RESIDUAL BIOGAS POTENTIAL

Transcription

1 RESIDUAL BIOGAS POTENTIAL OLDŘICH MUŽÍK, JAROSLAV KÁRA, JANA MAZANCOVÁ Research Institute of agricultural engineering in Prague, Energy and logistics of technological systems and biomass utilization for non-food purposes division Abstract: The contribution is focused on estimation of the biogas potential in the demonstration biogas plant Knezice. The estimation was calculated on basis of laboratory batch reactors experiments and on production of digestate in the real biogas plant. Prospective economical benefits calculation of the biogas utilization was also made. Keywords: biogas, batch reactors experiments, economical benefits calculations Introduction Controlled anaerobic digestion is perspective method how to ecologically friendly utilize biomass. Simplifiedly the anaerobic digestion is biological decomposition of organic matters in anaerobic environment. According to Pastorek (2004) the anaerobic digestion is a multiple-stage biological decomposition of witch the biogas is produced through activity of hydrogenic, acidogenic, acetogenic and methanogenic micro-organisms. The process is mainly used for treatment of sewage sludge, livestock waste, organic fraction of municipal waste, waste from foodprocessing and also for utilization of purposely growth phytomass. The residual biogas is produced in post-digestion stage (in digestate storage tanks) of biogas plants. The storage tanks are often not covered so the residual biogas is not collected and causes increase of methane emissions and economic waste. According to Angelidaki (2006) For thermophilic biogas plants 93% of the residual biogas potential was originating from particulate matter and 88% for the mesophilic biogas plants. This indicates that the residual biogas potential is mainly caused by insufficient retention time in the digester. Although average retention time in the digesters is usually sufficient (25-30 days), the out-going substrates from the digesters contains fractions of materials, which has been retained in the digesters for a much shorter time than the average retention time. The effect is multiplied when biomass contains a large fractions of particulate matters with relatively slow conversion rate, such as for manure some types of organic industrial waste (Angelidaki, 2006). This effect was also registered in co-digestion of phytomass with increased lignin, while lignin is material with the worst degradable plant cells. Material and Methodology The residual biogas potential experiments were conducted for purpose of demonstration biogas plant in Kněžice. The biogas plant consist of a reception tank, pasteurisation tank, mixing pit, mesophilic digester a two digestate storage tanks, as shown in Figure 1. Both storage tanks are not covered and the biogas plant operator consider investment in roofing of the first storage tank. The experiments were run in the three sets of batch reactors. Experiments were repeated twice with different feedstock. Each set contains 3 reactors of volume 1.5 litre. The reactors were located in tempered water bath at the temperature of 41 C, 20 C and 10 C (different temperature for each set). The methodology of experiments is in accordance with the methodology of VUZT commonly used for all the experiments testing the potential biogas production of different feedstock. The defined amount of feedstock (digestate) is dosed into the reactors of volume 1.5 litre and tightly closed and located in the water bath. The biogas production is continuously measured through the gas meter (SPEKTRUM Skuteč s.r.o.). The biogas composition is analyzed with the gas analyser (AIR LF). The lab operator registers the production regularly once a day at the determined time.

2 Figure 1 Scheme of biogas plant Kněžice Figure 2 - The testing reactors in the tempered water bath Two different samples of the digestate of known parameters (dry matter, volatile solids, ph). from the selected biogas plant Knezice were tested and used as the feedstock. Parameters of the digestate are shown in Table 1.

3 Table 1 - Parameters of the digestate Trial Batch digester volume Digestate Biomass stirring Trial Trial temperatureduration Initial DM Initial VS n (litres) (kg) yes/no C days (%) (% on DM) yes yes yes yes yes yes The defined amount of digestate (1 kg) was dosed into the reactors of volume 1.5 litre, stirred and tightly closed and located in the water bath. Each set of the reactors was located in the water bath of different temperature (41 C, 20 C and 10 C). The water bath temperatures were defined according to the possible temperature range in selected digestate storage tank during the year. The experiments are repeated twice. In case of the results under doubts, the other repetition is realized in order to confirm or disconfirm the previous results. Results and Discussion The results of the experiments expressed as a biogas production rated on dry matter or volatile solid are shown in Table 2. The results expressed the course of the biogas production of the same digestate under different temperature conditions. Cumulative biogas production of the first experiment is also graphically demonstrated in Figure 3. Table 2 - Results of Laboratory experiments with biogas production Trial Trial Trial temp. duration Initial DM n C days (%) Initial VS (% on DM) Final DM (%) Final VS (% on DM) Methan average biogas yield (%) l.kg VS -1 methane yield l.kg VS

