What type of Digester Configurations should be employed to produce Biomethane from Grass Silage?

Transcription

1 What type of Digester Configurations should be employed to produce Biomethane from Grass Silage? Nizami Abdul-Sattar, Biofuels Research Group, ERI T: 353 (0) F: 353 (0) E: Grass as a source of renewable gaseous fuel EPA funded conference 15 th April 2010, Brookfield, UCC 1

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3 Anaerobic Digester configurations Digester types may be characterized as below: wet or dry; single or multi stage; single or multi phase; thermophilic or mesophilic; batch or continuous; include for high rate reactors; combinations of different approaches 3

4 Stages and Phases Stage equates to a reactor; two stage = two reactor Phase is an anaerobic phase (acidogenesis or methanisation); two phase involves optimising a reactor for acidogenesis and another for methanogenesis. One-stage system popular at industrial scale; Dry and batch systems are most common. Two phase popular in academic literature 4

5 Recycle Design variations in One-Stage and Two-Stage Digesters* Recycle Substrate Effluent Substrate Effluent One stage; wet/continuous/batch/dry/ CSTR CSTR = continuously stirred tank reactor Two- stage; wet/ Continuous /batch/ CSTR *AS Nizami, JD Murphy; What type of digester configurations should be employed to produce biomethane from grass silage? Renewable and Sustainable Energy Reviews, 14 (2010)

6 Dry versus Wet Dry: 20-40% dry matter Wet: typically less than 12% dry matter Pre-treatment i.e. pulping and slurrying is required for grass silage in wet digesters. Dry continuous single stage digesters are most common for municipal solid waste (MSW). CSTR configuration is most common in 90% of the new wet digesters. 6

7 Batch versus Continuous Batch: vessel is loaded once with raw feedstock for a given retention time Continuous: feedstock is regularly fed Continuous digesters: one or two stage CSTR, or second stage Upflow Anaerobic Sludge Bed (UASB) Batch digesters: single stage, sequential batch and hybrid batch digesters are used. 7

8 One-Stage Dry Continuous Digesters * Effluent Substrate Mixer Kompogas Effluent Substrate Mixer Effluent Substrate recirculation DRANCO Valorga *AS Nizami, JD Murphy; What type of digester configurations should be employed to produce biomethane from grass silage? Renewable and Sustainable Energy Reviews, 14 (2010)

9 Dry Batch Digester * Substrate UASB Percolation liquid storage tank Pump One-stage dry batch digester New Mature Old Two-stage / Sequential-batch Hybrid batch-uasb Two-stage dry batch digesters Leach beds Substrate Sequencing fed leach bed digesters coupled with UASB UASB Leachate tank *AS Nizami, JD Murphy; What type of digester configurations should be employed to produce biomethane from grass silage? Renewable and Sustainable Energy Reviews, 14 (2010)

10 Preprocessing Systems for High Solid Substrates * * Weiland P, Rozzi A. The start up, operation and monitoring of high rate anaerobic treatment systems: discussers report. Water Science Technology 1991;24:

11 High Rate Bioreactors Treat high strength liquidy wastes through high solid retention time. This is achieved due to formation of granules or attachments of biomass to carriers. High Rate Bioreactors can withstand very high loading rates 20 kg COD/m 3 /day as compared to 2 kg COD /m 3 /day for CSTR. Have short retention times: 0.5 to 1 day compare to days for CSTR. High Rate Digesters: UASB or fluidized bed reactor. Typically employed after a leach bed stage. 11

12 Various types of High Rate Digesters * Effluent Effluent Effluent Sludge bed Recycle Recycle Influent Influent Influent UASB reactor Anaerobic packed-bed Anaerobic fluidised-bed reactor reactor *AS Nizami, JD Murphy; What type of digester configurations should be employed to produce biomethane from grass silage? Renewable and Sustainable Energy Reviews, 14 (2010)

13 Digester Configuration suitable for Grass Silage Dry batch digester Dry continuous digester Leach bed system connected with high rate digesters Wet continuous two-stage digester 13

14 Gas production from grass * Energy content of grass ~ 19 MJ/kg Volatile Solid (VS) Energy content of CH 4 ~ 37.8 MJ/m 3 1 kg VS destroyed = 19MJ = 0.5 m 3 CH 4 Max production of gas is 500 L CH 4 /kg VS added * AS Nizami, N E Korres, JD Murphy; A review of the integrated process for the production of grass biomethane, Environmental Science and Technology, 2009, 43 (22), pp

