Energy Generation by a Renewable Source Sewage Biogas

Transcription

1 Energy Generation by a Renewable Source Sewage Biogas Coelho, Suani Teixeira 1 ; suani@iee.usp.br Velázquez, Sílvia Maria Stortini González 1 ; sgvelaz@iee.usp.br Silva, Orlando Cristiano 1 ; gbntumbo@iee.usp.br Osvaldo Stella Martins 1 ; e mail: omartins@iee.usp.br Pecora, Vanessa 1 ; vpecora@iee.usp.br Abreu, Fernando Castro 1. fcabreu@iee.usp.br 1 USP Sao Paulo s University; IEE/CENBIO Brazilian National Biomass Reference Center; Av. Prof. Luciano Gualberto, 1289 CEP São Paulo SP Brazil; Phone: / Fax: Abstract This article will present two projects related to energy generation using sewage biogas as fuel and different engines, one is a Otto cycle engine and the other is a microturbine. The first one had as project proposal the sewage biogas use as fuel in an 18 kva Otto cycle generator, to produce electric energy, which development was a commitment of Brazilian Reference Center on Biomass (CENBIO). It was, in fact, one between other projects developed by a Sao Paulo University Program named Program of Rational Energy Use and Alternative Sources (PUREFA), whose financial backer was the Financier of Studies and Projects (FINEP), and aims to increase the renewable energy participation in the University s energetic matrix, as well as it allows new perspectives to renewable energy employment in Brazil. The second one is the ENERG-BIOG Project Installation and Tests of an Electric Energy Generation Demonstration Unit from Biogas Sewage Treatment, that uses biogas from sewage treatment for electricity generation with 30 kw (ISO) microturbine, at SABESP (Basic Sanitation Company of Sao Paulo State), located in Barueri, Brazil. This project, pioneer in Latin America, was accomplished together with BUN Biomass Users Network of Brazil (proponent), in association with CENBIO (executer), with patronage of FINEP / CT- ENERG (financial backer), by means of CONVENTION No: This paper describes the proposed systems to convert biogas in electricity and heat using those engines. Keywords: biomass, biodigestor, purification system, electric energy generation.

2 1. Introduction The biogas results from organic material anaerobic fermentation, which occurs inside the UASB biodigestor, and its chemical composition, that depends on several parameters, such as the biodigestor employed, the kind of organic material and the constancy of the feeding process of the biodigestor. The most important biogas components are methane (CH 4 ), carbon dioxide (CO 2 ) and sulfuric components (H 2 S). The biogas composition is an essential parameter, because it allows identifying the appropriate purification system, which aims to remove sulfuric gases and decrease the water volume, contributing to improve the combustion fuel conditions. Other data obtained from biogas analysis is referent to the low heat value, that combined to the efficiency and biogas consumption is important to estimate the electric generation potential. However, the biogas production is much variable because it depends on several parameters, such as the kind of organic material. The biogas production involves three steps: fermentation, which includes hydrolysis and acid genesis, acetone genesis e methane genesis. In the fermentation process, during the hydrolysis the organic material is converted into smaller molecules and this material is transformed in soluble acids by acidogenese process. After that it is initiated the acetanogenese process, transforming the products obtained in the first step in acetic acid, hydrogen and carbon dioxide. The last step is referent to metanogenese process, trough anaerobic bacteria action, producing methane gas. 2. The Most Used Technologies for Biogas Conversion There are different kinds of technology to convert the chemical energy in the biogas into electricity. In biogas conversion the chemical energy on the molecules is converted to mechanical energy in a controlled combustion system, then, this mechanical energy activates a generator producing electrical power. The gas turbines and the internal combustion engines are the most common technologies used to this kind of energy conversion. Figure 1 illustrates these technologies. Some characteristics of these technologies are show in Table 1. Figure 1 Engines, gas turbines and microturbines technologies, commercially available (CENBIO, 2002) Table 1 Commercial Technologies Characteristics (CENBIO, 2003) Power Efficiency Nox Emissions Gas Engines (Cycle Otto) 30 kw 20 MW 30 % - 40 % 250 ppm 3,000 ppm Gas Turbines 500 kw 150 MW 20 % - 30 % 35 ppm 50 ppm Microturbines (Small Scale) 30 kw 100 kw 24 % - 28 % < 9 ppm Even so, in general, engines are more efficient turbines may be more efficient when operating in a cogeneration cycle producing heat and electricity (COSTA et al., 2001). 3. PUREFA Case Study The PUREFA (Program of Rational Energy Use and Alternative Sources), is compound by 14 purposes. This project had three main objectives: to implement measures of management and

