Biogas and fatty acids produced from agricultural biomasses for industrial use.

The Swedish Industrial Biogas Commission is calling for 10 TWh of biogas/year (via digestion and gasification; by 2030). Current production is about 2 TWh/year, mainly from waste and sludge. The supply of organic waste is not sufficient to produce the required biogas.Agriculture has significant amounts of residual biomass that can be digested (mainly manure and straw). With this fact taken into account, this project report also assumesthat smaller parts of the arable land can be used for growing nitrogen-fixing grass/clover ley for biogas production, perhaps in combination with the production of protein feed for agriculture and fatty acids for industry in a biorefinery concept.It is possible to use manure, straw and ley with smaller amounts of waste in the western part of Sweden (Västra Götaland, Skåne and Halland) to produce 3.5 to 5 TWh of biogas/year in large biogas plants (approx. 100 GWh/plant and year) for use in industry. Co-production of fatty acids and biogas is also possible, e.g. at least 16 plants are needed to cover identified industrial needs.There are good opportunities for Bio-CCS, partly at the biogas plant, when biogas becomes biomethane, and partly in the industry where biomethane is used. Negative emissions possible, corresponding reduction of climate gases when biomethane replaces natural gas (5 TWh biomethane with CCS can reduce CO₂ emissions by about ¾ for the chemical and refinery industry segment). CO2 can also be used for production of emethane (Bio-CCU), but electricity shortages are a likely bottleneck.The price of natural gas (including tax) compared to biogas with existing subsidies is estimated to be relatively similar. The current subsidy system is directed towards manure digestion, which only produces about 1/5 of the potential biogas from agricultural biomass, which is why subsidies need to be modified to produce the biogas in demand. Fatty acids can also be produced using primarily pasture and waste via a biological process at a similar price level as today's fossil-based production method.A future investment in building biorefineries, which generate renewable commodities can be one solution for the industrial green transition, with agricultural biomasses, but this can also contribute to the green transition of the agriculture. Difficulties with the studied system is that it is large with many actors, significant investment is needed to be realized, and clear incentives are needed to become an actor in the system also includingthe farmers, and there are technical and biological uncertainties in function. A clear question is who is prepared to take the lead in realizing this?

This study conducted a life cycle assessment (LCA) of manure management, identifying transportation as a major contributor to global warming and freshwater eutrophication impacts. Transporting substrates to the biogas plant was the main hotspot, highlighting a critical area for improvement. The findings emphasize the importance of method selection in geographically dependent assessments, especially in the Baltic Sea region. Characterization factors specific to Sweden revealed higher environmental impact values than those produced by the ReCiPe method, underscoring the need for regional differentiation in LCA. By optimizing manure management practices and enhancing nutrient distribution, impacts on both climate change and eutrophication can be significantly reduced, thereby lowering nutrient flow to the Baltic Sea. Combining these optimizations with transportation impact reductions further amplifies these environmental benefits, demonstrating that geographically tailored approaches in LCA offer essential insights for managing regional-scale effects. 

The biomass potential in Swedish agriculture needs to be used more effectively to produce food, feed and energy in the future and to meet Swedish sustainability goals by 2030. In the project, a biorefinery concept was designed, that consists of the three processes biogas, ley protein and bio-oil. The concept was evaluated with costs and mass balances regionally for Västra Götaland Region. Lab scale trials to produce bio-oil and biogas were also carried out. The results showed that the concept has potential to produce biofuel, protein feed and plant nutrition from agricultural residues and the cultivation of ley and create a high degree of self-sufficiency. However, a more in-depth techno-economic analysis is required as well as an analysis of possible obstacles and bottlenecks. 

The biomass potential in Swedish agriculture needs to be used more effectively to produce food, feed and energy in the future and to meet Swedish sustainability goals by 2030. In the project, a biorefinery concept was designed, that consists of the three processes biogas, ley protein and bio-oil. The concept was evaluated with costs and mass balances regionally for Västra Götaland Region. Lab scale trials to produce bio-oil and biogas were also carried out. The results showed that the concept has potential to produce biofuel, protein feed and plant nutrition from agricultural residues and the cultivation of ley and create a high degree of self-sufficiency. However, a more in-depth techno-economic analysis is required as well as an analysis of possible obstacles and bottlenecks. 