15 Batch Digester BEKON or BIOFIRM processes are used widely in Europe for dry solids content up to 50%. Batch fed, leachate is recirculated/sprayed on to the feedstock, gas production begins, rises, peaks, declines, ceases, digester is reopened, half feedstock emptied, half left as inoculum, refilled with fresh feedstock and the cycle continues. Variations: garage door type digester with or without percolation, bag type digester with airtight plastic canvas, wet-dry combination digester Methane yield of fresh and ensiled grass species in the range of L kg -1 VS. (62 72% of maximum potential). 15

16 BEKON process * * BEKON. New BEKON biogas technology batch process dry fermentation (secured by various patents). BEKON energy technologies GmbH and Co. KG; 2008, Available from: 16

17 Dry Continuous Digester DRANCO, Kompogas and Valorga process are used at industrial scale. In DRANCO, the digestate/leachate is recirculated back vertically while in Kompogas it is horizontal. DRANCO plant at Nustedt, Germany* 12,500 t of agricultural crops; maize 6,200 t sunflowers 2400 t rye 2,000 t grass 600 ttotal Total biogas production 145 Nm3/t. Grass adds biogas at rate of Nm 3 /t *De Baere L. Dry continuous anaerobic digestion of energy crops; 2007, Publications ( 17

18 DRANCO process * *De Baere L. Dry continuous anaerobic digestion of energy crops; 2007, Publications ( 18

19 Leach Bed System connected with High Rate Digesters Leach beds connected with high rate digesters (UASB or anaerobic filter). High conversion of volatile solids to Chemical Oxygen Demand (COD) in the leach beds followed by a high rate of conversion of COD to methane in the UASB is achieved. Leach beds are batched loaded with recirculation of leachate. In such systems, 76 to 98% of the total methane yield originates from UASB. A methane yield of 0.39 m 3 CH4 kg -1 VS added (78% efficiency) after 50 days of digestion of grass silage at pilot scale batch leach bed connected with anaerobic filters. 19

20 Leach Bed Digester coupled with UASB for Digestion of Grass silage * Do not need to empty leach beds Leach Beds Grass Silage UASB Leachate Tank Hydrolysis and acidogenesis phases methanogenesis phase *AS Nizami, JD Murphy; What type of digester configurations should be employed to produce biomethane from grass silage? Renewable and Sustainable Energy Reviews, 14 (2010)

21 Wet Continuous Two-Stage Digester CSTR systems are ubiquitous; simplicity in design and operation, low capital costs. Long hydraulic retention times (HRT). The digestion of grass in CSTR is facilitated by the use of a separate preconditioning tank with chopper pump, screw-feeder and flushing system. Solid contents reduced by recirculation of leachate from second tank. Methane yield for fresh grass species as mono substrate was 335 L kg -1 VS added. At commercial scale CSTR in Eugendorf, Austria, the methane yield of CSTR grass silage as mono-substrate is 300 L /kg VS at OLR of 1kg VDS/ m 3 / day. 21

22 CSTR Systems * Grass Silage Water Digestate Pulping and Slurrying Digester at Eugendorf, Austria + *AS Nizami, JD Murphy; What type of digester configurations should be employed to produce biomethane from grass silage? Renewable and Sustainable Energy Reviews, 14 (2010) Energiewerkstatt. Projekt: Graskraftwerk Reitbach aus Wiesengras Energie ohne Ende, Technisches Bu ro und Verein zur Fo rderung erneuerbarer Energie, Energiewerkstatt; 7 November

23 Research Required Data from the literature on the best digester configuration using grass silage as a feedstock is inconclusive. Of significant concern is the comparison of data using different species of grass, at different dry solids content, cut at different times of year and times of day. There is a tendency to utilize data on digester configurations utilizing different feedstocks (OFMSW, green waste) and to apply the outcome to grass silage. It is suggested that a number of reactor configurations should be compared in real time treating similar quantities of grass silage under similar loading rates and characteristics to evaluate the optimal configuration. There is further need to compare the potential of various pretreatments options such as pressure, thermal, enzymatic, chemical in enhance the digesting and degradability of grass silage. 23

24 Anaerobic Digesters at UCC Sequential Batch Leach Bed- UASB (SBLB-UASB) Two-stage Continuously Stirred Tank Reactor (CSTR) 24

25 Sequential Batch Leach Bed- UASB (SBLB-UASB) Two-stage Continuously Stirred Tank Reactor (CSTR) 25

26 Thanks Department of Agriculture, Food and Fisheries (DAFF) for funding Green Grass project 26