BIOGAS PLANT OF VECAUCE FARM (LATVIA) TARGETED INVESTMENTS FOR IMPROVED ENVIRONMENTAL EFFECT

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TARGETED INVESTMENTS FOR IMPROVED ENVIRONMENTAL EFFECT By Sari Luostarinen, MTT AgriFood Research, Knud Tybirk, Agro Business Park & Iveta Grudovska, Vecauce Farm. The research and training farm ‘’Vecauce’’ built the first experimental biogas plant in Latvia in 2008. The problem with the plant is that it was designed as “a catalogue plant” without any tailoring to meet the needs of the farm and its location. Five years of operation clearly indicated that improvements are required to enhance its operation and the subsequent environmental services provided by the plant. Investments into the improved design and operation have been planned and partly implemented with support from the project Baltic Compact. The total amount of cattle on the Vecauce farm is 1100, half of which are dairy cows. The crop rotation on the 1700 ha of fields consists of 250 ha of maize, 500 ha of cereals, 250 ha of winter rape, 100 ha of pasture, 300 ha of grass and 100 ha of other production (orchard, experimental fields). Vecauce is located on the Latvian nitrate vulnerable zone (Nitrate directive) with strict fertilising plans. The farm fertilises the fields annually with 220 tons of NPK and 260 tons of N in addition to the digestate from the biogas plant. 2

BIOGAS PLANT ORIGINALLY The biogas plant in its original form consisted of a 2000 m3 mesophilic digester and a 4000 m3 open storage tank. Remote open storages for digestate were also available, adding up to 7 months of storage capacity per year. The digestate produced is used as a fertiliser on the farm fields. The digester is equipped with feeding devices for both slurry and solid feed materials. The solid materials are lightly chopped using vertical mixers. The average feed to the digester was originally 13 000 - 14 000 t/a of dairy cow slurry and 4 300 - 5 500 t/a of maize silage. Some grass silage was also used in years of low maize yield, even though the plant was not designed for it. The biogas is converted to energy with combined heat and power (CHP: 260 kWel + 350 kWth). The plant is expected to fulfil an annual quota of at least 1820 MWh of electricity produced to the grid. The heat (approx. 2500 MWh/a) from the CHP is mostly utilised on the farm to replace wood. During summer, some heat is wasted.

SILO FOR GRASS SILAGE SILO FOR MAIZE SILAGE

Electricity

Cutting with vertical mixers

Heat

CHP Biogas

SLURRY FROM ANIMAL SHELTER DIGESTER

Digestate use with broad spreading

OPEN STORAGE FOR DIGESTATE

PROBLEMS WITH THE ORIGINAL DESIGN

tions should be taken into account farm-specifically.

Several drawbacks have been identified in the original biogas plant on Vecauce farm. The problems date back to the fact that the plant was originally a basic prototype plant for any (German) farm and not at all tailored to the needs of this specific farm. Such an approach of “prototype” agricultural biogas plants is not recommendable. Instead, the requirements for the technical design and operation as well as the environmental and climatic condi-

The problems with the design and operation affect the environmental services the plant is expected to deliver as follows: RENEWABLE ENERGY One of the important environmental services expected of the original biogas plant in Vecauce is production of renewable energy as biogas. This has been of national interest In Latvia, due to the set goals in the share 3

of renewable energy in all energy consumption. However, the subsidy system for Vecauce supports only electricity production and is dependent on a pre-approved quota to be filled each year. The plant has to struggle to produce sufficient energy for the quota and this has required use of maize silage. Moreover, the heat produced was not subsidised and until 2014 there was no requirement for its utilisation. CHOICE OF CO-SUBSTRATE FOR SLURRY In order to fulfil the electricity quota, slurry requires maize silage as a co-substrate. The use of glycerine, a by-product from biodiesel production, was also originally considered. However, its quality was not stable and its high sulphur content caused 4

risks to the equipment. Moreover, Latvian biodiesel production was stopped and no glycerine has been available since. Maize is an annual energy crop which degrades readily in the biogas process and holds a high methane production potential. Moreover, the farm already produces it for cattle feed and has sufficient field area to produce maize also for biogas purposes. However, the biogas use of maize has several environmental consequences making it environmentally unsustainable despite the renewable energy produced. The most important drawback appears to be indirect land use change causing increased emissions in producing the substitute for the maize now utilised in biogas production1,2. However, also impacts on local scale may cause concern over maize utilisation. E.g. as an annual crop it leaves the soil bare

for the winter, resulting in higher potential for nutrient leaching and gaseous emissions from the soil. Annual soil tillage increase the GHG emissions and pesticide usage for annual crops is higher that for perennial grasslands.

