BIOGAS DEMONSTRATION PLANT AN ANAEROBIC DIGESTION TECHNOLOGY

Transcription

1 BIOGAS DEMONSTRATION PLANT AN ANAEROBIC DIGESTION TECHNOLOGY Budihardjo, PT Perusahaan Gas Negara (Persero), Pangasihan Nainggolan, PT Perusahaan Gas Negara (Persero), Jhan Binsen Purba, PT Perusahaan Gas Negara (Persero), Bukti Tamba, PT Perusahaan Gas Negara (Persero), Adil Abas Reksoatmodjo, PT Perusahaan Gas Negara (Persero), Nursubagjo Prijono, PT Perusahaan Gas Negara (Persero), 1. INTRODUCTION Many cities in Indonesia are facing a steadily growing production of waste, running a short of disposal capacities at the same time. Also the acquirement of new areas for dumping sites are difficult within the close surroundings of the city. At present the major part of the waste is still simply dump at waste disposal sites. The dumping of waste is limited by the capacity of the area used for disposal. The present sites fill up rapidly. New sites will be farther away from the city that increases the transport cost. The disposal site represent a potential danger to the environment. In addition the cities cannot be used for construction even years after waste disposal has stopped, due to the settlement behavior of the material. Furthermore the waste represents a large source of energy that remains unused. Since dumping can not serve as a long term solution, a technology is required to process waste without negative effects on the environment therefore eliminating the need for further dump sites and also utilize the energy of the waste. The continuous growth of the population, as well as the increasing economic activities, causes an enormous demand of energy. The possibilities to use the municipal and industrial waste as sources of renewable energy and to provide a solution to the environmental problem, at the same time, have been investigated in detail. The anaerobic digestion and the fermentation of the organic waste is the best solution to overcome the problem. 2. PROJECT S OBJECTIVE The main objective of this project is to promote the development of renewable energy sources and technology and to establish a basic concept and bench mark for the future biogas commercial plant. 1. The proposed project seeks to demonstrate a proven technology for anaerobic digestion system and to promote the development of renewable energy sources and technology and to provide an integral approach to the solution of environmental pollution problems by waste disposal. 2. To create local capacity for properly managing a treatment of municipal solid waste and draw lessons for replicate elsewhere. 3. To increase community awareness and participation in environmental management at the local government. 4. To support the country s policy in renewable energy program. 3. PLANT DESCRIPTION 3.1 Waste Treatment System The plant has been designed for the processing of municipal waste with a capacity of t/a (30 35 %TS). Most shares of this waste are organic matters suitable for fermentation and producing of biogas. For processing in a digester organic matters are separated from plastics, metals, papers and noxious matters. The GBU separation system is split into 2 subsystems, first being the manual sorting belt for separate large size not fermentable matters and in the second the presorted bio waste will be suspended with suspender system, consisting of a suspension buffer tank and a circulation system. The suspender is equipped with a integrated mixer and a cone bottom for sedimentation of inorganic solids. The circulation system mainly consist of an online cutter und a rotary piston pump is cutting coarse solids, homogenizing bio waste suspension and charging pre-acidification system. The filling of the suspender is controlled by a minimum und maximum level switch and an ultrasonic sensor. For charging the digester also an rotary piston pump is controlled by level sensors in the preacidification tank. 1

