Released under the GIC Framework
To identify the appropriate technologies for building a biogas plant to process cow manure and water hyacinth into biogas to feed a generator set capable of producing 500kw of continuous power to feed the grid and fertilizer for sale to farmers. Cogeneration will be employed to provide process heat to assist the digester process and surplus heat will be used to drive an absorption refrigeration unit to cool a cold storage warehouse for potatoes (and possibly carrots). An adjunct use of the refrigeration process could be to process milk products using milk being collected from the cows that will supply the manure.
The outstanding question is what digester technology to use. Dry fermentation would facilitate the production of dry fertilizer. Liquid fertilizer is more difficult to transport and not in popular demand. However, dry fermentation is less well understood and if not properly managed can result in lower yields of biogas.
The building of thousands of digesters for the processing of manure and bio-mass has been carried out in Bangladesh over recent decades. However most of these plants have been very small and are used to produce cooking fuel and in some cases to power domestic lighting. Many of these plants have lapsed into disuse either from a lack of substrate material or a lack of maintenance. Very few plants have been built for producing electricity and none on a large scale. We propose here to build a larger scale digester to produce fertilizer, and biogas to power a cogeneration plant to produce modest electric generation to feed the grid along with waste heat to drive a cold storage plant.
The proposed project location is in Dimla, Nilphamari District, Bangladesh. This site was chosen for the pilot due to the rapid expansion of the cattle population in recent years due to an income generation project of the Mennonite Central Committee Bangladesh (MCCB) in the Khoga Kharibari Union, north and northeast of Dimla. There is also a significant supply of water hyacinth vines on a water reservoir to the south of Dimla ranging in size from 350 to 1200 acres during the year. Poultry farms may be revived in the area in future to provide an additional source of substrate in future. Finally the location was influenced by the proximity to an REB substation on the grid periphery. (Power supplied to the peripheral areas has more value to the grid in terms of power quality and voltage regulation.)
The final size of the project has yet to be determined. We project the MCCB project population of cattle to be 3,000 and growing in the Khoga Kharibari Union. We assume that there are an additional 1,500 head of cattle in the Union. Surrounding Unions are estimated to have additional cattle, perhaps another 6,000 within easy reach. The plan is to buy and gather manure from farmers once a week by vangari to central points for transport by tractor to the plant. It is this collection system which constitutes the greatest untested methodology in this project proposal. Each cow will produce an estimated 10kg of manure per day (a large mature animal can produce 30kg per day or more).
We plan to supplement this manure with water hyacinth, gathered, chopped, crushed and aged appropriately. The optimum ratio of bio-mass to manure has yet to be determined but we are assuming a 50/50 ratio at the outset. We have then the potential for a 500kw (.5 Mw) continuous power plant if we assume 4,000 head of cattle and 40 tons/day of bio-mass for a total of 80 tons/day of substrate. Final sizing will be determined based on many factors including hauling distances for manure collection, optimum plant size of the digester, waste heat requirements for fertilizer production and cold storage unit(s), fertilizer marketing considerations, etc. It is assumed that the grid can absorb up to .5 Mw through 11kv lines in the area (there is a 33kv line feeding the REB substation in Dimla).
Revenue from the project will come then from four sources: fertilizer, electricity, cold storage and carbon credits. Cropping could provide a fifth revenue stream with the focus being on crops that could benefit from cold storage, namely carrots and onions. It is assumed that the government will approve a tariff for power supplied to the grid of $ .20/kwh, based on the very high cost of producing supplemental power from diesel fuel, and its commitment to alternative energy sources. A sixth income stream could result from milk processing using the refrigeration capacity.
The widely accepted method for producing biogas is based on wet slurry. This allows for stirring and a high level of capture of all methane available to the system. Although the slurry can be pressed to harvest a solid form of compost which needs less drying, there is inevitably a liquid output which is rich in nutrients but requires a different form of distribution than bagged compost. Bangladesh does not have a tradition of using liquid spray manure, despite the advantages of such application in certain situations. Our goal is to use vermi worm culture to further process the compost from the digester to produce a portable bagged fertilizer. The demand for such fertilizer is quite high.
This brings up the question of using so called dry fermentation techniques where the substrate is layered into a digester, often using front end loaders, and sprayed with water that is recirculated through the pile on a continuous basis. There are some issues around this method. First, it is not always possible to achieve even flow of re-circulated water through the pile resulting in less gas capture. Second, much of the material (50%) needs to be put through the system again to improve the gas capture. Third the end product is not truly dry, sometimes creating a mushy mess if not managed precisely. Fourth, this method does not benefit from the many years of experience accumulated with the wet slurry method.
The choice between wet and dry methods will drive many considerations in the project. For example if wet is chosen this will drive either a marketing effort to persuade farmers to give spaying a try (a high risk proposition) or an effort to develop a project component to utilize the nutrient rich effluent, such as growing carrots, a crop not generally found in the immediate area. Carrot production could fit in nicely with the cold storage component of the project, given the high seasonal variation in carrot market prices (up to 1000%).
Technology partners are needed. Beckon in Germany (dry) does not feel ready to tackle a project in Bangladesh. Bioferm of Germany will implement wet or dry systems and seem to be equally comfortable with either system. There is a trench dry method being promoted in Germany by a company Chiemgauer Biogas Anlagen. TES of Pune, India provide a connection to a German biogas company that works with both dry and wet fermentation.