3 action of energy efficiency, to increase the distributed generation in the USP from renewable resources and non conventional energy and to introduce incentive permanent politics to the efficient and rational use of energy. In this context, the Brazilian Reference Center on Biomass was responsible for two purposes, related to the biogas use for electricity generation. The first, purpose 11, had main objective to implant the generation system, to capture and to stock the biogas, produced by biodigestor in the Technological Hydraulically Center (CTH USP). The biodigestor is a UASB (Upflow Anaerobic Sludge Blanket), whose biogas outflow produce is near 4 m 3 /day, operates 24 hours per day and utilize the sewer from the residential buildings located in Sao Paulo University, inside the campus. For the biogas utilization, was made its outflow, chemical composition and heat value identifications, parameters that allowed to determinate the real potential for generation and to show the necessity of the previous treatment, as H 2 S removal. Finished this stage, started the purpose 12, regarding biogas used as fuel for electricity generation using a generator group Otto cycle. Nowadays this project is a demonstrative project Analysis and purification of the Biogas The presence of non-burnable substances in the biogas, like water and carbon dioxide, reduces the conversion efficiency. Incomplete combustion can occur, causing power reduction and corrosion, due to H 2 S presence. Most anaerobic digesters produce biogas with 0.3 to 2% H 2 S and significant amounts of nitrogen and hydrogen. The biogas purification system, which aims to remove sulfidric components and humidity from biogas, occurs in tree steps. In the first step most of the biogas humidity is removed, by a recipient where the water is condensed. After that, on the second step, biogas is directed to purification system, composed by two molecular screens, removing the remainder humidity and the sulfídrico components (H 2 S). In the third step, biogas passes trough an amount of iron chip, removing the H 2 S residual. To prove the purification system efficiency and to determine the biogas chemical composition, two analysis were required, one before the purification process and other after it. These results are presented on table 3. A biogas mass flow measurer was installed, allowing the electric energy potential determination, achieving values between 4 to 7 m 3 /day. Due to the low biogas mass flow values, it must be stored before its use as fuel to produce electric energy. Thus this project includes a gasholder, able to store 10 m 3 of biogas, made by PVC. Its dimensions are: external diameter 2 m, length 3,2 m and thickness of 2 mm Electric Generation Engine To procedeed the calculation above is necessary to admit the efficiency, which depends on the technology used in the biogas conversion, basically including three different technologies: gas turbines, microturbines and Otto Cycle engines. In this project, due to the low mass flow of biogas produced, Otto Cycle engine has been pointed as the appropriated technology. The Otto engine selected presents 18 kw of installed power and its biogas demand is around 5,6 m 3 / hour. After the biogas has passed in through the tubing, the water accumulator, absorbent material, the molecular screen and the iron chips, it is ready to be stored in a gasholder (flexible tank made with PVC) and be employed in Otto engine as fuel to produce electricity.