NUTRIENT RECYCLING Another environmental service intended for the biogas plant is to enhance reuse of slurry nutrients. The biogas process releases part of the organic nitrogen as soluble ammonium nitrogen. This increases the fertiliser value of the digestate as compared to untreated slurry. Simultaneously nutrient leaching is expected to decrease due to more efficient uptake of especially nitrogen.

gen losses as ammonia evaporation and nutrient run-off 3,4. The quantity of lost nitrogen depends on the temperature and pH of the digestate, on the formation of a possible natural crust as a storage cover, as well as on timing, conditions and method of the spread. The ammonia emissions from storage and spread are potentially higher than with untreated slurry due to the higher pH and ammonium content of the digestate. Any lost ammonia decreases the fertilising value of the digestate and causes several harmful environmental impacts, such as acidification, eutrophication and formation of unhealthy particles. Moreover, broadband spread results in ammonia loss during spread and does not deliver the nutrients to the root of the plant efficiently, causing higher risk of nutrient run-off especially during rainy conditions.

However, the original uncovered and insufficient storage capacity for the digestate and its original broadband spreading will result in nitro5

GHG EMISSIONS The biogas plant is also expected to reduce direct and indirect GHG emissions from slurry handling. The closed process enables controlled degradation of the organic material and collection and utilisation of the methane produced. However, the original short retention time of the digester (approx. 33 d) and the uncovered storage afterwards most likely result in higher GHG emissions than when storing raw slurry. The degradation of the organic material and thus biogas production continues in the storage tank, especially during warm seasons and subsequent high microbial activity. The resulting methane emissions can be significant. 6

Also nitrous oxide is formed directly during storage and indirectly in field soils when part of the nitrogen in the digestate is nitrified into nitrite and nitrate.

CALL FOR IMPROVEMENTS With targeted changes, however, the plant design and subsequently its environmental impacts in Vecauce can be improved. Thus, new investments were planned to improve its design and operation during Baltic Compact project (2012-2014). Several options were raised: 1. Addition of an efficient pre-treatment to enable deep litter and grass silage as substrates instead of maize silage. 2. Addition of macerating pump in order to avoid constant pump breakages due to impurities in slurry. 3. Addition of post-digestion in order to increase retention time for improved degradation and to collect residual methane for energy use. 4. Increased digestate storage capacity in order to improve timing of spread on fields. 5. Covering the digestate storages. 6. New spreading machinery in order to improve nutrient use and to decrease emissions. FOUR INVESTMENTS MADE, TWO UNDER PLANNING Within the project lifespan, four of the improvements proposed were done: • • • •

increased storage capacity, addition of a macerating pump, a new spreader with trailing hoses, pre-treatment as a new biomass feeding system with integrated chopping system.

The storage covers and post-digestion were not yet realised. The planning to purchase them later does, however, continue.

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HOW ARE THE ENVIRONMENTAL SERVICES IMPROVED? In the following, we go through all the improvements proposed, including also the as-of-yet planned post-digestion and pre-treatment of solid substrates. How would they affect the environmental services provided by the biogas plant as compared to the original design? What can be improved? RENEWABLE ENERGY In order to enable more versatile use of available substrates and to ensure reaching the quota for electricity SILO FOR GRASS SILAGE STORAGE FOR DEEP LITTER

production, improved pre-treatment of solid substrates and the prolonging of the retention time with post-digestion are recommended. For instance in Denmark, extrusion of grass materials has resulted in 15-27% higher biogas production as opposed to no pre-treatment5. According to experience in a farmscale biogas plant in Finland, co-digesting dairy cattle slurry and grass silage, approximately 20% of the all methane produced originates from post-digestion. For Vecauce, this would mean an additional 360-550 MWhel and 500-750 MWhth annually, when Electricity