The agricultural industry plays a crucial role in transitioning towards a sustainable and fossil-free future. This article explores the potential of biorefineries using biomass from agriculture to reduce emissions and promote self sufficiency. Regarding a concept that integrated anaerobic digestion, grass and legume protein production, and hydrothermal liquefaction. A case study was conducted in the southwestern part of Sweden, involving interviews with a biogas plant and local farmers. The study analyzed the utilization of input goods in agriculture and evaluated the potential of biomass in the area. To assess the potential for farms to become self-sufficient in fuel, protein feed, and plant nutrients. The results show an overall positive outlook of the biorefinery concept. By utilizing 20% of the available biomass in the area can the biorefinery concept annually produce 100 GWh of biogas, 3800 tonnes of grass and legume protein concentrate and 1200 GWh bio-oil. This could theoretically cover 100 % of the need of soy meal, 44% for nitrogen, 50% for phosphorus and 100% for potassium.

Agricultural Biorefinery - combining local and regional scale In order to achieve Sweden's sustainability goals and an increased degree of self-sufficiency, our resources need to be used in an innovative way. Resources that today are classified as residual streams can be used in a smarter way to produce the future's food, feed, fuel and energy. There is a great potential in utilizing agricultural biomasses. In the project, the potential of agriculture to supply ILUC-free feedstock to a local and regional biorefinery concept was calculated and the system was evaluated through mass and energy flow calculations, cost calculations and case descriptions on Vårgårda Herrljunga Biogas Plant (VH Biogas). In addition, practical tests were carried out on bio-oil production from dewatered digestate from participating biogas plants. Quantifications were also carried out of how the concept contributes to more resource-efficient crop cultivation with maintained humus content in soil despite increased removal of biomass from the farm. ...

Ett tjugotal lantbrukare i Mönsterås kommun planerar för att bygga en stor biogas­anläggning som kommer att ha fastgödsel från höns som huvudsubstrat. Den gemen­samma biogasanläggning kan bli den största i sitt slag i Sverige, med en biogas­produktion på ca 70 GWh biogas/år för produktion av drivmedel. Den genererade rötresten kan kväve­försörja ca 12 000 ha/år växtodling och fosforförsörja ca 20 000 ha/år. Den rötade gödseln bidrar i princip till all växtnäring i rötresten, där fjäderfä­gödseln kommer att bidra med ca 80 % av rötrestens innehåll av kväve och fosfor. Ett hinder för att realisera denna anläggning är att det idag saknas erfarenhet från praktisk drift av rötningsanläggningar med betydande inblandning av fjäderfägödsel. Fjäderfä­gödsel är både ett fosfor- och kväverikt substrat, och innehåller dessutom både tunga och lätta partiklar som riskerar att bilda sediment och svämtäcken i rötkammaren. Idag betraktas fjäderfägödsel framför allt som ett fosforgödselmedel eftersom huvuddelen av kvävet inte är direkt upptagbart av växter, men rötning av fjäderfägödsel ökar andelen direktverkanande kväve. För att optimera hantering, spridning och utnyttjande av kväve och fosfor i fjäderfägödsel är det önskvärt att processa denna gödsel för att generera fyra stycken gödselmedel med olika kväve- och fosforkvot. För att kunna studera dessa fråge­ställningar och bredda gödselbasen för rötning har Vinnova, under 2016-2018, finansierat denna tillämpade studie.

Under ett tillämpat försök genomfördes rötning i pilotskala av kväverik hönsgödsel som huvudsubstrat. Rötningsprocessen gick att driva stabilt vid en ammoniumkvävehalt på närmare 6 g/l inom det mesofila temperaturområdet i en totalomblandad rötkammare med volymetrisk metanproduktion som uppgick till 1,1 m³ CH4/m³ slamvolym & dag. Järnklorid tillfördes rötkammaren för att hålla biogasens innehåll av svavelväte vid ca 100 ppm. För att hålla ammoniumhalten vid angiven nivå tillfördes vatten motsvarande halva inflödet in i rötkammaren, för att späda ner kvävet till denna nivå. Efter rötningen av substratblandningen ökade dess ammoniuminnehåll med 3,7 gånger.

