SWEET SORGHUM BIOGAS PLANT IN TEMPERATE REGIONS (BELGIUM) - DEMONSTRATION PLANT FOR BIOGAS AND HIGH VALUE BIOFERTILIZER PRODUCTION

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1 SWEET SORGHUM BIOGAS PLANT IN TEMPERATE REGIONS (BELGIUM) - DEMONSTRATION PLANT FOR BIOGAS AND HIGH VALUE BIOFERTILIZER PRODUCTION Andrea Salimbeni European Biomass Industry Association Rond-Point Schuman 6, 1040, Brussels, Belgium ABSTRACT: Sweet sorghum is one of the most interesting energy crops. Due to its high photosynthetic efficiency (3-4%) S.Sorghum hybrids have an impressive biomass yield per hectare at all latitudes ( up till 95 tons of fresh biomass per ha per cycle). S. Sorghum is often cultivated in intensive plantations for the well known bioethanol production. Anaerobic digestion is becoming a valuable alternative for s.sorghum processing due to the lower costs and more simple installation. However, electricity and heat are still the main product obtained by biogas plant where s.sorghum is processed. Objective of this study is to focus on high value biofertilizer production from sweet sorghum energy crop, in order to evaluate the application of a new advanced anaerobic digestion technology to the large sweet sorghum plantations as solution for biofertilizer production on site replacing artificial fertilizers demand, thus reducing costs and GHG emissions. All of these aspects well be studied in a demonstration plant to be installed in Belgium. The following paper can be easily divided in five main studies: 1-Evaluation of Sweet Sorghum Yield in Temperate region; 2-New anaerobic digestion technology for fast sweet sorghum digestion; 3-First evaluation of digester dimension and energy yield; 4-Biofertilizer production technology; 5-Preliminary economic evaluation. Keywords: biofertilizer, sorghum, anaerobic digestion, biogas, digestate, pretreatment 1 SWEET SORGHUM IN TEMPERATE REGIONS S. Sorghum hybrids are often cultivated in tropical region for sugar and ethanol production. Intropical region this plant provides two cycles per year and an impressive biomass yield per hectare per cycle. However, the high photosynthetic efficiency makes this biomass energy crop a valuable solution for many bio-products as well as for many European region. Trials made in Belgium showed a biomass yield of about 70 tons of fresh biomass per hectare, with a total grain production of about 5,4 tons per ha. Of course only one cycle can be provided in this cold weather country 1.1 Suitable Land Sorghum is mainly grown on low potential, shallow soils with high clay content, which usually are not suitable for the production of maize. S.S. usually grows poorly on sandy soils, except where a heavy textured subsoil is present. Optimal clay percentage in soil for sweet sorghum plantation is 10 % - 30 %. At the same time, this plant is more tolerant of alkaline salts than other grain crops and can therefore be successfully cultivated on soils with a ph (KCl) between 5.5 and 8.5. Source: FAO 1.2 Behaviour in low temperature conditions Water does not represent a problem. Sweet Sorghum requires about 4000 cubic meter of Water per ha. Irrigation is required but rainy days in Belgium are estimated to provide a valuable amount of water to be stored. Source: FAO The lower temperature for germination is 7-10 ºC. However, the biomass yield in these condition is very much lower than the real potential of this energy crop. In order to have a suitable growing rate of Sweet Sorghum, a temperature of 15 ºC is recommended to have a 80 % of seed germinate. The best time to plant is when there is sufficient water in the soil and the soil temperature is 15 C Minimium sowing temperature is 14 C, so spring time is recommended for firs sowing activity. However, the optimum temperature for growing period are C. These high temperature required can represent a problem in a cold region as Belgium is. However, many trials demonstrated that a general temperature of 20 C can provide a valuable result in terms of biomass yield and sugar content. 1.3 Belgium cultivation parameters With relative high temperatures registrated only in summer months like august and july, The plant yield decreases of about 15%!. Sweet Sorghum Intensive plantations provide about tons/ha with ideal climate conditions. With a very fast growing period of 120 days. The estimated cultivating condition for sweet sorghum hybrid in Belgium are defined as follows: Sowing: May Days requie before harvesting: 150 Harvesting: September / October Due to the low temperature in September /october, the sugar content fermentation does not start immediately and does not affect the storage. Thus, the harvesting period can be extended of about 50 days, also using specific different varieties. Expected yield: 70 fresh tons per hectare. Figure 1. Belgium average monthly temperature. Source: Belgium climate forecast 725

