Energy Fuels 2010, 24, 4459–4469 Published on Web 08/02/2010
: DOI:10.1021/ef1003039
Difficulties Associated with Monodigestion of Grass as Exemplified by Commissioning a Pilot-Scale Digester T. Thamsiriroj†,‡ and J. D. Murphy*,†,‡ †
Department of Civil and Environmental Engineering and ‡Environmental Research Institute, University College Cork, Cork, Ireland Received March 15, 2010. Revised Manuscript Received July 13, 2010
Grass is potentially a crop that in temperate oceanic climates (such as Ireland) offers an excellent yield of gaseous biofuel per hectare. The biogas/biomethane industry in Ireland is in its infancy as compared to its neighbors in continental Europe; Germany, in particular, has a thriving biogas industry. As a result, there is a tendency for digesters that were developed in continental Europe based on high solids content feedstocks, such as maize and the organic fraction of municipal solid waste, to be imported to Ireland for use as grass digesters. This paper examines the difficulties associated with monodigestion of grass through the design and commissioning of a two-stage wet continuously stirred tank reactor. The functional specific gravity of grass silage in a digester remains below 1 for a period of 24 h. This results in accumulations of undigested floating grass, which can be problematic. The agitation system for a grass digester must be designed to overcome this tendency for grass to float. Methane yields for Irish grass silage were found to be particularly high in the experimental period. Expected values for the design of 300 L of CH4 kg-1 of volatile solids added (VSadded) were based on analysis of the literature and with functional full-scale grass digesters. The value of 455 L of CH4 kg-1 of VSadded was measured, equating to 91% destruction of volatile solids. The solids content of digestate was estimated at 5% dry solids (from a feedstock with 30.7% dry solids), which would suggest handling difficulties if used in a vertical garage-door-type dry batch process.
complete with an upflow anaerobic sludge blanket (SBLBUASB).4 1.2. Biogas Potential of High Solids Content Feedstocks. The biogas potential can be stoichiometrically estimated by the chemical equation developed by Buswell and Boyle.5,6 The chemical compositions of the substrate need to be determined through ultimate analysis. Subsequently, biogas composition (CH4, CO2, H2S, and NH3) can be determined. The potential methane yield may not be realized. The actual yield (m3 of CH4 kg-1 of VSadded) is influenced by a number of factors. On the feedstock side, these factors include biodegradability, which is encouraged by the adsorption of hydrolytic enzymes to the biodegradable surface sites,7 and inhibition to microorganisms, which is due to the high concentration of some compositions in the substrate (such as ammonia, sulfide, and long-chain fatty acids)8,9 or the lack of some essential nutrients.10 On the technology side, some
1. Introduction 1.1. Different Digesters for High Solids Content Feedstocks. The production of biomethane through digestion of high solids content feedstocks [including the organic fraction of municipal solid waste (OFMSW) and energy crops] has significant potential to substitute for natural gas; Singh et al. suggested a potential substitution in the range of 7-33% for Ireland.1 A range of digestion technologies are available that may be generally categorized as single-stage/two-stage, dry/ wet, and batch/continuous.2 An issue in Ireland at present is that technologies designed for high solids content feedstocks, such as OFMSW and maize, are being imported to digest grass silage in Ireland. It is important that the digester configuration is suitable for the particular feedstock. The objectives of good design are to increase solids retention, decrease reactor size, and reduce process energy requirement.3 For high solids content feedstocks, technologies are either based on intermediate solids content at 5-10% dry solids (DS) through dilution or high solids content systems (>10% DS) through sprinkling with water/leachate in a closed loop. Two-stage processes include a two-stage continuously stirred tank reactor (CSTR), a two-stage two-phase digester (acidogenic and methanogenic phases separated), and sequencing batch leach bed reactors
(4) Nizami, A. S.; Korres, N. E.; Murphy, J. D. Review of the integrated process for the production of grass biomethane. Environ. Sci. Technol. 2009, 43, 8496–8508. (5) Buswell, A. M. Anaerobic fermentations. Bulletin 32; Division of Illinois State Water Survey, University of Illinois, Urbana, IL, 1936. (6) Boyle, W. C. Energy recovery from sanitary landfills. Microbial Energy Conversion; United Nations Institute for Training and Research (UNITAR), Geneva, Switzerland, 1977; pp 119-138. (7) Mata-Alvarez, J.; Mace, S.; Llabres, P. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresour. Technol. 2000, 74, 3–16. (8) Gebauer, R.; Eikebrokk, B. Mesophilic anaerobic treatment of sludge from salmon smolt hatching. Bioresour. Technol. 2006, 97, 2389– 2401. (9) Chen, Y.; Cheng, J. J.; Creamer, K. S. Inhibition of anaerobic digestion process: A review. Bioresour. Technol. 2008, 99, 4044–4064. (10) Angelidaki, I.; Sanders, W. Assessment of the anaerobic biodegradability of macropollutants. Rev. Environ. Sci. Bio/Technol. 2004, 3, 117–129.