2 3.2 Anaerobe System During fermentation the disintegration of the organic substances is performed under exclusion of oxygen inside of a completely capsulated digester. Fermentation requires a liquid media with a drymatter-content from 8-12 %. The process is divided into two principal phases: The 1st phase in the pre-acidification buffer tank is a hydrolisation process where facultative anaerobic bacteria start the hydrolisation and disintegration of the organic substances. Complex organic compounds are split under production of organic acids, CO2, H2S and alcohol. Dissolved oxygen is consumed by the bacteria, nitrates and sulphates are reduced. The ph value is lowered. In the 2nd phase in the digester, methane bacteria digest the products of the metabolism of phase 1. Final products of the process are methane gas, CO2, and mineral salts. Due to the reducing atmosphere inside the fermenter and the production of ammonium out of the anaerobic degradation of proteins, the ph-value is raising continuously. The ph-value of the fermented substrate ranges from 7.5 to 8.5. In contrary to the fast growing aerobic bacteria, methane bacteria are not present in the initial substrate, but have to be bred inside the fermenter. Degradation rates of organic substances are very high in between the temperature ranges where methane bacteria have their maximum activity. Mesophilic methane bacteria have a temperature optimum of about 32 C to 35 C, while thermophilic stems require substrate temperatures of 50 C to 55 C. Beside the temperature, degradation rates depend mainly on digestibility of the organic substances, the specific organic load of the fermenter (kg of organic dry matter per m³ of fermenter volume) and the retention time of the substrate inside the fermenter. Gas yields are directly correlated with the decomposition rates. Each substrate has a specific gas yield expressed in liters of biogas produced per kg of decomposed organic matter. Performance data and energy yields of the biogas system are shown in the following table: Data Bio waste Process Water Digester Input Volume 3,5 t/d 7,0 t/d 7,0 t/d Total Solid (TS) 35%TS 2,0 %TS 12,9 %TS Organic TS (OTS) 50%OTS/TS 45 %OTS/TS 45 %OTS/TS 80% of OTS 80% of OTS Spec. Gas Yield 0,9Nm³/kgOTS 0,85Nm³/kgOTS Digester Volume 152 m³ 152 m³ Amount of Biogas 413 Nm³/d 413 Nm³/d Energy Content kwh/d kwh/d Production of Electricity 680 kwh/d 680 kwh/d Table 1. Performance Data 3.3 Digester The technology of biogas production is a complex one, since biological processes need to be optimized, taking individual structural and hydraulic requirements into account. Perfect thermostatisation, continuous blending, homogenization, reduction and injection of the substrate are all vital preconditions. At the same time the consumption of process energy for heating and especially for the mixing of the substrate has to be minimized. The choice of the appropriate digester type, therefore, is very important for an economic operation of the plant. The fully mixed and fully hydraulic digester type is designed for the treatment of sludge with high dry matter contents (5%-12%). As opposed to the anaerobic filter it features long retention times and high gas yields. The bacteria do not live on carrier material but float freely in the substrate. The sludge form surface scums and sediments resulting in a very inhomogeneous distribution of the organic matter inside the digester. It must be mixed and blended regularly in order to achieve optimum gas production within the total digester volume. The way mixing is performed determines the digesters consumption of electrical process energy and is therefore a crucial point of its operational economy. As the methane bacteria have generation times as long as 30 days, they show little adaptability to varying process conditions, especially to the substrate temperature, organic load and ph-values. These process conditions have to be kept constant all the time. The digester therefore must be completely insulated. A variation of the inside temperature of more than 1 C per day already has negative impacts on gas yields. Another criterion is the hydraulic 2

3 structure of the digester. Simple constructions consist of only one fermentation chamber, which is fully mixed. As a result freshly fed material is blended with already fermented substrate and carried out of the digester. Consequently lower gas yields are obtained and the effluent often strongly smells. The GBU digester meets all these requirements, featuring optimum substrate management and blending without any built-in moving parts. As a unique feature of this digester type no electrical energy is needed for mixing of the substrate, but mixing is done using the pressure of the produced biogas. 4. SYSTEM DESCRIPTION The gas produced causes the pressure in the main fermenting chamber to rise, which in turn leads to a drop in the fluid level combined with a rise in the level in the second fermenting chamber. Once the two chambers have reached a certain predetermined level, a defined volume of fresh material is fed into the outer cylinder. Exactly the same volume of fermented substrate flows over from the inner cylinder into the effluent pipe. After feeding the gas mixing flap opens, causing instantaneous pressure equalization. The returning substrate is guided in such a way that it destroys both surface scum and sediment layers and ensures that the mixture is reliably blended. - The organic load of the digesters is max. 4,0 kg OTS/m³. - The dry matter content of the bio waste suspension is between %. - The retention time is about 20 days. The resulting total digester volume is calculated to be 150 m³. The digester will be constructed in steel sheets with an integrated heat exchanger, a completely insulation by foamglas or polyurethane layers and covered by trapezoidal steel sheets. 3