Under pilotförsöket producerades ca 13 m³ rötrest som sparades och lagrades 4–8 månader innan den förädlades i två steg till två olika gödselmedel. I första steget genomfördes fasseparation med en dekantercentrifug följt av indunstning av surgjord tunnfas. Baserat på dessa försök beräknas fassepareringen kunna generera ett fast gödselmedel vars vikt motsvarar 23 % av i rötningsprocessen producerad rötrest. Det fasta gödselmedlets innehåll av fosfor utgör dryga 70 % av rötrestens fosforinnehåll. Vidare beräknas indunstningen kunna generera ett uppkoncentrerat flytande gödsel­medel vars vikt motsvarar 2o % av i rötningsprocessen producerad rötrest. Det flytande gödselmedlets innehåll av ammoniumkvävet utgör dryga 70 % av rötrestens ammonium­innehåll.      

Det producerade koncentratgödselmedlet liksom tunnfasgödselmedlet hade en hög andel lättillgängligt kväve, vilket är en viktig delförutsättning för att uppnå höga skördar vid växtodling. Gödselmedel från Mönsterås Biogas planerade biogasanläggning kan i framtiden bidra till att jordbruket i Kalmar län med omnejd kan gå mot en hållbar intensifiering av livsmedelsproduktionen.

Digestion of poultry manure with digestate processing in pilot scale tests

Twenty farmers in the municipality of Mönsterås in southern Sweden are jointly planning to build a large biogas plant. The plant will digest a very high proportion of poultry manure, resulting in significantly greater biogas yield than normally expected, and higher nitrogen contents in the digestate. A major obstacle to realising the biogas plant is controlling digestion operation under high nitrogen levels resulting from the considerable amounts of poultry manure substrate. Poultry manure also contains both heavy and light particles that can cause challenges with poor stability in the digestion process, formation of sediments and crust in digester tanks. The high nitrogen contents together with high phosphorus and solids concentrations will also create difficulties for digestate management and use as fertiliser. 

Today solid poultry manure is mainly used as a phosphorus fertiliser, not only due to high concentration of phosphorus but also due to relatively poor utilisation of the nitrogen by crops. However, due to the high concentration of phosphorus, the application rate needed to meet plant needs is lower than modern solid manure spreaders can evenly apply. Over application and inefficient use of nitrogen increases risk of nutrient losses to waters and the environment.

To study these issues for biogas production, Vinnova (Sweden's innovation agency) has supported this research project in applied digestion and digestate processing.

The project contained the following components: i) 6 months digestion tests with prospective substrates in a pilot-plant with 5 m³ active digesting volume, provided with mixers and pumps commonly used in full-scale plants, ii) laboratory tests to determine biogas potential for feedstocks, and to determine the potential for increased gas production by post-digestion, iii) applied trials of separating and concentrating the digestate with centrifuge followed by industrial evaporation of the liquid phase, iv) analysis of the nutrient value and the function of the concentrated fertilizer in organic farming.

The biological and technical operational performance in the pilot test was evaluated in a complete stirred tank reactor at mesophilic temperature during co-digestion of poultry manure, liquid manure and glycerol. The poultry manure contributed with approx. 70% of dry matter in the substrate mixture and 80% of the nitrogen and phosphorus. The digestion process was stable with NH₄-N levels close to 6 g/l. To control hydrogen sulphide in the biogas to approx. 100 ppm, ferric chloride was added to the digester. Volumetric methane production reached 1,1 m³ CH4/ m³ digester and day. Significant formation of sediment occurred in both digester and in pipes, however, no crust formation was observed in the digester. The outflow of ammonia from the digester by the digestate was 3.7 times higher than the ammonia inflow by the substrate mixture. Thirteen tonnes of digestate was produced during the pilot test. The digestate was separated with a decanter centrifuge generating a solid fraction corresponding to 23% of digestate weight and approx. 70% of the phosphorus in the digestate. Sulfuric acid was added to the liquid fraction generated by the centrifuge before evaporation to stabilize the ammonium nitrogen. Industrial evaporation of the liquid fraction produced a concentrate corresponding to 23% of digestate weight and containing approx. 70% of the ammonium nitrogen in the digestate.