4 90 Cumulative Biogas production [l.kg DM -1 ] day digestate 41 C digestate 20 C digestate 10 C Figure 3 Cumulative biogas production rated on dry matter under different temperature conditions The results show that biogas production depends especially on the temperature of the water bath and also on the initial dry matter and the volatile solid of the digestate. The results correspond with those which were run in other laboratories. According to Angelidaki s batch experiments (2005) temperature has also a very significant effect on recovery of the residual biogas. The biogas yield was significantly reduced when the temperature was lowered to 15 C and anaerobic degradation was nearly ceased when temperature lowered more than 10 C. Estimation of biogas production Potential residual biogas production from the demonstration biogas plant Knezice was estimated on the basis of the laboratory experiments, monthly digestate production in last year and its characteristics and an average air temperature in Knezice. The estimation of biogas production is shown in Table 3. The estimated residual methane potential is about 3% of yearly methane production of the biogas plant Kněžice. The percentage is comparable with other studies, but is on the bottom bound. According to Nielsen s study (2008) methane loss of followed Danish biogas plants ranged between 2.9 and 25%. Table 3 - Estimation of biogas production Month Digestate volume [tons] Average air temperature [ C] Average digester inner temperature biogas yield [m 3.ton DM -1 ] Biogas Methan yield [m 3.ton DM -1 ] Methan [ C] nov dec jan feb , mar , apr , may ,

5 jun jul aug sep oct nov Economical benefits The estimation of the residual biogas production was converted into the electricity production and its financial value. Estimated residual biogas production and incomes in separate months are shown in Figure 4. Calculations were made using guaranteed purchasing prices which were increased for year Exchange rate of CZK per EUR (according to Czech National Bank, January 2009) was counted. The residual biogas production depends on digester inner temperature and its oscillation. That is the reason why Financial benefits are unstable during the year and more than half extra electricity would be produced in summer. Yearly estimation of the residual biogas production and Incomes for extra electricity are summarized in Table 4. Investment costs of the first digestate storage tank roofing are about EUR. Total yearly costs of the residual biogas pumping were calculated as a roofing amortization, increased maintenance and insurance costs and increased energy costs. biogas production biogas production Incomes for extra electricity production in 2009 Incomes for extra electricity production in 2008 dec 07 jan 08 feb 08 mar 08 apr 08 may 08 jun 08 jul 08 aug 08 sep 08 oct 08 nov 08 Month Incomes for extra electricity production Figure 4 - Estimated residual biogas production and Incomes for extra electricity production Table 4 - Estimation of yearly financial benefits from residual biogas Biogas Methane Extra kwh from biogas Income from 2008 Income from 2009 Costs of residual biogas pumping Profit from Profit from 2009

6 Conclusions Comparing the results of the experiments, methane yield, dosage of digestate, costs of the residual biogas pumping and financial benefit from the extra electricity and heat production, the coverage of the first digestate storage tank and the utilization of the residual biogas could bring about EUR per year. As mentioned above the estimated residual methane potential was on the bottom bound so we could expect better results at the real condition of the demonstration biogas plant. References ANGELIDAKI, I., HEINFELT, A., ELLEGAARD, L Enhanced biogas recovery by applying post-digestion in large-scale centralized biogas plants. Water Science & Technology, 2006, Vol 54, No 2, pp IWA Publishing. ISSN ANGELIDAKI, I., BOE, K., ELLEGAARD, L Effect of operating conditions and reactor configuration on efficiency of full-scale biogas plants. Water Science & Technology, 2006, Vol 52, No 1-2, pp IWA Publishing. ISSN NIELSEN, H. B., ANGELIDAKI, I., Codigestion of manure and industrial organic waste at centralized biogas plants: process imbalances and limitations. Water Science & Technology, 2008, Vol 58, No 7, pp IWA Publishing. ISSN PASTOREK, Z., KÁRA, J., JEVIČ, P Biomasa obnovitelný zdroj energie. Praha FCC Public ISBN Published results are part of the research project EU-AGRO-BIOGAS No Contact address: Ing. Oldřich Mužík, Ing. Jaroslav Kára, CSc., Ing. Jana Mazancová, Ph.D. Research Institute of agricultural engineering, Drnovská 507, Praha 6, Czech Republic