*To whom correspondence should be addressed. Telephone: þ35321-490-2286. E-mail:
[email protected]. (1) Singh, A.; Smyth, B. M.; Murphy, J. D. A biofuel strategy for Ireland with an emphasis on production of biomethane and minimization of land-take. Renewable Sustainable Energy Rev. 2010, 14, 277–288. (2) Lissens, G.; Vandervivere, P.; De Baere, L.; Biey, E. M.; Verstraete, W. Solid waste digestors: Process performance and practice for municipal solid waste digestion. Water Sci. Technol. 2001, 44, 91–102. (3) Chynoweth, D. P.; Owens, J. M.; Legrand, R. Renewable methane from anaerobic digestion of biomass. Renewable Energy 2001, 22, 1–8. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 4459–4469
: DOI:10.1021/ef1003039
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fractions of substrate are lost in the effluent because of a limited retention time or poor design and some small portion of substrate which is used to synthesize bacterial mass.10,11 The percentage degradation is determined by comparing the actual methane yield with the potential methane yield. This tends to decrease with increased solids content in substrates.12 Pretreatments may improve methane yield because they increase the accessibility for enzymes and bacteria to the core substrate within ligno-cellulose complexes.4 1.3. Difference between Maize and Grass as Feedstock for Digesters. Maize is an annual crop grown on arable land; typically this involves annual ploughing of soil. This is problematic in an Irish context because only 9% of agricultural land is arable and has already been fully used. Ploughing of pasture land is not acceptable for the production of sustainable biofuels.4 In contrast, grass is a perennial crop;13 91% of Irish agricultural land is under pasture. In central Europe, maize provides 15-18 tons of DS ha-1 annum-1, while grass provides relatively less yield (8-10 tons of DS ha-1 annum-1).13 However, in western Europe and, particularly, in Ireland, maize yields are lower, while grass yields are higher; grass yields between 11-15 tons of DS ha-1 annum-1.14 The dry solids (DS) and volatile solids (VS) content of maize and grass silages can be similar, about 30% DS and more than 90% of VS in DS (depending upon ensiling methods).14,15 Maize can be harvested as whole maize crop, maize corns, or corn cob mix.15 Standard harvesters are commonly used, simultaneously chopping the whole maize plant for subsequent ensiling.16 This is similar to grass harvesting systems, in which precision-chop harvesters are used and grass is cut to about 20 mm long.17,18 Maize is the preferred feedstock for biogas production in Europe.12 Whole crop maize produces the highest methane yield (205-450 m3 of CH4 kg-1 of VSadded) as opposed to other maize combinations.15 This may be compared to grass at 298-467 m3 of CH4 kg-1 of VSadded.16 In Europe, wet process digester types are the dominating technology for the digestion of maize and grass.19 This is because most anaerobic
digestion plants are designed to treat liquid manure with a smaller proportion of energy crops added.20 Grass silage is more fibrous than maize silage.21,22 This can be problematic for mechanical mixing/agitation systems of the digester. The problem associated with fibrous substrates is that they tend to accumulate with interlocking between particles. A robust system and extra energy are thus required to ensure a proper mixing of fibrous substrate.23 An accumulation of undigested grass in the digester because of inhibition of the system leads to increased fibrous content and can result in the failure of the agitating system. One significant source of inhibition in anaerobic digestion of grass is a high ammonia concentration. Of the total nitrogen in dry matter grass silage, ammonia comprises about 12%; ammonia associated with maize silage is negligible.21,24 These factors limit the organic loading rate (OLR) of grass in monodigestion to below 3 kg of VS m-3 day-1.25,26 In monodigestion of maize, ammonia is not problematic but trace elements are more important in longterm operation.27 In wet digesters, both maize and grass substrates tend to float on the liquor surface initially. This is indicated by their initial functional specific gravity (FSG) of below 1.0 (FSG represents the density of individual feed particles along with associated gas-filled spaces and bound water28). However, maize tends to saturate and submerge after a few hours in the digester (FSG rises above 1.0), while grass remains floating for more than a day.28,29 Without proper mixing, buoyant grass particles form a floating mass that accumulates in size, rising above the liquor layer, which gradually dries, and becomes an indigestible scum.30 A horizontal plug flow digester plant in Germany, visited by the authors, was successful in maize digestion but had a difficulty with grass because of this floating characteristic. However, a two-stage CSTR plant in Austria, visited by the authors, was successful in the digestion of grass as monofeedstock because of the particular arrangement of mixers. (20) Baere, L. D. Dry continuous anaerobic digestion of energy crops. Organic Waste Systems (OWS): Ghent, Belgium; http://www.ows.be/ pub/Dry%20AD%20of%20energy%20crops.Papenburg.March07. pdf/ (accessed on Feb 2010). (21) De Boever, J. L.; De Smet, A.; De Brabander, D. L.; Boucque, C. V. Evaluation of physical structure. 1. Grass silage. J. Dairy Sci. 1993, 76, 140–153. (22) De Boever, J. L.; De Brabander, D. L.; De Smet, A. M.; Vanacker, J. M.; Boucque, C. V. Evaluation of physical structure. 2. Maize silage. J. Dairy Sci. 1993, 76, 1624–1634. (23) James, S.; Wiles, C.; Swartzbaugh, J.; Smith, R. Mixing in largescale municipal solid waste-sewage sludge anaerobic digesters. Biotechnol. Bioeng. Symp. 1980, 10, 259–272. (24) Daniel, B.; Branislav, G.; Miroslav, J.; Milan, S.; Jaroslava, M.; Erika, G.; Michal, T. Fermentation process characteristics of different maize silage hybrids. J. Cent. Eur. Agric. 2008, 9, 463–468. (25) Gollackner, M. Projekt: Graskraftwerk Reitbach. Biogas aus Wiesengras—Energie ohne Ende. GRASKRAFT Reitbach reg.Gen.m. b.H.: Friedburg, Austria; http://www.klimaaktiv.at/filemanager/download/19237/ (accessed on Jan 2010). (26) Marı´ n-Perez, C.; Lebuhn, M.; Gronauer, A. Mehr Gas aus Gras. Biogas J. 2009, 4, 72–75. (27) Lebuhn, M.; Liu, F.; Heuwinkel, H.; Gronauer, A. Biogas production from mono-digestion of maize silage—Long-term process stability and requirements. Water Sci. Technol. 2008, 58, 1645–1651. (28) Siciliano-Jones, J.; Murphy, M. R. Specific gravity of various feedstuffs as affected by particle size and in vitro fermentation. J. Dairy Sci. 1991, 74, 896–901. (29) Hooper, A. P.; Welch, J. G. Effects of particle size and forage composition on functional specific gravity. J. Dairy Sci. 1985, 68, 1181– 1188. (30) Chanakya, H. N.; Sharma, I.; Ramachandra, T. V. Micro-scale anaerobic digestion of point source components of organic fraction of municipal solid waste. Waste Manage. 2009, 29, 1306–1312.
(11) Sialve, B.; Bernet, N.; Bernard, O. Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable. Biotechnol. Adv. 2009, 27, 409–416. (12) Bauer, A.; Leonhartsberger, C.; B€ osch, P.; Amon, B.; Friedl, A.; Amon, T. Analysis of methane yields from energy crops and agricultural by-products and estimation of energy potential from sustainable crop rotation systems in EU-27. Clean Technol. Environ. Poliy 2010, 12, 153– 161. (13) Gerin, P. A.; Vliegen, F.; Jossart, J. M. Energy and CO2 balance of maize and grass as energy crops for anaerobic digestion. Bioresour. Technol. 2008, 99, 2620–2627. (14) Smyth, B. M.; Murphy, J. D.; O’Brien, C. M. What is the energy balance of grass biomethane in Ireland and other temperate northern European climates? Renewable Sustainable Energy Rev. 2009, 13, 2349– 2360. (15) Amon, T.; Amon, B.; Kryvoruchko, V.; Zollitsch, W.; Mayer, K.; Gruber, L. Biogas production from maize and dairy cattle manure— Influence of biomass composition on the methane yield. Agric., Ecosyst. Environ. 2007, 118, 173–182. (16) Braun, R.; Weiland, P.; Wellinger, A. Biogas from energy crop digestion. International Energy Agency (IEA) Bioenergy Task 37— Energy from Biogas and Landfill Gas; IEA: Paris, France; http://www. iea-biogas.net/Dokumente/energycrop_def_Low_Res.pdf/ (accessed on Feb 2010). (17) O’Kiely, P.; McNamara, K.; Forristal, D.; Lenehan, J. J. Grass silage in Ireland. Farm Food 2000, Winter, 33–38. (18) Homepage of Offwell Woodland and Wildlife Trust. Precision chop silaging—British intensive agricultural grasslands. Offwell Woodland and Wildlife Trust: Offwell, U.K.; http://www.countrysideinfo.co. uk/ag_grasslnd/silage2.htm/ (accessed on Feb 2010). (19) Heiermann, M.; Plochl, M. Biogas farming in central and northern Europe: A strategy for developing countries? CIGR J. 2006, 8 (8).