The pilot test generated four different fertilisers, (digestate, solid fraction, liquid fraction and concentrate) each with very different physical and chemical properties. Digestate processing increased the N/P ration of the liquid fraction and concentrate allowing more balanced N and P supply to crop demand, reducing the risk of nutrient losses to waters but also increasing the resource use efficiency of the plant nutrients. Processing was also successful at concentrating the two of the fertilisers, enabling cost effective long-distance transport for use in areas with low animal density and a need for the soil amendment properties of from manure.

These project results have contributed to plans for a full-scale plant by developing the basis of design and the credibility for implementation, resulting in an investment grant and the formation of a new economically stronger company.  Based on the results from the project, the estimated production of biogas in a full-scale plant is 70 GWh /year for renewable automotive fuels. Post-digestion of the digestate with 10 days retention time can increase biogas production with an extra 3 GWh/year (4%). The solid and liquid products can fertilise 12 000 ha/year of organic cultivation with nitrogen and up to 20 000 up to ha/year with phosphorus.

Kunskap om effektiva, funktionella och ekonomiska åtgärder krävs för att säkerställa små utsläpp av växthusgaser från lager med både orötad och rötad gödsel. I detta treåriga projekt har olika tänkbara åtgärder i flytgödsellager studerats genom mätning av växthusgaserna metan och lustgas under sommarförhållanden. Åtgärder som förlängd utrötningstid och surgörning av gödsel med svavelsyra, har utvärderats i RISE pilotskaleanläggning för lagring av flytgödsel. Åtgärder för att minska lustgasemissioner bildat i svämtäcke på gödselyta i ett fullskalelager har studerats på gårdsnivå. Kompletterande teoretiska beräkningar har utförts för att bedöma effekten av att täcka flytgödsellager samt laboratoriestudier av temperaturens påverkan på metangas-emissionerna.

Grundläggande är att temperaturen har stor betydelse, vilket visades i laboratorieskalan. Vid ökad temperatur ökade metanproduktionen exponentiellt för rötad gödsel medan för orötad gödsel var ökningen betydligt mindre. De teoretiska värmebalansberäkningarna för lager med gödsel visade att beskuggning av gödselytan eller täckning av lager med vitt tak bör kunna reducera denna uppvärmning kraftigt på våren eftersom värmeinstrålningen från solljus till gödsellager kan förklarade största delen av gödselns uppvärmning.

Studierna under första och sista året visade att metanemissionerna var signifikant större från gödseln när den var rötad än om den var orötad. Sammanlagda förlusterna av metan var 2,5 respektive fyra gånger så höga från den rötade gödseln under sommarlagringarna (ca fyra månader). Det betyder att det är speciellt viktigt att sätta in åtgärder vid lagring av rötad gödsel för att begränsa utsläppen av metan och därmed minska klimatpåverkan.

En åtgärd för att få lägre metanemissioner från den rötade gödseln är att förlänga utrötningstiden, dvs. den hydrauliska uppehållstiden i rötkammaren. Studierna år 1 visar att vid en fördubblad uppehållstid, 48 dagar istället för 24 dagar, minskade metanemissionerna från lagret med 30 procent. På gårdar med rötningsanläggningar är ett gastätt tak med uppsamling av biogasen också en bra åtgärd för att effektivisera anläggningen och förhindra utsläpp av klimatgaser från lagret.

Surgörning av flytgödsel med svavelsyra praktiseras främst i Danmark för att minska ammoniakavgången från flytgödsel, i stall, lager och vid spridning. Resultaten visar att det är en mycket effektiv metod för att minimera metangasemissionerna från lager med en reduktion med mer än 90 procent både för orötad och för rötad gödsel. Speciellt för gödselslag där det inte bildas naturligt svämtäcke kan surgörning vara intressant för att minska både ammoniak- och metanemissioner.