2 2 NEW CONCEPT S. SORGHUM PRETREATMENT SYSTEM BEFORE ANAEROBIC DIGESTION Even with low temperature, the main problem of sweet sorghum silage usually produced for the biogas production in many countries is represented by sugar losses and time of retention: Sugar Losses: 10% after two weeks storage Mesophilic bacteria anaerobic digestion. RT: days The strategy for reducing the sugar losses and increasing retention time is focused on a quenching treatment of Sweet Sorghum sugar stalks. Immediately after harvesting process. As first step, an overview on Sweet Sorghum advanced harvesting logistic system is shown below: Harvesting Machine: CASE 7000 adopted and modified for sorghum cultivation Table I. Sweet Sorghum Harvester parameters. Source: Case Max high 4,5m Max efficiency of stalks harvesting Max harvesting speed Fuel Capacity Fuel consumption: Average Harvesting speed 1,5 t/min 9 km/h 480 l ~ 80 l/ha 1 hour/ha Estimated price Mantenaince cost 2% Sweet Sorghum harvesting machinery utilized will allow to separate three different products. A tractor will operate next to the harvester in order to collect all the plant components. In addition, a Baler will be used to collect leaves left on the ground. Grains & Panicles: Trailer 1 Leaves: Ground. Bailing machine Stalks: 30 cm cut in trailer 2 A specific machinery will be used to separate grains from panicles: Grain Cleaner: Cicoria Atx 2000 C. Table II. Grain cleaner parameters Function: Separation of grains to panicles Cost Placement: Trailed by harvester or fixed in the factory Grains, separated from panicles, represent a valuable source of income due to their (underestimated) 200 /tons market price as animal food. Leaves will be collected by a Claas Baler Table III. Leaves baler data Capacity: Weight per bale Speed 9 Bales/hour 600 kg/bale 1,5 ha/hour Cost The total biomass harvested using this advanced machinery is 70 fresh tons /ha, divided as follows: Table IV. Sweet Sorghum plant main parts Stalks Leaves Pannicles Grains 51,1 t/ha 11,5 t/ha 2,31 t/ha 5,46 t/ha Two main substates are recovered by Sweet Sorghum fresh biomass: 1. Sugar juice 2. Solid biomass Stalks are quenched with water to extract sugar juice from fiber and separate bagasse from liquid part. The raw Sugar Juice properties are listed below: Total fresh substrate: 30 tons per hectare. The simple crushing technology provides: Total Solid: 16% t ST / TT Total digestible sugars: 84% Total Ashes: 2,8% Total organic content: 97,2% tsv/ tst These physical compound allows to estimate the biogas yield of sugar juice, which is made by mainly carbohydrate and has at very fast rate digestion: Table V. Sugar Juice digestion data Biogas yield of carbohydrates fibre: 790 l/kgsv Methane related yield: 50 % The second substrate obtained from sugar stalks crushing system is the solid bagasse. The bagasse together with leaves and panicles represent the most energetic content in terms of anaerobic digestion potentials with a valuable nutrient content of about 4-5 kg/m 3 726

3 The Solid biomass substrate presents the following parameters: Bagasse + leaves + panicles: 34,8 fresh tons Total solid biomass: 50% tst/tt Ashes: 9% Organic Biomass: 91% tsv/st Diluited with sugar juice digestate The total Amount of biogas production and related methane concentration is estimated below: Table VI. Bagasse digestion data Cubic meters of Biogas 550 m3 biogas/tsv Methane related yield 60% 3 BIOGAS PRODUCTION YIELD and MIXING SUBSTRATE PRODUCTION The two substrates ( Juice based and bagasse based) need to be integrated with liquid substrates in order to reduce the solid content and to increase the digestion rate. The concept is based on the integration of cow sludge and water in both of the substrate in different concentration Sugar Juice Based Substrate Production An Biogas Yield Estimation Sugar juice solid content is 16%. The aim is to reduce the total amount of solid content up till 10% in order to be digested in a semi-liquid reactor already used for many biogas installation. The table below shows the energy efficiency of Sugar Juice: Table VII. Sugar Juice biogas yield Sugar Juice: Solid Biomass: Organic Biomass: Biogas yield: Toal biogas per ha: 30,15 fresh tons 4,82 tons 4,69 tons 483 m3 Biogas/t SV 2264,68 m3/ha Sugar Juice is mixed with water and sludge in order to reduce the total solid content from 16 to 10%. The amount of sludge will also increase the biogas production due to a solid content of 0,4 ST/TT. In the table below the total substrate estimated for 25 hectares: Table VIII. Sugar Juice mixture details Total Sugar Juice (0,16): 753,75 Total Water (0) : 301,5 Total Sludge (0,04): 150,75 Total fresh tons : 1206 Mixture total solid: 0,105 The total amount of 1,206 fresh tons (=cubic meters) will be processed in a thermophilic reactor at 55 C for ten days. The total percentage of cow slurry put in the digester is 20% of juice, while the percentage of water is 40%. The different substrates (Sludge and Juice, has different biogas potentials, different solid content but