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Thus, the main differences between maize and grass as feedstocks in wet digestion are as follows: (i) High fibrous content in grass requires a robust mechanical mixing/agitating system designed specifically for grass silage. (ii) High ammonia content of grass causes an inhibition in anaerobic digestion especially at high OLR. An accumulation of ammonia at low OLR operation can also effect the anaerobic process in the long term. (iii) Grass tends to float for a longer period than maize, causing an indigestible scum layer. 1.4. Difference between OFMSW and Grass as Feedstock for Digesters. The organic fraction of municipal solid waste (MSW) typically comprises the organic element of MSW; it includes food and garden wastes.1 While grass is homogeneous in composition, OFMSW is widely variable and can contain slowly degrading lignocellulosic material, such as branches, and non-organic material, such as plastic. The composition of OFMSW varies from source to source and over the year. For example, source-separated OFMSW can be high in grass cuttings in the summer and high in hedge pruning in the winter.31 There is also a significant difference between OFMSW from a materials recovery facility and from a source-separated system. The moisture content of OFMSW also significantly varies; the DS content of food waste is about 30%, and garden wastes are about 40% DS. Thus, the composition decides the DS content of the OFMSW. The DS content of grass silage is dependent upon the ensiling method. For example, in temperate oceanic climates, such as Ireland, pit silage has a solids content of about 21%, while bale silage has a solids content of 32%. Grass as previously stated has a VS content of about 92% and methane yields in the range of 298-467 m3 of CH4 kg-1 of VSadded. The volatile content of OFMSW is of the order of 60%, and thus, the methane yield from OFMSW is lower than grass at about 240 m3 kg-1 of VSadded.32 This has a significant effect on the DS content of the digestate. About 90% of full-scale OFMSW digestion plants in Europe are one-stage systems.2 Two-stage digester systems are preferred for grass, consisting of a high-loaded main digester and a low-loaded secondary digester in series. The second digester treats the digestate from the first stage,33 and as a result, the solids concentration is considerable lower than that of the first digester. Floating grass is not a problem; residual biogas is collected; and in the spring, liquid digestate may be drawn off for land application. Wet digesters may not be suitable for OFMSW, unless the heavy materials (such as glass and metals) are fully removed and the feedstock is chopped to fine particles; this level of quality control in waste management is extremely difficult to achieve. Any hard material in the feedstock can be extremely problematic to pumping and agitation systems. Also, similar to grass, lignocellulosic material in OFMSW can accumulate at the surface, causing a scum layer. As for grass, a well-designed mixing system would be required. As a result, dry digesters are more common for OFMSW, typically dry batch digesters. In dry batch systems, a vertical gastight door is opened to feed the digester and remove the digestate. This is very
suitable for digestion of OFMSW because the digestate will have a high solids content (40% of solids are not volatile, and conversion of VS to methane is lower than that of grass) and an associated angle of repose. This is not the case for grass. The high volatile content and the high methane production can result in grass digestate having a very low solids content, with a slurry consistency that may flow out a vertical door upon opening. Dry digestion offers great advantages, in that the digester volume can be minimized by avoiding a large addition of water.20 However, it is unsure whether the technology is suitable for monodigestion of grass. The main differences between OFMSW and grass for anaerobic digestion are as follows: (i) OFMSW is more variable in composition and solids content than grass. (ii) OFMSW has a lower percentage VS content and a lower methane yield per kilogram of VS than grass. (iii) OFMSW may not be suitable for wet digestion unless pretreatments are extremely effective in removing hard contaminants and homogenizing the feedstock. (iv) Dry batch processes are suitable for OFMSW but may not be for grass because of the solids content of the digestate. 1.5. Focus of the Paper. German digester technology, developed using feedstocks such as maize and OFMSW, is being imported into Ireland. Many of the proposed Irish facilities use either monodigestion of grass silage or feedstocks dominated by grass silage. Anecdotal information particularly from Germany (where digestion of energy crops, such as maize, is ubiquitous) suggests that grass is a very difficult feedstock for monodigestion. A review of the scientific literature shows a number of studies on anaerobic digestion of grass; however, not many are based on a CSTR system using grass as monofeedstock.34-37 This paper describes the detailed design and commissioning of a pilot-scale two-stage CSTR digester. Problems experiencing and modifications to the design are described in detail. This is also the first study outlining the production of methane from Irish grass silage, in a country where 91% of agricultural land is under pasture and yields per hectare are among the highest in the world. 2. Production of Biogas from Grass 2.1. Energy Balance of Grass Biomethane. Over 90% of Irish agricultural land is under grass;38 there is thus significant potential for Ireland to use this indigenous feedstock in energy production, reducing dependency upon importation. In the Irish context, Smyth et al.14 investigated the energy balance of grass biomethane as a transport fuel using life cycle analysis (LCA) methodologies. Results of the analysis showed that a gross energy of 122 GJ ha-1 annum-1 can be obtained from grass biomethane; this is far superior to indigenous European biofuel systems, such as rapeseed (34) M€ahnert, P.; Heiermann, M.; Linke, B. Batch- and semi-continuous biogas production from different grass species. CIGR J. 2005, 7. (35) Stewart, D. J.; Bogue, M. J.; Badger, D. M. Biogas production from crops and organic wastes. 2. Results of continuous digestion tests. N. Z. J. Sci. 1984, 27, 285–294. (36) Wichern, M.; Gehring, T.; Fischer, K.; Andrade, D.; L€ ubken, M.; Koch, K.; Gronauer, A.; Horn, H. Monofermentation of grass silage under mesophilic conditions: Measurements and mathematical modeling with ADM1. Bioresour. Technol. 2009, 100, 1675–1681. (37) Jarvis, A˚.; Nordberg, A˚.; Jarlsvik, T.; Mathisen, B.; Svensson, B. H. Improvement of a grass-clover silage-fed biogas process by the addition of cobalt. Biomass Bioenergy 1997, 12, 453–460. (38) Murphy, J. D.; Power, N. M. An argument for using biomethane generated from grass as a biofuel in Ireland. Biomass Bioenergy 2009, 33, 504–512.