PRETREATMENT

Heat

CHP Biogas

SLURRY FROM ANIMAL SHELTER

MACERATING PUMP

DIGESTER

Digestate use WITH TRAIL HOSES

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TWO STORAGES FOR DIGESTATE (covered) POSTDIGESTER

estimating with an average 20-30% increase from the original biogas production. The environmental service of saved GHG emissions via replacing fossil energy would be significantly higher than originally. Moreover, the quota of subsidised electricity would be met without problems. Thus, the already added pre-treatment of solid substrates and the planned addition of post-digestion maximise the environmental services for additional energy, additional income and minimised emissions. As the pre-treatment was installed just before writing this article, there is no data on its actual effect on methane production yet. CHOICE OF CO-SUBSTRATE FOR SLURRY

ble replacing maize silage with grass silage as the co-substrate. Grass silage contains a high methane production potential, but it is somewhat less readily degraded than maize. Hence, the need for pretreatment and longer retention time in order to ensure at least as high biogas production as with maize. The environmental sustainability of perennial grasses is better than that of annual crops, as maize, due to offering covered soils during the winters and especially on organic soils. They are also expected to improve the soil structure and efficiently take-up nutrients. Grass silage may be produced on existing grasslands by enhancing the production for higher yields and thus causing no indirect land use change.

In order to diminish the environmental effects of using energy crops, the new pre-treatment and prolonged digestion time will ena9

NUTRIENT RECYCLING Increased storage capacity enables better timing of digestate spread, i.e. during vegetative stage when the plants take up the nitrogen directly and emissions both as ammonia and as leaching into waters are minimised.. The new storage tank already built increases the storage capacity of the Vecauce farm to 8 months per year. This is one month more than the required minimum in Latvia. Covering the storages would improve ammonia preservation, but has not yet been done. However, the natural crust formed on top of the digestate serves as a cover and decreases ammonia volatilisation. The pending investment into postdigestion would further improve ammonia utilisation as fertiliser. This

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is further support for its implementation. The new trailing hoses improve the precision of digestate spread and improve digestate absorption into soil. Injection would improve nutrient take-up even more. Also, acidification would decrease ammonia emissions. From agro-technological point of view, injection technology is not applicable for leguminous cultures. Since Vecauce is growing alfalfa, the choice of trailing hoses spreading system allowed balancing environmental, economic and technological aspects. GHG EMISSIONS The already made investments do not directly improve GHG mitigation in the Vecauce biogas plant. Pre-treatment may decrease potential emissions due to the expected

improved degradation in the digester. However, if the pending investments into post-digestion and covered storages are implemented, the GHG emissions would be reduced to a minimum, as follows:

thus having a maximum positive impact on biogas technologies used in Latvia.

The longer retention time, which post-digestion provides, further increases degradation. The digestate to be stored has little degradable organic matter left and thus little methane emissions from the storage can be expected. Covering the storages further reduces direct GHG emissions.

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Additionally, with the correct timing and improved method of the spread, less nitrogen is converted to nitrite and nitrate in the soil, thus decreasing the indirect emissions as nitrous oxide.

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REFERENCES Hamelin 2013. PhD thesis. University of Southern Denmark. Meyer-Aurich et al. 2012. Renewable Energy 37, 277-284. 2

Amon et al. 2006. Agriculture, Ecosystems and Environment 112, 153-162. 3

Amon et al. 2006. Agriculture, Ecosystems and Environment 112, 153-162. http://agro-technology-atlas.eu/ techdescs.aspx?techgroup=500 5

PERSPECTIVES FOR FURTHER IMPROVEMENTS OF BIOGAS IN LATVIA As Vecauce is a training and research farm of the Latvia University of Agriculture, the investments create optimal opportunities for disseminating environmental issues to students in agricultural, engineering and biological sciences. In addition, the experiences from the investments will reach farmers and policy makers through numerous delegation visits at Vecauce, 11

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