a common methane concentration of 50%. Sludge biogas potential and methane concentration estimated for this study are: Biogas potential: 400m 3 biogas/ tst Methane concentration: 50% The mixed substrate biogas potential has been estimated with a total Biogas production estimation for 25 hectares from sugar juice: Table IX. total biogas production from sugar juice mixture Total biogas from juice: m3 Total biogas from sludge: m3 Total biogas production: m3 Total Methane production: m Bagasse Based Substrate Production And Biogas Yield Estimation Bagasse + leaves + panicles provide a total of 34,8 fresh biomass at 50% moisture. The objective is to reduce the solid content to 20-22% of Moisture in order to digest the biomass in a semi-solid reactor. The solid biomass biogas yield is the following: Table X1. Bagasse biogas yield Total fresh biomass: 34,8 t/ha Total Solid 17,4 t/ha total volatile 15,834 t/ha Total biogas 8708,7 m3/ha Solid bagasse is mixed with slurry, water and a high percentage of digested juice (5% solid) to reduce the solid content from 45% to 21%. The table below shows the final substrate components: 727

4 Table XI. Bagasse mixture details Total Bagasse (0,45) Total Water (0) Total Sludge (0,04) 870 t 435 t 261 t Table XIII. Sugar juice biogas digester and storage tank Digester dimensions 251 m 3 Storage tank: 460 m 3 Tank for water 420 m 3 Digested Sugar Juice (0,05) Total: 609 t 2175 t Mixture total solid 0,21% The total concentration of different substrate mixed with bagasse is ìthe following: Water: 50% of solid biomass Sludge: 30% of solid biomass Sugar Juice digestate: 70% of solid biomass Considering Sludge to have the same energy potential defined above, the total energy provided by the anaerobic digestion of bagasse based mixture substrated is: Table XII. Biogas production from bagasse mixture Total Biogas from bagasse: m 3 Total Biogas from sludge: m 3 Total biogas production: m 3 Total methane production: m 3 The substrate of 21% solid is digested in a two stages thermophilic semi-solid reactor. The two stages are represented by a first hydrolyser in micro-aerobic conditions with general losses of about 3% in 2-3 days retention time, and a second digester where the substrate must remain for 35 days at 55 C. 4 DIGESTION LOGISTIC. ENERGY PRODUCTION & ELECTRIC POWER INSTALLATION. (see the scheme illustrated in Image 1 at the end of the paper) The different substrate are connected with pipeline in order to mix sugar juice digestate with the bagasse in the second semi-solid digester. However, the digestion time and the impossibility to store the sugar oblige to digest all the milled sugar immediately Sugar Juice The previously mentioned harvesting time of 50 days, and the very low retention time of 10 days due to the high digestibility of the juice allows to reduce the digester dimensions. The total cubic meters are 1,206. To be divided in 5 times due to the possibility to harvest in 50 days what can be digested in 10 days. Here below the data concerning digester dimension for juice-based substrate The estimation above are calculated basing on the installation of a CHP engine with 52% efficiency for thermal energy production and 32% efficiency of electric engine. The total amount of hours per year working of the engines is very low, due to the need to process all a big part of the biogas produced in 60 days. The sugar juice digester is a thermophilic down-right digester with 4 meters radius and 5 meters high. For a total of 250 cubic meters The total electric power is 68 kwe. However, two solutions are under estimation: Utilization of biogas to produce heat and supply the required thermal energy for digestate drying, digesters heat demand etc.. Store the biogas onsite mixing the biogas with the amount of biogas produced by the other digester. 4.2 Bagasse The total amount of Solid substrate (21,1%) is 2,175 fresh tons per year. Part of this substrate is represented by Sugar Juice digestate, which is recirculated in a prehydrolyser and than in the semi-solid digester with a 70% of bagasse input. The sugar losses during hydrolisys retention time of 2days are estimated to be around 2-3%. (Wellinger et al., 1999; Capela et al., 1999). Thus it is not considered as affecting the biogas yield. The total of digested sugar juice is so about 609 fresh tons per year (mix of water, juice and slurry). This digestion will take place during all along the year and the digestion retention time is estimated to be around 35 days, thanks to the prehydrolysis, the mix of sugar juice and a 55 C of thermophilic digestion. Here below a table with the parameters defined for digester volume calculation. Table XIV. Solid bagasse digestion parameters Temperature 55 C Retention time (thermophilic) substrate heating capacity 35 days 4 Kj/kg K Temperature substrate 12 C The digester, as well as the hydrolyser is very small consutruction insulated on lateral and base surface in order to maintain the thermophilic temperature inside. The reactor is a dry solid horizontal cylinder of 8 meters length and 3 meters radius (total 226 cubic meters). 728