(31) Murphy, J. D. Visit to German biogas plant, hosted by BEKON Energy Technologies GMBH. Summary Report; University College Cork: Cork, Ireland, Dec 2008. (32) Murphy, J. D.; Power, N. A technical, economic, and environmental analysis of energy production from newspaper in Ireland. Waste Manage. 2007, 27, 177–192. (33) Weiland, P. Biogas production: Current state and perspectives. Appl. Microbiol. Biotechnol. 2010, 85, 849–860.
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Figure 1. Mass balance of the designed two-stage CSTR system.
biodiesel (46 GJ ha-1 annum-1) and wheat ethanol (66 GJ ha-1 annum-1).14 The gross energy is comparable to tropical biofuels, such as palm oil biodiesel from southeast Asia (120 GJ ha-1 annum-1)39 and sugar cane ethanol from Latin America (135 GJ ha-1 annum-1).40 Biofuel from grass does not displace food production nor cause the loss of biodiversity by habitat destruction; neither does it impact the visual environment.38 Grass yields and compositions vary from country to country. In Finland, biomethane from reed canary grass yields 122 GJ ha-1 annum-1.41 Lower gross energy from grass biomethane is reported in regions of central Europe, such as Belgium (90 GJ ha-1 annum-1).13 2.2. Integrated Agronomy and Process for Grass Biomethane. Ireland has the fourth highest level of bovinity in the world,4 leading to very high per capita greenhouse gas emissions. A reduction of the national beef herd is in line with Irish government policy.42-44 Conversion of surplus grass land to arable land is not permissible because of crosscompliance and release of carbon;4 thus, surplus grass is proposed for biogas production.14 Established knowledge in grass production is beneficial to the grass biomethane industry. For example, the lignocellulosic content of grass increases with the maturity of grass; the first cut offers more
methane potential than the later cuts; and the water-soluble content is higher in the afternoon than at other times of the day.4 2.3. Digester Scheme. A two-stage CSTR system with two digesters operated in series is proposed (Figure 1). Warm liquid digestate may be taken from digester 2 to dilute feedstock inserted to digester 1. The DS content of the digester may thus be kept below 10%. Hydrolysis and methanogenic processes take place in both digesters 1 and 2. The agitating/mixing system is designed to prevent the buildup of a scum layer. Up to 90% of biogas plants use mechanical stirring equipment. For an operation at high solids content, slow rotating paddle stirrers are preferred with a horizontal, vertical, or diagonal axis and large paddles.33 Biogas produced is more intensive in digester 1 than in digester 2. The two-stage CSTR system results in a higher biogas yield in comparison to the single-stage CSTR and, thus, reduces residual methane potential of the digestate,45 which significantly improves the sustainability of the whole life cycle of the system (less methane is released when digestate is land-applied). 2.4. Full-Scale Grass Digester Facility. A farm-scale digester plant in Austria was visited by the authors in 2008. The two-stage CSTR digester using grass as monofeedstock had a methane yield of approximately 300 m3 ton-1 of VSadded.14 The silage had a solids content of about 40% because of wilting in warm summers on land (in an Irish context, pit silage has a solids content of 21%, while bale silage has a solids content of 32%4). The silage was macerated prior to insertion to the digester. Feedstock was fed to digester 1, while preceding substrate flowed to digester 2. Some liquor left digester 2 as the digestate, while some was recirculated to digester 1 to keep the total solids content of the input feedstock below 10%. The digester operated at a typical hydraulic retention time (HRT) of 78 days (for two digesters
(39) Thamsiriroj, T.; Murphy, J. D. Is it better to import palm oil from Thailand to produce biodiesel in Ireland than to produce biodiesel from indigenous Irish rape seed? Appl. Energy 2009, 86, 595–604. (40) Bourne, J. Green dreams. Biofuels; National Geographic: Washington, D.C.; Oct 2007; http://ngm.nationalgeographic.com/2007/10/biofuels/ biofuels-text/ (accessed on Feb 2010). (41) Tuomisto, H. L.; Helenius, J. Comparison of energy and greenhouse gas balances of biogas with other transport biofuel options based on domestic agricultural biomass in Finland. Agric. Food Sci. 2008, 17, 240–251. (42) Department of the Environment, Heritage, and Local Government (DEHLG). Ireland National Climate Change Strategy 2007-2012; DEHLG: Dublin, Ireland, 2007. (43) Department of the Environment, Heritage, and Local Government (DEHLG). National Climate Change Strategy Ireland; DEHLG: Dublin, Ireland, 2000. (44) Department of the Environment, Heritage, and Local Government (DEHLG). National Climate Change Strategy. The Plain Guide; DEHLG: Dublin, Ireland, 2000.
(45) Kaparaju, P.; Ellegaard, L.; Angelidaki, I. Optimisation of biogas production from manure through serial digestion: Lab-scale and pilot-scale studies. Bioresour. Technol. 2009, 100, 701–709.
