Performance Evaluation of Novel Low Cost Biogas System for Rural Application

Sethumadhavan Pa*, Arul Mozhi Selvan Vb

a Research Scholar, Department of Mechanical Engineering, National Institute of Technology,Tiruchirappalli, India b Assistant Professor , Department of Mechanical Engineering, National Institute of Technology,Tiruchirappalli, India

*Corresponding Author. Tel:+91 (431) 2503417, Fax: +91 (431) 2500133, E-mail:qualitysethu@gmail.com

Abstract

Biogas is a renewable and economical source of energy for the rural community. It addresses the twin problems of energy and climate change. Deployment of biogas systems for combined heat and power generation for rural community not only helps to avoid the fossil fuel emissions, but also provides employment opportunities. Agriculture dominating developing countries can develop biogas systems that utilize waste from agricultural processes. The result of comparison of performance and economic viability of a novel low cost biogas system applicable for the rural community has been presented in this paper. The result shows the possibility of easy acceptance by the rural community.

Keywords: Biogas, Cooking, Low cost digester, Renewable energy, Rural development

1. Introduction

Rural households in most of the developing countries use about 90% of their total energy consumption for meeting their cooking needs[1]. Commonly used fuel in rural community is biomass based materials such as firewood, charcoal, agricultural wastes and animal wastes. Among these fuels, firewood occupies more than 34% of the total fuel consumption by the rural domestic sector and has been a dominant energy source. It contributes to the problems of forest depletion, global warming and climate change. Oflate there is energy crisis due to shortage of firewood for cooking purposes .This shortage forces rural women to travel longer distances everyday to collect firewood. Burning wood for cooking purpose is not only inefficient but also harmful to the rural women and children as they are exposed to the firewood smoke for prolonged hours during cooking [2]. It has been reported that smoke can lead to birth defects and even cancer. Hence availability of clean form of energy for the rural community has become an important issue. Level of economic development of a country can be evaluated by the amount of energy usage by a rural community. Only with adequate energy supply, people can fulfill their basic needs like cooking, lighting up their homes, communication, medical attention and preserving the essential commodities. Energy availability has become the most important differentiating factor between the living styles of Urban and Rural communities. Governments make policies and takes steps to improve energy supply to the rural community. Even today, India’s 80% of energy needs are fulfilled only by fossil fuels [3]. Fossil fuels are non-renewable sources with limited availability which can last only for next 100 or 150 years. Even in developed countries fossil fuel dominates the energy supply with contribution from renewable energy sources as low as only 11 % of its total demand [4]. It has been estimated that atleast 620 kcal per capita per day of useful energy is required for cooking in India[2]. In the rural areas people are forced to consume the available domestic energy sources like biomass, dried animal wastes, dried agro wastes in the absence of clean energy source at affordable cost. With the commercial availability of renewable energy technologies like Solar photovoltaic power plants, Small wind turbine generators, biomass gasification systems, biogas plants, the possibility of generating and providing clean energy to the rural community at their own place is becoming possible. Solar energy is a promising solution to rural community areas with bright sunshine, but for using it as cooking fuel, the following obstacles are faced. a) Solar thermal energy for cooking purpose is deployed through solar parabolic cookers which utilizes solar beam radiation. But beam radiation is available only between 9 am and 3 pm even in tropical region. This limits the cooking time. b) For using the solar thermal device like solar cooker, user has to take the cooking vessel to open space with beam radiation. c) The user needs to attend to the cooker at regular intervals to focus the parabolic collector towards the moving sun. These aspects slow down the deployment of solar cookers at rural level. Wind energy systems and Solar Photovoltaic systems can generate electricity, which is a clean form of energy, but the initial cost of the systems and converting the electrical energy to heat energy for cooking purpose make these systems economically unviable for the rural cooking application. Biogas is a promising renewable energy source for supplying clean energy to the rural community. The aim of the present work is to design a small biogas digester, which can be fabricated at an affordable cost by the rural community itself and to study its performance characteristics for successful deployment.