5 Table XV. Bagasse digester and storage tank Digester dimensions: 226 m3 Storage Tank: 240 m3 Hydrolyser: 230 m3 The Hydrolyser is a downright micro-aerobic digester with a high of 3 meters and a basis of 5 meters radius. The Electric power engine is a CHP for raw biogas internal combustion with 35% electric efficiency and 52% thermal efficiency. The total estimated electric power is: 63kWe 4.3. Total Amount of energy production. The methane produced from the two digesters is used to produce electricity and heat. Here below the total energy production estimation: Total Energy production: kWh Total Heat production: kWht Total Electricity: kWhe The electricity produced with two engines of 68 and 63 kwe is mostly sold to the national grid (estimated feed in tariff 0,14 /kwh) The total heat produced is used all for the digesters heating and for the bio-fertilizer production. 5 ANAEROBIC DIGESTER HEATING, DIGESTATE DRYING SYSTEM AND MECHANICAL SEPARATION ENERGY DEMAND: The energy demand for heating tanks and moving pumps, mechanical conveyor, etc represents a large part of total thermal energy produced. 5.1 Digesters heat demand The total amount of substrate to be heated is 2,772 m3 The heat demand has been calculated as follows: Total mass: 2,772 m 3 Total KJ/Kg K of substrate: 4 Total T : 55 C (thermophilic) 12 C(T of substrate) The total heat required by the digesters is: 2,772 m 3 4 KJ/Kg K (55 C-12 C) 1,3 Juice digester required heat: ,00 kwht Bagasse digester required heat: ,67 kwht The Total thermal energy required for digester heat supply is about 300,000 kwht per year. It is estimated that the mechanical separation consume about 1,4 kwhe per ton of fresh substrate, the total consumption is around 3880 kwhe. Through the mechanical separation, the water content can be reduced from 90% to 50%, in the solid part, while the liquid part contain 0,02% of solid. The liquid part can be evaporated to recover all the solid matter, or used as water supply for digester, irrigation or other purposes. The recommendation is to use the liquid as liquid part to be put in the digester, in order to reduce the environmental impact of water utilization. A further amount of electricity must be considered for the biofertilizer production, and for the total electricity supply on site (cooling, other pumps, light, etc.. According to the literature, a total amount of 10% of electricity produced is considered to be used to feed all the utilities of the biogas plant and biofertilizer production installation. For a total consumption of: 53,199 kwhe per year 5.3 Heat required for digestate drying system After the mechanical separation, the digestate has a 50% moisture content. The objective is to reduce the total moisture up till 10% -15% in order to be able to store the biofertilizer. The drying system consumption is about 800 kwht 7ton of water to evaporate. Basing on this data, the total amount of heat required has been calculated evaluating the total amount of water to evaporate, which is 390 ton per year. The result is: 312,362 kwht to be used for digestate drying system Considering the total heat demand required by Biogas plant facility, the total amount of heat is: Table XVI. Heat required by digesters and drying systems Drying required heat: 312,362 kwht Digester required heat: 74,906 kwht Digester required heat: 135,091 kwht Total Heat Required: 522,360 kwht The resulting remaining energy available is shown in the table below: Table XVII. Total Remaining Net Energy Total remaining heat: 279,064 kwht Total remaining electricity: 439,985 kwhe 5.2 Digestate treatment electricity demand. In order to reduce the thermal energy required for digestate drying activity, a mechanical conveyor for the separation of a large part of water would be utilized. 729

6 6 THE PRODUCTION OF BIOFERTILIZER 6.1 Sweet Sorghum digestate available nutrients The Nitrogen content in Sweet Sorghum digestate is estimated to be around 4% (literature). The digestate solid content ratio on fresh cubic meters output is underestimated to 10% (16% is the solid content of bagasse digestate, 5% is solid content of sugar juice digestate). The theoretic output should be considered around 13% but an underestimation have been considered in order to include drying losses, liquid part separation etc.. The percentage of Nitrogen is always considered as 50 kg/m3 of dry matter (10% solid). The total amount of fresh biomass output is: 2,716 tons per year. The estimated solid content, as mentioned above, is 10%. With 25 hectares, nitrogen results to have a total amount of tons per year. 150 kg of N per hectare per year are required for Sweet Sorghum Cultivation The total is 3850kg of N = 3.8 tons/year for cultivation. The total remaining biofertilizer for sale is 196 tons, with a N percentage of 5%. The price