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Table 1. Characteristics of Grass Silage in the Study pH ammonia (% of total N) protein (% DM) ME (MJ kg-1 of DM) DMD or D value (% DM or D value) silage intake or palatability (g kg-1 of W0.75) lactic acid (% DM) lactic acid (% total acids) VFA (% DM) PAL (mequiv kg-1 of DM) NDF (% DM) soluble sugars (% DM) FME (MJ kg-1 of DM) FME/ME ratio oil (% DM) C (% DM) H (% DM) N (% DM) DS (%) VS (%)
Table 2. Provisional Design of the Pilot-Scale, Two-Stage CSTR System
4.3 9 9.5 10 64 89 4.3 7.3 1.6 821 59 5 8.2 0.81 3.3 43.035 5.82 1.61 30.66 92.46
Substrate active volume of reactor: 2 tanks at 300 L OLR: 1 kg of VS m-3 day-1 at 600 L active volume grass silage: 0.6 kg of VS at 30.66% DS and 92.46% VS water added: 10% DS in total substrate total substrate: 2.12 kg of silage and 4.37 kg of water HRT: 600 L volume at 6.49 kg of substrate day-1
600 L 0.6 kg of VS day-1 2.12 kg day-1 4.37 kg of water day-1 6.49 kg of substrate day-1 92 days
Expected Gas Production methane produced: 0.3 m3 of 180 L of CH4 day-1 -1 CH4 kg of VS at 0.6 kg of VS day-1 biogas produced: 180 L of CH4 327 L of biogas day-1 day-1 at 55% CH4 biogas produced in digester 262 L of biogas day-1 1: 80% total biogas produced in digester 65 L of biogas day-1 2: 20% total
in total) with an OLR of less than 3 kg of VS m-3 day-1, to ensure high process stability and biogas yield.25 The two digesters were originally installed with mechanical stirring systems, consisting of agitators with both vertical and diagonal axis propellers. They also experimented with pneumatic and hydraulic stirrings. The pneumatic stirring facilitated the upflow of gas bubbles, which kept the floating mass wet and active. When required, liquor can also be pumped by the hydraulic system from the bottom half to the top of the digester to stop the formation of a scum layer.25
2 based on a methane content of 55% (Table 2). The DS content in the digesters 1 and 2 are predicted to be consistent at about 8 and 5%, respectively, for the long-term operation (Figure 1). 3.3. Digester Components. 3.3.1. Control System and Sensing Devices. The digester scheme was equipped with basic sensing devices, including pH and temperature sensors, and a biogas flow meter. All data was recorded and retrievable from the data log system. A PLC control automated the sensor measurements. The composition of biogas was tested by a portable biogas analyzer model PGD3-IR, supplied by Status Scientific Controls Ltd. 3.3.2. Heating System. Each digester tank sat on a hot water bulk equipped with an internal core heating element (Figure 1). The heating coil was controlled by the PLC control to gradually bring the digester to meet the target temperature. A venting pipe allows contact between the heating water and the outside air to prevent a pressure rise because of an expansion of the heated water volume. 3.3.3. Substrate Flow. The selected quantity of grass was placed in the reception hopper on a daily basis. It was pushed vertically down into the digester by a movable steel rod; the rod was tipped with a circular meshed bearing plate (diameter slightly less than the insertion pipe), which allowed liquid through when pushing the silage feedstock into the digester, preventing a sharp increase of pressure in the digester during feed. It was maintained in a downward position after feed, as to prevent grass floating up and gathering in the inlet chamber. Recirculated liquor could also be pumped at a known quantity into the main digester through the feed inlet pipe if required (Figure 1). Digesting liquor was transferred from the main digester to the secondary digester via a U-tube. Both ends of the U-tube are positioned just above the liquor level in the digesters (Figure 1). The hydraulic gradient thus allowed the digesting liquor to flow naturally from one tank to the other when the new feed was added and the digestate was removed. 3.3.4. Agitating System. Digesters were equipped with a mechanical stirring system, which consisted of a vertical shaft with horizontal mixer blades (Figure 2a). Mixer blades were of an airfoil shape twisted along the longitudinal axis. The blade movement created a downward resultant force that pushed the floating substrate into the digesting liquor. Mixer blades were installed at two levels: partially above the liquor level and above the tank bottom level (Figures 1 and 2a). The fabricator placed the central shaft of the mixer off center of the tank to
3. Detailed Design of the Pilot-Scale Grass Digester and Methodology Proposed 3.1. Characteristics of Grass Feedstock. Grass feedstock used for the period of the experiment was bale silage obtained from the Irish Agricultural Institute (Teagasc). The herbage was harvested on June 2, 2009 (1st cut, early mature) from a homogeneous perennial ryegrass dominant plot. The herbage was field-wilted for 24 h before being baled. The bales of herbage were wrapped in 6 layers of polythene stretch film and stored for around 5 weeks (to allow ensilage to take place). They were then repackaged as small square bales, producing a quantity of silage that once the bale was opened would not “go off” before use in the pilot digester. Samples were taken from 18 of the bales at random and composited. These samples were further processed to produce 6 silage samples. Characteristics of silage samples are reported in Table 1. 3.2. Design of the System and Predicted Mass Balance. The pilot-scale digester (Figure 1) was provisionally designed to have an active volume of 300 L in each tank, with an aspect ratio (height/diameter) of 1:2. An OLR of 1 kg of VS m-3 day-1 was initially proposed. The DS content of substrate inserted to digester 1 was limited to 10% through dilution with the recirculated liquor from digester 2. The total substrate added was estimated to be 6.49 kg day-1 (2.12 kg of silage and 4.37 kg of water). At 30.66% DS and 92.46% VS, this equated to 0.6 kg of VS day-1. The HRT was designed to be 92 days, 46 days in each digester (Table 2). This parameter was given slightly above the typical value (70-80 days), because this digester will be loaded over time in a manner to increase OLR and reduce HRT. Biogas and methane produced from the system were estimated on assumptions of the specific methane yield of 0.30 m3 of CH4 kg-1 of VSadded based on the yield of the visited full-scale plant in Austria. The methane potential of grass silage was estimated in Table 3 as 0.50 m3 of CH4 kg-1 of VSadded. The total destruction of VS is thus expected to be 60% of VSadded. The total biogas produced was calculated to be 327 L day-1, with production of 262 L from digester 1 and 65 L from digester 4463
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Table 3. Methane Potential from Grass Silage silage sample on a dry basis: C, 43.035%; H, 5.82%; VS, 92.46%; ash, 7.54% (Table 2) Sheng’s formula:46
energy content of biomass ðMJ kg- 1 DSÞ ¼ - 1:3675 þ 0:3137C þ 0:7009H þ 0:0318O ¼ - 1:3675 þ 0:3137ð43:035Þ þ 0:7009ð5:82Þ þ 0:0318ð100 - 43:035 - 5:82 - 7:54Þ ¼ 17:6 MJ kg- 1 DS ¼ 19:03 MJ kg- 1 VS (note: O* = 100 - C - H - ash) energy content of methane gas = 37.78 MJ m-3 methane potential from grass silage = 19.03/37.78 = 0.50 m3 kg-1 of VS
Figure 2. (a) Components in the digester tank. (b) Gas seal between the digester tank and mixer shaft in the original design. (c) Modification to the gas seal between the digester tank and mixer shaft.
4. Problems and Modifications
centralize the gas exit port. Mixers turned in one direction with an adjustable rotational speed to a maximum of 3 rpm. In each digester tank, a set of motor and gearbox was externally equipped to drive the mixer shaft. The digester tanks were sealed on the top, where an end of the mixer shaft left the tank to connect with the motor. Originally, the seal was made by wearing the shaft to a deep cup; the open end of the cup was sunk into filled oil and turned along with the shaft; and the filled oil surrounding the cup was designed to prevent biogas escape (Figure 2b).