4 4.1 Gasholder After passing the digester fermented substrate and biogas will be buffered in a combined buffer tank. These are constructed in glass fiber coated steel with integrated pressure less biogas balloon. The PE membrane is fixed at the center of the roof and the bottom edge hanging under the effluent level. Due to this construction following processes are possible: - continued fermentation of substrates and production of biogas - biological desulphurization The gasholder is equipped with gas filter and safety device. The biogas flows pressure less from the gasholder through the filter to the biogas blower. The gas filter fulfils following functions: - Separation of coarse solids - Condensate drain - Overpressure protection for the gasholder: The siphon system which is open to the atmosphere and filled with water. If for any reason the gas pressure exceeds 10 mbar, the water is blown out of the siphon and the gas is released to the environment in order to protect the PE membrane of the gasholder. The biological desulphurization requires oxygen dosing about 3-4 % of biogas flow, realized with an air injection pump into the gasholder. The cleaned gas is transported to the energy station. The following parameters are stirred or controlled: - Gas flow - Content of methane and carbon dioxide in % 4.2 Generator Station The energy station is integrated in the building. The generator room contains the co-generator, the gas blowers, the warm water distribution system and the ventilation system. The ventilation system supplied generator station with fresh air for combustion and for outlet of heat radiation of the engines. The ventilator is controlled by the room temperature about 35 C. 4.3 Co-Generator The co-generator with an electrical maximum output of 4 kw at an electrical efficiency of about 35 % if operated with biogas. The co generator is equipped with a synchronous generator and operates in a net parallel installation. They are capable to supply emergency power by automatically switching to asynchronous mode whenever the net fails without interruption of the power supply of the plant. If the power supply by the public grid is detected, it automatically switches back to the netparallel mode. The co generator is equipped with own control panels providing an interface to the central PLC. They can be regulated continuously from % of their capacity. Their actual output is controlled by the filling level of the gasholder. The cogeneration is started at a filling level of the gasholder of about 25 % and than continuously controlled. If the gasholder reaches 60 % of its capacity the co generators will operate at full power. 4.4 Heating Water Distribution an Cooling System The engine cooling circuit is directly connected of the plate heat exchanger of the module. The cooling water has a temperature of 85 C and the required backflow temperature is 70 C. The cooling water line is connected to the heat exchanger in the digester and the fan coil as security cooling system over a hydraulic distributor and 3 separate pumps for circulation. 4.5 Decantation The effluent flow pressure less to the intake of the decanter which is mounted beyond maximum level of the buffer tank of fermented substrate. The decanter is designed to separate a solid fraction with a dry matter content of 30% and a liquid fraction with max. 3% DM without any additive of polymers or other chemicals. 4.6 Process Water Supply Process water is needed for suspending of bio waste. The decanter flotate will be stored in the process water shaft. An integrated loading pump charge the process water into the suspender before filling pre-sorted bio waste. 4.7 Automatisation The plant is operated under the control of a PLC system. A PC based visualization software allows 4

5 permanent control of all relevant process data as well as remote control and maintenance via the telephone line. The following data will be monitored and stored for further evaluation: Filling level of the pre-acidification tank Temperature of the pre-acidification tank Digester feeding volumes Digester temperature Differential pressure of in between inner and outer cylinder PH-value of the digester effluent Gas production Methane content of the biogas (%) H2S-Content of the biogas Electricity production Engine cooling water temperature Room temperature of the engine room 5. EXPECTED RESULTS The realization of a small scale anaerobic digestion plant of municipal solid waste by means of wet process. The pilot plant will be designed for processing 3.5 ton of organic waste a day. The expectation of 413 Nm 3 biogas will be produced per day. The outcome of biogas pilot plant gives a description of the technical possibilities and economic feasibility of construction of a commercial biogas plant. The proposed biogas plant will be constructed in old disposal dump site TPA Pasir Impun Bandung West Java. There are several reasons that make municipal solid waste (MSW) become interesting object to be used as a raw material for the production of biogas, i.e.: 1. Potentially, the MSW, which exists in every city, has a huge capacity. The MSW will constantly increase day by day. 2. Technologically, the MSW could be processed into biogas, because it contains % organic matters. Its production is absolutely in line with diversification program, and therefore gives maximum added value. 3. Economically, MSW is cheap because has no value on its original forms. It can be accumulated in one location, so this could reduce collection and transportation cost. 4. Socially, the MSW contains inorganic wastes such as papers, glasses, plastics, iron, aluminium, etc. that come from the pollutant and files. This will definitely help the garbage collectors to raise their income after those wastes are collected, sorted and sold. 5. Environmentally, the MSW management could reduce solid, air and water pollution problems as well as diseases originated from the pollutant. 6. Aesthetically, the MSW management makes the city healthy, attractive and more civilized. REFERENCES 1. Goetz GmbH, Perum Gas Negara,PT Bumaka Ripta, GBU mbh (1995), Organic Waste Treatment Plant for Energy Source Alternatives, Final Report, Jakarta, Indonesia. 2. GBU mbh (2001), Proposed Biogas Pilot Plant - Plant Description, PT Perusahaan Gas Negara (Persero), Jakarta, Indonesia. 5

6 Figure 2. Process Flow Diagram 6