power, 462 kW of it is utilized for agitator inside the digester. So the energy that can be transfer to the grid is 963.42 kW. The temperature of flue gas which leaves the system is 556.84 K. Capacity factor of gas turbine is assumed to be 25% (R. Kakka Dewayan, et al., 2014) so that the electricity generation of centralized biogas power plant is 2.109 GWh/year. 5
CONCLUSION 1.
Centralized Biogas Power Plant in Bantul is design to use feedstock from 25,000 cattle manure. The net electricity from Centralized Biogas Power Plant in Bantul is 963.42 kW. 2. Process design of Centralized Biogas Power Plant in Bantul begins with digestion. For 25,000 cow head manure, it takes place in 15 first stage and 10 second stage digesters which both are continuous flow ones. This process produces 18,270 m3 of biogas daily. This raw biogas is filtered using carbon active and water scrubbing technology. Then, it is ready to be used in gas turbine (Brayton Cycle). This plant utilizes regenerative Brayton Cycle. 3. There are 3 alternatives area to build Centralized Biogas Power Plant in Bantul, which are Panjangan, Bantul and Pleret. Feedstock will be gathered from nearby farm. This plant use 12,000 liter vacuum tanker to transport the feedstock to plant area. Centralized Biogas Power Plants can raise energy sovereignty in Indonesia. It also enhances integrated farming in Indonesia. Biogas in general is one of solutions to waste management and energy scarcity in Indonesia. But to build Centralized Biogas Power Plant still needs more research from all education background, such as microbiology, chemical engineering, mechanical engineering, economic, etc. 6
ACKNOWLEDGEMENTS
Lars Ellegard from BWSC Denmark who willing to discuss about process parameter of centralized biogas power plant and centralized digester sizing.
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