4.1. Commissioning Period. Initially, both digesters were filled to the operational level with innoculum from an anaerobic digester plant in Ireland, which used food waste and manure as a feedstock. Both digesters were heated to 35 °C and operated as a batch for a period of 2 months. Grass silage was added to one of the digester vessels. The difference between the biogas produced in the two vessels would equate to the biogas yield purely from grass. The duration of these 4464
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Figure 3. (a) Gas transfer vessel. (b) Characteristics of U-tube and the modification.
batch experiments was 2 months, which was sufficient to release most of the biogas produced from both digesters. The biogas produced in the main experiment starting later would be considered without the residual biogas left over from this commissioning period. Several problems with the digester system were observed during the initial 2 month commissioning period. These included (i) data lost because of the data log system collapsing, (ii) accumulated floating layer of grass in the shadow area, where the mixers did not sweep (because of off central position of the vertical mixing axis), (iii) evaporation losses of heating water through the ventilation pipe, which caused the water to dry out in a very short time, and (iv) leak of biogas through the seal between the mixer shaft and tank roof. The digester system was subsequently modified after 2 months of the commissioning period to overcome these problems as follows. 4.1.1. Data Log System. During this 2 month period, the observed pH, temperature, and gas flow rate were not recorded because of the demise of the data log system. The problem occurred a week after the startup. Rectification of the problem required a specific expertise. It took 3 months to organize the contractor and reinstall the data log system. 4.1.2. Agitating System. Because the agitator was not centralized in the digester tank, there was an unreached crescent-shaped surface area in which floating grass accumulated. To overcome the problem, an extra mixing blade made from flexible plastic was added to the vertical shaft, which enabled mixing in the previously unreached crescentshaped area (Figure 2a). 4.1.3. Heating System. In the original design, the heating system employed an open contact between the heated water and the outside air; this allowed the water to expand without increasing pressure. However, it also caused a loss of water
through evaporation, requiring a refill on a weekly basis. The modification employed a closed system by installing a pressure storage tank to absorb the pressure arising from the water expansion; evaporation of water outside the system was not allowed. 4.1.4. Gas Seal. The original gas seal design on the vertical mixing axis using oil to prevent an escape of biogas proved unreliable (Figure 2b). The gas pressure in the digester tank caused the oil to continually spill out. Thus, a different seal type was designed and inserted. The new seal comprised a compact rubber bellows seal (top seal) sitting on a polished Teflon bottom seal. They were compressed by a stainlesssteel retaining spring to produce an airtight seal (Figure 2c). The top seal moved along with the mixing shaft, while the bottom seal was fixed to the digester tank. 4.2. Main Experiment Period. Continuous feeding initially started at an OLR of 1 kg of VS m-3 day-1. Although a number of modifications were undertaken during the commissioning period, problems continued to arise in the main experiment. Additional modifications made to the digester system during the main experiment are described below. 4.2.1. Incorrect Gas Flow Meter. It was realized that the wrong gas flow meter was delivered; it did not have the capacity to measure the design flow of gas. This required operation at an OLR target of 0.5 kg of VS m-3 day-1. The quantity of grass feed was revised to 1.1 kg day-1. The quantity of liquid recirculation was also reduced because of the initial low solids content in the digester. This resulted in an extended HRT of 221 days. The new expected biogas production was estimated to be 136 L day-1 (74.8 L of CH4 day-1) for digester 1 and 34 L day-1 (18.7 L of CH4 day-1) for digester 2, respectively. The new gas flow meter required 11 weeks to reorder and install. 4.2.2. Feeding System. An increase in biogas in the digester resulted in a buildup of pressure in the digester tank, which caused difficulty in feeding the digester. The pressure increase resulted in a downward pressure on the liquor level in the digester tank and resulted in a raised level in the feed inlet pipe, which was small in diameter relative to the digester. The
(46) Sheng, C.; Azevedo, J. L. T. Estimating the higher heating value of biomass fuels from basic analysis data. Biomass Bioenergy 2005, 28, 499–507.
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liquor level in the inlet pipe could increase up to the feed hopper level (Figure 1). The problem was exasperated as more biogas was produced. A modification employed involved installing a gas transfer vessel (Figure 3a). The mechanism of the gas transfer vessel was simple but very effective. The vessel comprised a tall stainless-steel chamber filled with water to 2/3 of its height. A deep cup was placed facing downward in the chamber and completely immersed in water. The cup was lifted up to create a vacuum and allowed for a suction of gas from the digester tank to the space inside the cup. With biogas removed from digester 1, the pressure abated; the liquor level in the feed inlet pipe reduced, and feeding could be performed. Upon completion of feeding, biogas could be simply transferred back to the digester tank by pressing down the cup to the initial position. An expandable solid stopper was used to completely seal the top of the feed inlet pipe. This prevented the liquor level from rising during the day. 4.2.3. U-Tube Connection. The U-tube connection, which allowed flow between digesters 1 and 2, was removed after 3 weeks of the digester operation because of its functional failure. Two problems arose with the U-tube. The first problem was the blockage of the U-tube through accumulation of grass in the U-tube; the second problem was the transfer of biogas from digester 1 to digester 2 through the U-tube. The U-tube connection was installed at the liquor surface level of the two digester tanks (Figure 1). Upon feeding digester 1 and withdrawing of digestate from digester 2, the liquor was allowed to spill from digester 1 into one end of the U-tube and the same quantity was pushed from the U-tube into digester 2. However, the grass substrate, which accumulated and floated on the liquor surface, also spilled into the U-tube and caused a blockage. As a result, the liquor level in digester 1 gradually increased and flooded the U-tube, while the level in digester 2 dropped significantly. The gas flow meter mounted to each digester was manufactured to have a flow restriction to provide an accurate flow measurement. This restriction particularly in the biogas active digester 1 resulted in biogas leaving digester 1 via the U-tube to digester 2. To overcome the problems, the U-tube was removed and the digester tanks were linked through a straight connection pipe at a low level of the digesters instead (Figure 3b). The connection was well under the top liquor level, removing potential for the biogas volume to migrate from digester tank 1 to digester tank 2. Movement of liquor from digester 1 to digester 2 cannot include for floating fibrous grass. The connection pipe used two valves to control flow. A stainless-steel box was fitted in the middle of the pipe (with a valve on either side) to allow for cleaning in the case of blockage. 4.2.4. Stirring System. The new elastic blade installed (Figure 2a) did not solve the problem of floating grass accumulation as expected. The floating mass continued to accumulate around the digester tank and accumulated volume and height to the extent that damage was caused to the digester system. The buoyant mass was relatively dry and of a high solids content. It blocked the mixing shaft from turning and damaged the agitating motor. It distorted the temperature sensor reading. The temperature of the floating substrate was different to the liquor, and the temperature sensor was installed near the liquor surface (Figure 2a), near the floating grass. This caused the temperature to rise from 35 to 60 °C and resulted in temporary failure of the biological process.