4 3.6. Results and Discussions The table 3 present the results 2 of the biogas 4 composition 1 before and after the purification 7 system, respectively. Table 3 - Biogas Composition Before and After the Purification Process Chemical Components Before After O 2 (Oxygen) 1,23% 0,89% N 2 (Nitrogen) 15,50% 13,20% CO 2 (Dioxide Carbon) 4,75% 4,07% CH 4 (Methane) 75,80% 80,80% H 2 S (Sulfate Hydrogen) 649 ppm < 1,0 ppm H 2 O (water) 2,62% 0,98% According to that analysis occurred a significant reduction of H 2 S and water in biogas composition, achieving safe values to engine operational conditions, allowing the continuity of this project. The civil construction, where the biogas engine and gasholder would be installed, could be done. Having all equipments installed and the gasholder full of biogas, the engine was started. The electric demand was simulated by a test panel. The demand was 2,4 kw. It is in this scene that the analyses of the exhaustion gases had been made and its results are presented in the table 4. Table 4 - Exhaustion gases Analyses Component 10:38 h 11:00 h 11:10 h 11:20 h Total hydro-carbons as CH 4 3,1% 0,67% 2,7% 0,19% Carbon monoxide (CO) ppm Carbon dioxide (CO 2 ) 8,8% * 8,4% * Nitrogen oxides (NOx) 5 ppm 15 ppm 48 ppm 65 ppm Oxygen (O 2 ) 3,7% 5,4% 5,7% * Sulphur dioxide (SO 2 ) < 1 ppm < 1 ppm <1 ppm <1 ppm At first, the engine functioned pretty well, but when the pressure inside the gasholder goes down, the engine starts to fail. Therefore we had to put some weights on top of the gasholder to increase its pressure, and so, the problem was solved PUREFA s Conclusion Despite the fact that this project has been developed experimentally in Sao Paulo University s campus, in urban area, one of the expectations is that the results obtained provide information about biodigestor s operational conditions. This allows defining appropriate areas where this project could be applicable. Especially in rural areas, the use of swage as fuel to produce electric energy is able to contribute with electrification programs already structured in Brazil, increasing decentralized electric generation, what creates important benefits to the country. The most important environmental contribution associated to this project is the mitigation of greenhouse gases (GHG) emissions, especially verified trough methane conversion in carbon dioxide, which presents a dangerous level around twenty five times lower than methane.

5 4. ENERG-BIOG Case Study This study was held at SABESP (Basic Sanitation Company of São Paulo State), located in Barueri, Brazil. This plant operates with anaerobic digestion process (using UASB digestor), which has as mainly products biogas (composed mainly by methane) and sludge. The ENERG-BIOG Project aims to analyze the use of sewer biogas to electricity production in Brazil. The study in being held in a sewer treatment plant located in Barueri, State of São Paulo. This plant operates with anaerobic digestion process, which has as mainly products biogas (composed mainly by methane) and sludge. The main advantage of using anaerobic digestion process is that the sludge treatment process is followed by energy production as biogas. Currently, part of the methane produced is burnt in a boiler used to increase digestors temperature and so, the process efficiency. The methane remnant is burnt in flare to reduce the impacts caused by gases emissions. An alternative to burn it in flare is the biogas conversion into electricity through engines or microturbines. This paper describes the proposed system to convert biogas in electricity and heat using microturbine. The first survey indicated an average production of 24,000 m 3 (secondary treatment) per day of biogas (reaching 28,000 m 3 /day in some periods), with a LHV (lower heat value) of 5,300 kcal/nm 3 (22.2 MJ/Nm 3 ), whose composition (%) are presented in Table 5 and other biogas characteristics are presented in Table 6. Table 5 Biomass measure composition in % of SABESP STS at Barueri (CENBIO, 2003) Gas mixture measure Composition Methane (CH 4 ) 66.5% Carbon Dioxide (CO 2 ) 30.5% Oxygen (O 2 ) + Nitrogen (N 2 ) 0.5% Humidity (H 2 O) 2.5% Table 6 Others Characteristics (CENBIO, 2003 and SABESP,2001) Others Characteristics 4.1. Biogas Cleaning Sulfuric Acid (H 2 S) 134 ppm or 0.01% LHV 5,300 Kcal/m 3 or 22,195 kj/m 3 Relative Density 0.86 a 15ºC kpa Pressure 250 mm c.a. (Gas tank measure) Produced Volume 24,000 m 3 /dia (approximately) As in the PUREFA project, the biogas generated in SABESP s, sewage treatment station in Barueri contains impurities that can compromise the operation of the installation, damaging the cleaning system, the compression system and the electric energy generation system (microturbine). The humidity can compromise the operation of microturbine s internal parts (injector, combustion chamber, turbine rotor), besides reducing the biogas heating value; the H 2 S can damage drier s internal parts, as well as the compressor and the microturbine, because H 2 S is corrodible; air presence into the pipeline: reduces the biogas heating value; and the CO 2, that is a inert gas that also reduces the biogas heating value; however, the microturbine was projected to operate with CO 2 levels between 30% and 50%. So, the withdrawal of this element did not become necessary. For the humidity withdrawal present on the biogas, coalescent filters were used on the line and two refrigerated driers, one before and another after the compressor. To deal with the H 2 S

