Enhancing methane production in a farm-scale biogas production system

Abstract

Biokaasuteknologiaa pidetään maataloudessa lisääntyvässä määrin lupaavana tekniikkana tuottaa uusiutuvaa energiaa ja minimoida päästöjä ympäristöön. M.Sc. Prasad Kaparaju arvioi väitöstyössään menetelmiä, jotka tehostaisivat metaanintuottoa tilakohtaisessa biokaasuntuotantojärjestelmässä.Hyvin suunniteltu agroteollisuuden jätteiden ja sivutuotteiden käyttö lisää biokaasun tuotantoa maatilakohtaisissa biokaasulaitoksissa ja parantaa ravinteiden kierrätettävyyttä. Maatilakohtaisten prosessien metaanintuotannon maksimoimiseksi työ kohdistui erityyppisten jätemateriaalien metaanintuotannon ja prosessivaikutusten tutkimiseen. Lisäksi tutkittiin erialaisia fysikaalisia ja kemiallisia menetelmiä, joilla erityisesti metaanintuottoa voitaisiin lisätä ja nopeuttaa.

This work highlights the application of biogas technology in a farm biogas system to generate renewable energy and to reduce environmental impacts and GHG emissions from high strength organic wastes ranging from agricultural, animal wastes and agro-industrial organic wastes. In addition, the thesis also emphasizes various measures to enhance methane production especially in a farm-scale biogas production system through co-digestion of manure and industrial organic waste/energy crops and recover the residual methane of the already digested manure through post-methanation. It was found that farm manures as such would produce methane, a potential GHG, if not recovered. Farm digesters could be started successfully without inoculation but use of acclimatized inocula would fasten and increased the methane production. Cow manures were found to be more benefited from inoculation than pig or sow manures. This study also revealed that under similar process conditions, pig manures would be better substrates than dairy cow manures. Specific methane yields obtained under semi-continuous digestion were 0.30-0.32 and 0.13-0.16 m3 kg-1 VSadded waste for pig manure and dairy cow manure respectively at 2 kgVS m-3 d-1 loading rate, 30 d hydraulic retention time (HRT) and 6.0% feed VS while in batches yields were 0.14, and 0.36 m3 kg-1 VS for dairy cow and pig and manures respectively. These yields were lower than the theoretical yield of 0.4 m3 kg-1 VS reported for cow manure. Possible co-substrates to enhance the methane production were investigated. Results from batch, semi-continuous and farm-scale studies suggested that energy crops (clover, oats and grass hay) and confectionery by products (e.g. chocolate) could be considered as potential co-substrates to be digested with manures. Especially, confectionery by-products showed potential for highly enhanced methane yields compared to digestion of cow manure alone. Energy crops could also be used as co-substrates. The produced methane yields (m3 kg-1 VS) were 0.14 to 0.35 for three different energy crops and 0.32-0.39 for confectionery by-products. Pre-treatment of energy crops by reducing particle size (2, 1 and 0.5 cm) did not influence methane yields in oats while, 1 cm particle size seems to optimal for grass hay and least optimal for clover . Energy crops harvested during vegetative stage resulted in higher methane yields than those harvested during the flowering period. On full-scale application, cow manure alone and co-digestion with energy crops produced 0.22 m3 CH4 kg-1 VS and co-digestion with confectionery by-products (20% of feed biomass) about 0.28 m3 kg-1 VS. However, the proportion of waste in a co-digestion process appears to be important e.g. feed VS in co-digestion of pig manure and potato waste may contain up to 15-20% of potato waste. Laboratory co-digestion of pig manure with potato tuber or its industrial by-products (potato peel or potato stillage) at loading rate of 2 kg VS m-3 d-1 produced methane yields (m3 kg-1 VS) of about 0.22 at 85:15 and 0.31 at 80:20 feed VS ratio (VS% pig manure to potato co-substrate) compared to 0.14 for pig manure alone. Application of co-digestion technology of safe industrial by-products such as confectionery by-products or potato industrial by-products in the farm-scale biogas digesters will offer a number of benefits for both farmer and industry by not only generating on-farm renewable energy but also enabling treatment of waste for industry.Post-digestion of digested materials sampled from laboratory continuously stirred tank reactors (CSTRs) during semi-continuous digestion/co-digestion or during full-scale digestion from farm digester and its associated post-storage tank indicated that the digested materials still contained some degradable material and would produce an appreciable amount of methane during post-methanation. During the long-term incubation, the effect of post-methanation and temperature on methane yields of digested materials sampled from farm digester and post-storage tank indicated that highest methane potentials up to 0.20 m3 kg-1 VS were obtained at 35°C, and could be considered as the maximum obtainable methane potential from these materials. The effect of temperature on methane potential of digested materials (farm digester or post-storage tank) was significant as the achieved potentials for both materials were less than 50% at 20°C and even less than 10% at 5°C of those achieved at 35°C. Increase in temperature after incubated for 345 d at temperatures ≤ 20°C to 35°C, improved methane production. Thus, these results suggest that the untapped methane potential in the digested manure cannot effectively be recovered at temperatures prevailing in the post-storage tank (5-10°C) during the winter in the Northern latitude biogas production system. Nevertheless, as the ambient temperature in post-storage tank increases e.g. with increase in day length, an increase in methanogenesis can be expected.The results from separation of digested materials into solids and liquid fractions to optimize the methane and nitrogen recovery showed that materials obtained from farm biogas digester (35°C) and post-storage tank (5-10°C) would differ significantly. Although digester material found to contain high methane potential, its methane potential cannot effectively be recovered as separation of the material would distributes its methane potential equally depending upon the distribution of its’ fractions. On the other hand, only a part of the methane potential could be recovered from the post-storage tank material. This study also showed that the digester material had high ultimate methane potential both in solids and liquid fraction while the methane potential of the material from post-storage tank was mainly concentrated in the solids. Separation of digested material into a nitrogen rich or poor fraction would not be feasible as both TKN and NH4+-N were equally distributed along the segregated fractions in both materials. Thus, separation of digested materials into solids and liquid fractions to recover methane may be feasible only with post-storage tank material but not with digester material. On the other hand, nitrogen management would not be feasible with neither material.The effect of post-treatments and temperature on post-methanation to recover the remaining methane potential of >2 mm fraction of the digested manure indicated that at 5-20°C maceration or thermal treatments and at 35-55°C chemical treatment with or without a thermal treatment would improve the methane yields to a smaller extent. However, the feasibility of the tested treatment methods in a full-scale application needs further evaluation.The feasibility of on-farm AD process through estimating energy production on the studied dairy, sow and pig farms indicated that dairy farm can be self sufficient in electricity and heat requirements and even sell surplus energy to local grid or heating system. While the benefits of adoption of AD process on mitigating GHG emissions was mainly through replacing fossil fuel consumption followed by reduction in emissions due to manure management (sow farm) and from fertilizer use and production (dairy and pig farm). This study also suggested that AD of farm manures offers both treatment of the manures and, produces carbon dioxide neutral methane from renewable sources and aid in reducing GHG emissions due to replacement of fossil fuels, manure management and inorganic fertilizer use and production.13.11.2003