From detailed observations, it was realized that the accumulation of the floating substrate was influenced by the position of the temperature sensor (Figure 2a). This in effect acted as a hook and caused floating grass to accumulate at the sensor probe. The modification was 2-fold, involving the installation of a new more rigid elastic blade and allowing clockwise and counter-clockwise rotation of the mixer. The accumulation of buoyant mass was completely removed, and floating grass consistently submerged below the liquor level. The mixer was set at a continually rotational speed of 2.4 rpm with timed variation in the direction of rotation. 5. Results 5.1. Digester Operation. This paper is concerned with the commissioning of the anaerobic digester system and obtaining a steady-state process in monodigestion of grass silage. The results of the process for 101 days, in which the digester was operated in series as a two-stage CSTR system, are plotted in Figure 4. For the first 20 days of operation, the biological process was not fully functional. Although the CH4 content in the biogas rose, a very low volume of biogas was produced. The pH in digester 1 dropped to 6.0, indicating the instability of the anaerobic digestion process. An assumed factor in this was that the inoculum may have become relatively inactive after stabilizing in the 2 month commissioning period; the inoculum was also diluted because of some losses during the modifications in the commissioning period. Feeding began at an OLR of 1 kg of VS m-3 day-1 for the first 5 days but reduced to 0.5 kg of VS m-3 day-1 because of the shortfall in the measuring capacity of the gas flow meter (as described earlier). Feeding was stopped for a week to replace the U-tube. Bicarbonate was added to digester 1 during the period, increasing the pH to 6.7. The operation started again from the 25th day. The operation continued with an OLR of 0.5 kg of VS m-3 day-1 to the end of the 101 day period. Feeding at 0.5 kg of VS m-3 day-1 continued for 73 consecutive days. The biogas yield continually increased, indicating the process stability improvement. A leak of biogas from the digester was noted on the 45th day; this leak was due to the oil seal at the vertical mixer shaft, which was modified as described previously. However, the operation was again temporarily stopped on the 50th day. Although the digester was fed as usual, biogas was discharged from the digester but not through the gas flow meter and was thus unrecorded. The problem occurred because of the collapse of the accumulated buoyant mass of floating grass that resulting in damage to the mixing system and the previously mentioned rise in the temperature of liquor to 60 °C. The problem with the floating grass was not fully resolved until the 64th day when the mixer shaft was modified to allow for rotation in two directions. 5.2. Methane Yield. The methane yield can be considered in three scenarios or time frames. Scenario 1 (days 56-101): This scenario began when the biological process resumed stability after the mixer failure and the rise of the temperature to 60 °C and continued for 45 consecutive days. Methane production of 457 L of CH4 kg-1 of VSadded was recorded. Scenario 2 (days 64-101): This scenario began when the mixer was put on dual rotation, overcoming any residual potential for accumulation of floating grass, and continued for 37 consecutive days. Methane production was recorded as 460 L of CH4 kg-1 of VSadded. Scenario 3 (days 86-101): This scenario began 60 days after the U-tube 4466
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Figure 4. Results of the experimental two-stage CSTR operation.
removal, thus allowing for complete digestion of any accumulated floating grass, and lasted for 15 consecutive days. Methane production of 455 L of CH4 kg-1 of VSadded was recorded. In scenarios 1 and 2, it could be argued that the accumulated grass that was floating before the U-tube was removed was not available for digestion until day 26 and, thus, would
add to the later methane yield. Scenario 3 is suggested to be the most likely steady-state methane yield of the grass silage in the two-stage CSTR operation. It should be noted however that the variance between these scenarios is very small, at about 1%. The average methane content in biogas corresponding to the methane yield of 455 L of CH4 kg-1 of VSadded is 52.2%, yielding 872 L of biogas kg-1 of VSadded. 4467
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Lehtom€ aki et al. reported a yield of 306 L of CH4 kg-1 of VSadded from ensiled mixture of timothy and meadow fescue in a laboratory-scale batch experiment. Stewart et al.35 reported a yield of 310 L of CH4 kg-1 of VSadded from ryegrass plus clover in a single-stage CSTR system at an OLR of 2.5 kg of DS m-3 day-1 (2.25 kg of VS m-3 day-1 at 90% VS). Higher yields were reported by M€ ahnert et al.34 in a batch experiment ranging from 310 to 360 L of CH4 kg-1 of VSadded for fresh cut meadow, foxtail, and perennial ryegrass. Pakarinen et al.48 used laboratory batch digestion of a mixture of grasses (timothy, red clover, and meadow fescue, henceforth, grass) and ryegrass. The grasses achieved a yield ranging from 320 to 510 L of CH4 kg-1 of VSadded, while the ryegrass yielded values ranging from 390 to 510 L of CH4 kg-1 of VSadded. Pouech et al.49 reported a high range of methane yield between 423 and 627 L kg-1 of VSadded from ryegrass cut at various maturity stages. 6.2. Digester Performance. According to M€ ahnert et al.,34 the biogas yielded from a single-stage CSTR digester is 80-85% of the yield from a batch experiment. Stewart et al.35 indicated that the biogas yields from both systems are very similar. This study on two-stage CSTR suggests agreement with Stewart and co-workers, in that a high performance was achieved with VS destruction of 91% based on the methane yield of 455 L of CH4 kg-1 of VSadded and the maximum methane potential of 500 L of CH4 kg-1 of VSadded (Table 3). The high yield of methane gas in our study may be associated with the low OLR (0.5 kg of VS m-3 day-1), high HRT (221 days), and low solids content in the digester (e.g., 2.13% DS in digester 1 at the end of operation). A similarity is found in a study by Jarvis et al.,37 which stated that a temporary increase in methane production in a singlestage CSTR was always preceded by periods of low OLR or intermittent feeding. Recirculation of liquor also improves the performance of the system. Nordberg et al.,50 in their anaerobic digestion study using alfalfa silage, indicated an increase of the OLR limit to 3 kg of VS m-3 day-1, as compared to 2.25 kg of VS m-3 day-1 in a control reactor without recirculation. 6.3. Solids Content of Digestate. The grass silage digested had a 30.7% DS and 92.5% VS content (Table 1). Thus, in 1 ton of feedstock, there is 307 kg of solids, of which 284 kg are volatile. With a 91% destruction of volatiles, 258 kg of volatiles are converted to biogas and only 48 kg of solids are left from the original ton of feedstock. The DS content of the feedstock is 4.8%, which is about half that of pig slurry. It is suggested that this feedstock as a result may not be suited to handling with a vertical garage door arrangement, as is ubiquitous in dry batch digestion of OFMSW. 6.4. Operation of the Pilot-Scale Grass Digester. 6.4.1. Digester Operation. The operation of an automated pilotscale digester can generate more problems than a lab-scale digester. In effect, the biological process is under investigation 47