6 gas removal, a carbon filter was used, operating by absorption principle. For the H 2 S in water solution were used refrigeration drier and coalescent filters. The purification system used in this project, also the first one in Latin America, was designed to guarantee that the biogas characteristics would accomplish the microturbine s specifications, what happened indeed. The gas analysis results shows that the gas cleaning system used fulfills the turbine requirements. The microturbine consumes an average of 20 m 3 /h or 480 m 3 /day ENERG-BIOG s Conclusion The microtubine electricity cost is higher then the electricity produced in conventional generators the emissions, mainly NOx, are significantly lower. Table 7 Comparison between installations costs relations for both technologies (Capstone Microturbine and Trigas Generation Group) (CENBIO, 2004) Relation between Capstone Microturbine Trigas Generation Group Initial investment and installed power R 1 = 2.195,28 US$/kW R 2 = 358,69 US$/kW Initial investment and liquid installed power R 1 = 3.377,36 US$/kW R 2 = 430,43 US$/kW Operation and maintenance costs by the electricity production R 3 = 0,0989 US$/kWh R 4 = 0,0148 US$/kWh Total costs by the electric energy production R 5 = 0,2045 US$/kWh R 6 = 0,1224 US$/kWh The exhaustion gases analysis showed NOx emissions of less then 1 ppm (parts per million). Then the large advantage of using this technology is directly tied with the environmental benefits, when these emissions are compared with the internal combustion engines ones, approximately 3,000 ppm NOx. It is necessary to consider in this scenario the potential of emissions reductions and the carbon credits in a Kyoto Protocol CDM (Clean Development Mechanism) project where each kwh produced using biogas, in Brazilian conditions, avoids emissions of 0,5 tc. 5. Conclusions The energetic use of biogas causes different environmental and economical impacts depending on witch system is used. The electricity generation using biogas in landfills fulfills the electricity requirements of the plant and a surplus of energy can be delivered to the grid. In the agricultural sector the biogas produced, mainly in anaerobic digestors feed by manure residues, can provide energy surplus to, depending one the number of animals and the technology used to treat their residues. In sewage treatment plants the biogas use to electricity production allows a reduction of 20% in electricity consumption. This relation between the electricity production and consumption don t change due the size of the facilities. 6. References CENBIO. Nota Técnica VII - Geração de Energia a Partir do Biogás Gerado por Resíduos Urbanos e Rurais, São Paulo, CENBIO. Relatórios de Atividades Projeto ENERG-BIOG, São Paulo, COSTA et al. Produção de Energia Elétrica a partir de Resíduos Sólidos Urbanos, Trabalho de Graduação Interdisciplinar/FAAP, São Paulo, SABESP. Companhia e Saneamento Básico do Estado de São Paulo, 2001.