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2. Biogas systems as renewable and rural energy source

Biogas is a renewable energy source as the feedstock used is replenished every year and their availability is sustainable. Biogas is generated through anaerobic digestion of different biodegradable material such as animal dung, sewage, industrial effluents, municipal waste and kitchen waste. Anaerobic digestion is a natural process which does not affect the ecological balance of the environment. Biogas can be used as gaseous fuel in specially designed stoves for fulfilling cooking needs. It has been reported that the energy consumption is least in biogas cooking with specific fuel consumption of 3185 KJ when compared with LPG where the value is 10070 KJ and firewood where the value is 216000 kJ for cooking same quantity of food [1]. It is also noted from literature that there is no significant difference in nutrient and carbohydrate content in the food cooked with biogas, LPG and firewood [1]. The only disadvantage that can be seen from the literature for using biogas as a cooking fuel is the increase in time taken for cooking, which can be attributed to the presence of CO 2

and H2S in biogas along with Methane ( CH4). This can be overcome by refining the biogas before usage [1]. Once the biogas produced is refined, the same can be used effectively as Dual fuel along with diesel or petrol for electricity generation in Internal Combustion engines. It is estimated that, to achieve a good standard of living, developing countries like India needs to plan for per capita electrical energy consumption of 1,840 kWh/Yr [3]. The present level of per capita consumption of electricity in India is not even 23% of the world average [5]. Thus to meet higher GDP growth rate targets of India, decentralized renewable energy generation systems like biogas system could be a possible solution [6]. India has demonstrated its commitment to adopting renewable energy by according ministerial status to renewable energy at federal level. An economic evaluation of three major renewable sources that can be applied for rural community is presented in Table 1.

Table. 1. Economic evaluation of Renewable energy systems (RE systems)[7][8][9][10][11][12]

RE system Specification

Avg Electricity generated/year

(kWhr/Year)

Capital cost in Indian Rupee

(Lakhs of Rs.)

Expected life of the plant

(years)

Initial cost of the plant per unit of electricity generated

(Rupees)

SPV with battery 1 kW 1500 1.40 25* 93

Wind 1.2 kW 1200 1.80 20 150

FRP Biogas plant 5 m3 1800 1.05 20 58

* only for SPV panel. Batteries are to be replaced once in four to five years adding to capital cost.

It is observed from the above table that biogas system has the lowest financial input per kWh output. The only significant operation cost of biogas plant is the cost of feedstock. This can be overcome by utilizing the agricultural and domestic organic wastes as feedstock material, thus making biogas system easily adoptable by the rural community. Considering the above points, it is observed that biogas system has the potential to be used as cooking gas which can replace the dry biomass fuel thereby reducing the indoor air pollution and making the rural kitchen hygienic. The performance of a novel low cost biogas system installed on experimental basis is presented in this paper.

3. Material and Methods

3.1. Biogas Plant

Biogas is a combustible gas produced by the biological process of degrading organic materials [7]. India has pioneered in installing biogas system with rich experience gained over the last 100 to 150 years of its usage. India is credited for the contribution it has made in developing the floating drum model. Most small type biogas plants working on waste to energy principle are based on the floating drum design. The floating drum adds to significant cost as it requires mechanism to keep its movement without gas leakage. In addition, the open fluid to keep the drum moving contributes to breeding of mosquitoes which can spread diseases like Dengue. Hence, the experimental biogas plant has been made as a fixed drum design to evaluate its performance to digest the food and kitchen waste generated in rural area. The photographic view of the biogas digester is shown in the Fig. 1. and the specification of the same is shown in the Table 2.The advantage of this digester is the availability of the cement material at rural level itself and the design of the outlet pipe which can expel the digestate without losing any gas. The pipelines are also designed with plumbing pipes which are easily available at rural area at affordable cost. Altogether the overall cost of the biogas plant which can deliver 1 m 3 biogas per day which can meet the needs of a rural family works out to only Rs 4500/-. This is a significant contribution to the society as the cost of the plant compared to existing commercially available plants is lesser by more than 50%. The stove used has been specially designed for biogas systems. The top cover is also made using cement slab with provision for gas outlet valve thereby arresting the chance of the fluid in contact with ambient air and eliminating the chance of breeding of mosquitoes. These

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features make the system novel in achieving the objective of providing clean energy at affordable cost.