5.3. Other Measurements. 5.3.1. pH. Digester 1 appeared to have a slightly higher pH value than digester 2. Average pH values for 45 consecutive days of operation (scenario 1) were 7.39 and 7.25 for digesters 1 and 2, respectively. The maximum pH of digester 1 during the period was 7.5, as compared to 7.34 of digester 2. 5.3.2. Solids Content. The DS content of the initial inoculum when starting up was 0.55% in digester 1 and 0.76% in digester 2. For digester 1, it increased to 2.13% at the end of operation, for digester 2, it increased to 0.93%. The VS content (as a percentage of DS) was in a range of 62.6-66.8% for digester 1 and in a range of 53.9-59.1% for digester 2. No significant trend was discernible. 5.3.3. Alkalinity. Alkalinity values increased with operating time at a higher rate for digester 1 than digester 2. At the end of operation, values of 800 and 510 mg L-1 for digesters 1 and 2 were recorded, respectively. This suggests an increase in the stability of the anaerobic digestion process as compared to the first 20 days of operation, in which the alkalinity values did not appear to improve. 5.3.4. Volatile Fatty Acids (VFA). VFA values were relatively constant for the last 30 days of operation. This indicated a good stability of the system, in which a steady stage was approached. The average values for the period were 70 and 29 mg L-1 in digesters 1 and 2, respectively. A sudden increase of VFA indicates instability. Such an increase can be noticed, for example, after the 50th day, when the system was in a recovery from the process failure because of the temperature rise to 60 °C in digester 1. New grass was still fed to the digester, converted to VFA, but not converted to CH4 as the methanogens were inhibited because of the temperature increase. 5.3.5. C/N Ratio. The C/N ratios decreased with time, especially in digester 1; this indicates an increase in nitrogen in the digester. The ratios decreased from 16 to 10.1 for digester 1 and from 12.6 to 11.6 for digester 2. This was due to an increase in ammonia, an important factor that can cause inhibition to the anaerobic digestion process. This was not sampled during the operation of the digester. However, values were measured at the experiment end, indicating values of 1.2 and 1.0 g of (NH3 þ NH4)-N L-1 for digesters 1 and 2, respectively. 6. Discussion 6.1. Methane Yield. Our production of methane from ensiled perennial ryegrass grown in a temperature oceanic climate using a two-stage CSTR experiment equates to 455 L of CH4 kg-1 of VSadded. From our calculations (Table 3), the maximum potential associated with 100% destruction of volatiles is 500 L of CH4 kg-1 of VSadded. A literature review indicates that the range of values (on the basis of different species, different maturity stages of grass, different grass storage methods, and different digester types) varies from 306 to 627 L of CH4 kg-1 of VSadded. The authors are sceptical about values that result in more energy produced from a system than is input to a system; the values in excess of 500 L of CH4 kg-1 of VSadded however may be based on grasses with higher methane potential.
(48) Pakarinen, O.; Lehtom€aki, A.; Rissanen, S.; Rintala, J. Storing energy crops for methane production: Effects of solids content and biological additive. Bioresour. Technol. 2008, 99, 7074–7082. (49) Pouech, P.; Fruteau, H.; Bewa, H. Agricultural crops for biogas production on anaerobic digestion plants. Proceeding of the 10th European Conference on Biomass for Energy and Industry; CARMEN: W€urzburg, Germany, 1998. (50) Nordberg, A.; Jarvis, A.; Stenberg, B.; Mathisen, B.; Svensson, B. H. Anaerobic digestion of alfalfa silage with recirculation of process liquid. Bioresour. Technol. 2007, 98, 104–111.
(47) Lehtom€ aki, A.; Huttunen, S.; Rintala, J. A. Laboratory investigations on co-digestion of energy crops and crop residues with cow manure for methane production: Effect of crop to manure ratio. Resour., Conserv. Recycl. 2007, 51, 591–609.
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as well as up-scale issues. Attaching automated controls can help to simplify the operation. However, as attested to in this paper, management of the system especially in a commissioning period can be very problematic. An in depth review on control systems and sensors for an anaerobic digester can be found in the work by Ward et al.51 6.4.2. Substrate Mixing. Grass as outlined in the Introduction has been viewed as a troublesome substrate, especially because of floating effects. Bigger issues are related to the potential damage that may occur to the mixing systems. Providing an effective mixing is thus significantly important. 6.4.3. Inhibition. An anaerobic digestion process results in a mineralization of nitrogen and, consequently, a lowering of the C/N ratio.33 A high ammonia content associated with grass can significantly increase the total ammonia concentration in the digester. This is the case when grass is used as the only substrate and operated at a high OLR.26 In our study, inhibition due to high ammonia concentration was not an issue. The value recorded of 1.2 g of (NH3 þ NH4)-N L-1 (digester 1) was well below the inhibition level for grass digestion of 4.7 g L-1.26 Recirculation of digesting liquor is indicated as an accelerant to the increase of the ammonia concentration, causing instability in the long-term operation.50 Intelligent monitoring of operation will be required in long-term monodigestion of grass silage with recirculation. Inhibition caused by ammonia results in rising VFA and a subsequent decrease in the biogas yield. Adjustment of recirculation should be performed on a regular basis to balance the level of VFA in the two vessels and prevent a sudden increase of VFA in any vessel causing instability. Ongoing work on these digesters will investigate the potential level of OLR before inhibition occurs.
the feedstock. Grass digesters are different to maize and OFMSW digesters. In a wet continuous-stirred process (such as a CSTR, which is ideally suited to maize digestion), grass, which has a functional specific gravity of less than 1 and remains less than 1 for 24 h, has the potential to form accumulated floating fibrous agglomerations. This is particularly problematic to the mixing system. The mixing/ agitation system must overcome the tendency of grass silage to float. The flow of digesting material from one digester to another in a grass digester is also affected by the functional specific gravity; flow from chamber to chamber must take place under the liquor level. This prevents fibrous grass accumulations from blocking the pipes between the digesters and the escape of gas from one digester chamber to another. Methane production from Irish silage as digested under these experimental conditions is shown to be very high (455 L of CH4 kg-1 of VS) in comparison to the scientific literature. Methane production equates to about 91% of maximum potential. As a result of the high biological conversion, the digestate from grass silage has a solids content of less than 5% and, as such, may not be suited to handling with a vertical garage door arrangement, as is ubiquitous in dry batch digestion of OFMSW. A further experimental study on the operation of the digester at higher loading rates is under preparation. This paper will investigate the mass balance, the proportion of biogas produced from the primary and secondary digesters, and the ultimate OLR for grass monosubstrate in the twostage CSTR digester. Acknowledgment. Research funding was obtained from the Department of Agriculture, Fisheries, and Food (DAFF) Research Stimulus Fund Project “GreenGrass” and the Higher Education Authority Programme for Research in Third Level Institutes Cycle 4 (HEA PRTLI Cycle 4). We thank Eddie Applebe and Erneside Engineering for all of the input in fabrication, in modifications, and Padraig O’Kiely and Joe McInerney from Teagasc, Grange, for the supply of grass silage.
7. Conclusions A major thesis of this paper is that digesters designed for high solid content feedstocks should vary depending upon (51) Ward, A. J.; Hobbs, P. J.; Holliman, P. J.; Jones, D. L. Optimisation of the anaerobic digestion of agricultural resources. Bioresour. Technol. 2008, 99, 7928–7940.
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