Åtgärder som surgörning av svämtäcket för att minska lustgasemissioner visade sig inte behövas eftersom lustgasemissionerna var relativt låga, trots att svämtäcket var bortåt en halv meter tjockt. Den finhackade halmen som användes som strö, bildade ett slätt och tätt svämtäcke på gödselytan vilket troligen hämmande lustgasbildningen, till följd av att luften inte kunde penetrera skiktet. Så finhackningen av halmströ kan eventuellt vara i sig en tänkbar åtgärd, vilket också kan minska ströåtgången.

Metanproduktionen från en rötkammare är ofta svår att mäta, och beräknas därför ofta indirekt utifrån producerad elproduktion. Ett exempel på nyckeltal för att visa klimateffektiviteten hos anläggningen visas där metanemissionerna från lager under sommaren var 10,2 % av producerad mängd metan från rötkammare vid enstegsrötning under 24 dagar respektive 5,5 % vid tvåstegsrötning under 48 dagar. På årsbasis blir procenttalen betydligt lägre eftersom emissionerna är låga under vintern.

Ensuring low emissions of greenhouse gases from both undigested and digested animal slurry in storage requires a knowledge of effective, functional and economic measures. This three-year project has studied various potential measures for use in slurry storage. The greenhouse gases methane and nitrous oxide have been measured under summer conditions. Measures such as extended digestion time and acidification of slurry with sulfuric acid have been evaluated in a RISE pilot-scale plant for slurry storage. Measures to reduce nitrous oxide emissions formed in floating crust in a full-scale storage have been studied at farm level. Complementary theoretical calculations have been carried out to assess the effect of covering slurry stores. The impact of temperature on methane emissions has been studied in the laboratory.

The fundamental point demonstrated on the laboratory scale is that the temperature is highly significant. As the temperature rose, methane production increased exponentially for digested slurry. For undigested slurry, the increase was considerably less. Most of the heat gained by the slurry can be attributed to solar radiation. Theoretical thermal balance calculations for slurry in storage indicated that it should be possible to reduce this heating significantly in spring by shading the slurry surface or provide the storage with a white roof.

The studies in years 1 and 3 showed that methane emissions were significantly greater from digested than from undigested slurry. The total loss of methane from digested slurry was 2.5 and four times higher, respectively, during summer storage (approx. four months). It is therefore particularly important to implement measures to limit methane emissions from digested slurry in storage, thereby reducing the impact on the climate.

One way to achieve lower methane emissions from digested slurry is to extend the duration of digestion, i.e. the hydraulic retention time in the digester. The studies in year 1 showed that doubling the retention time from 24 to 48 days reduced methane emissions from storage by 30 percent. At farms with digestion plants, a gas-tight roof with biogas collection is also an effective way to make the plant more efficient and prevent emissions of greenhouse gases from storage.

Acidification of slurry with sulfuric acid is practiced in Denmark, to reduce ammonia emissions from slurry in housing, in storage and during spreading. The results show that it is also a very effective method for minimizing methane emissions from storage, with a reduction of more than 90 percent for both undigested and digested slurry. Acidification may be of interest as a way of reducing emissions of both ammonia and methane, particularly for types of slurry that do not naturally form a floating crust.

Measures such as acidification of the floating crust to reduce nitrous oxide emissions did not prove to have effect because nitrous oxide emissions were relatively low, despite the floating crust being nearly half a metre thick. The chopped straw used for litter formed a smooth and dense floating crust on the surface of the slurry, and probably inhibited nitrous oxide formation because air was unable to penetrate the layer. Chopped straw litter in itself could therefore be a potential measure. This might also reduce straw consumption.

Methane production from a digester is often difficult to measure and is therefore often calculated indirectly from the electricity produced. An example of key indicator for the climatic efficiency of the plant is given. For storage in summer, 10.2% of the methane produced was emitted during one-stage digestion over 24 days, and 5.5% during two-stage digestion over 48 days. The annual percentages are considerably lower because of low emissions in winter.