Fig. 1. Constructional features of Novel Low cost Biogas Digester

Table. 2. Specification of Low Cost Biogas Digester

Parameter Size Unit

Material of construction Cement concrete

Diameter of Digester 1000 mm

Volume of Gas Holder 0.5 m3

Volume of Digester 722 Litres

Height of Digester 1.22 m

3.2. Method of biogas digester start up and performance measurement

The experimental setup can utilize 2 kg of organic waste generated in rural household. For initial startup, 20 bags of cowdung weighing about 450 kg were obtained from nearby Ghosala housing cows. This quantity of waste was fed into the empty digester with addition of water to dilute the cowdung to liquid stage. The entire quantity is fed to the level of 3/4 th of the digester height. The digester was kept open for one day and on the next day the gas holder was placed on the digester and sealed. The valve on the top of the gas holder drum was opened to release out the air inside. On the next day the gas started coming out through the valve. This gave a confirmation that gas has started generating. The gas was let out for few more days till the rotten small indicating methane generation has been felt. To confirm the availability of methane, a tube was connected to the gas holder valve and a biogas stove was used to light up the gas. The gas flowing out of the gas holder started burning giving slight bluish color indicating the generation of biogas with methane content. From that day a quantity of 2 kg of organic kitchen waste was diluted with water and fed into the inlet port for continuous biogas generation. The biogas generated was used for boiling of 5 liter of water so that a performance study could be carried out of the gas generated over a period of time. The quantity of waste fed inside the digester, the quantum of gas formed every day and the time taken to boil 5 liters of water on the biogas stove has been recorded.

3.3. Evaluation of Biogas for cooking purpose

Percent heat utilization (PHU) test measures the thermal efficiency of stove [2]. PHU is a measure of useful heat available at the cooking pot to the heat available as input from the fuel supplied. This is measured by heating a known quantity of water in the stove with the test fuel. The quantity of fuel supplied was measured before starting the test. A literature survey shows that PHU value for biogas stove based on Khadi and Village Industries Commission (KVIC) has been reported as 45.1% when compared with 14.2 % for firewood stove and 60.2% for LPG stove[2]. The mean specific fuel consumption (SFC) per meal has been reported 0.06 m3 per kg of cooked food for the biogas stove. For comparison of different fuels, SFC has been converted in terns of energy units , into MJ fuel/ kg of cooked food. It is reported that, for biogas stove the SFC value is 1.3 MJ/Kg which is comparable to that of kerosene stoves with 1.2 MJ/Kg and is only slightly lower than LPG with 0.92 MJ/Kg [2]. Thus it is evident that biogas is a best cooking system that can be focused for further research to make it adoptable by the rural community.

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4. Results and Discussion

The performance of the biogas digester show that once the methane formation starts, then it is possible to get cooking quality gas everyday by maintaining the feed rate. The feed rate of 2 kg for this size of plant has shown the formation of gas on the next day. However during the experimentation, the usage was kept only once in a day to analyze the performance. From the results of this experimentation, it is evident that a constant rate of around 0.4 to 0.5 m 3 of gas for cooking purpose can be obtained from this type of biogas plant. The time taken for boiling of water varies from 30 to 45 minutes. The time started decreasing after few days of operation showing improvement in biogas quality after initial startup and there is no much difference in performance after a week of regular operation. The trend of biogas formation has been placed in graph as Fig 2.

Fig. 2. Trend of biogas generation

Fig. 3. Time taken for water boiling

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From the graph at Fig 2, it is evident that the biogas generation depends on the feed rate and the process stabilizes after a period of 10 to 15 days based on the start up conditions. Once the methane formation starts, then feeding at a constant rate with organic content provides gas output. It is also recorded that after 16 days of feeding of kitchen waste, the digested liquid started flowing from the output port provided on the side of the digester. This liquid is a organic fertilizer that can give value to the rural community. The trend of time taken for water boiling, which is an indicator of biogas is placed at Fig 3.

5. Conclusion

It is understood that the acceptance of renewable options depend on the understanding of the process by the community, lower initial cost for establishing the plant and the usage value of the output for their day to day needs in rural areas. From the performance study of this novel experimental biogas digester, it is observed that there is a possibility of using the biogas formed in the morning for cooking purpose. Immediately after usage further quantity of waste can be fed inside the digester so that equal quantity of gas can be produced , which can be used in the evening. This present work will throw light on the rural community to design own biogas plants to collect waste at community level and generate clean burning fuel for their rural household cooking needs thereby converting waste into useful energy to satisfy their day to day needs. It is planned to conduct experiments with refining of biogas before usage in stove and report the specific fuel consumption of the biogas stove so that the improvement in performance with a refining method can be compared with previous works.

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