has still to be estimated. The NH4-N content is 70-80% of total organic N thanks to the anaerobic digestion process, this make the substrate a more valuable fertilizer with fast absorption rate of N in the soil. Furthermore, the total P is 0,8 kg/m3 of fresh biomass. The recycling of Phosphorus (up till 90%) makes this biofertilizer very attractive in terms of cultivation sustainability. Of course a valuable production and control system is included in the biofertilizer plant installation. The different steps for evaluating the Digestate nutrient content is: Total Nitrogen(TN) persulfate digestion, colorimetric method Total Ammonia Nitrogen (TAN) was measured with an ion-selective electrode Phosphorus persulfate digestion, colorimetric method Potassium was measured with an ion-selective electrode In addition a new technique would be applied in order to increase the digestate nutrient content and provide a real high value biofertilizer (Estimated up till 6-7% N): 6.2 New technology for biofertilizer production: New systems based on bacteria and fungi inoculation highly increase biofertilizers nitrogen content. The sustainability of biofertilizer derives mostly from the Phosphorus content. The production of fertilizer fom digestate include the recirculation of Phosphorus, which is one of the most importan nutrinet in nature. It is thus very important to concentrate the attention on the increase of Nitrogen content of substrate. The technology identified is the following: Integration of anaerobic digestion technology improving digestate nutrient content with inoculation of cellulolytic fungi (T. harzianum )and Azotobacter (or Azospirillium) for highest value biofertilizer production. [2] Incoulation data source: Bayani M. Espiritu Figure 2. Bio-Fertilizer Production Concept Scheme In addition, A new pelletization machinery has been created by a consortium of industries and investors which is able to pelletize biomass with very high moisture content (trials with peat have been made) This machine operates at about 80 C, this process avoids ammonia losses and reduce pelletization costs. 7 COSTS, INCOMES & ESTIMATED RETURN OF INVESTMENT 7.1. Estimation of costs The total investment has been evaluated considering the following values: Table XVIII. Biogas plant components cost Digester price 180 /m3 Engine Price 800 /kwe Insulation digester lateral 400 /m3 surface (s=0,06): Insulation digester basis 200 /m3 (s=0,08) Heating system connection 1500 pipeline for substrate : 4,5 /m Pressure pump (5,5 kwe) 2800 /each Secure Flame (>400kWt) /each Electricity connection 20,000 Annual Maintenance (% total costs) 1,5 % total The total digesters cost has been calculated considering the two tanks, the bagasse storage tank, and the hydrolyser. The average cost for this type of digester is 190 /m3. While the sugar juice tank has been defined as cheaper because of it is designed without a heating system and without post-digestion biogas storing system. The final cost of this tank is defined as 120 /m3. Internal combustion engines cost is estimated to be aroung 850 /kwe installed power for both the engines. The efficiency is estimated to be quite low (32%) due to the small size of the engines. In addition, lateral surface insulation cost is estimated to be 5,802, while the basis of the sugar juice digester insulation is 1,256. Here below a resuming table including all the costs estimated for digester and power engines installation. The cost of harvesting machineries and utilities has not been 730

7 evaluated in this study due to the general idea that this type of installation should be integrated in a pre existing farm. Table XIX. Total estimated investment costs COSTS Digester Engine Sugar juice storage tank Insulation: Heatin system connection: Pipeline: Secure flame: Pumps: Electricity grid connection: Fertilizer plant cost: Total: Estimation of Incomes Three different main products are salable on the market every years from this type of biogas plant: o o o Electricity Sweet Sorghum Grains Bio-Fertilizer Electricity: Net electricity produced is estimated to be kwhe per year, divided between the two different Internal combustion engines operating independently. The estimated Feed in tariff in Belgium is not so high. The average is /kwhe for those biomass plant operating with residues, using CHP engines, fully sustainable. These parameters match perfectly the operating conditions of Sweet Sorghum co-production of Biogas and Biofertilizer. The average feed in tariff identified is defined as 0,14 /kwhe. For a total annual income of 61,598 /year. Grains: Grains for animal feed: Sweeto Sorghum grains are rich of sugar and can be used as feedstock for different utilizations. Sugar market can easily produce valuable sugar from grains. In additions, sweet sorghum grains nutrient content makes them a valuable product to be sold as animale feed. The average low price for a new market is estimated around 230 /ton (0,23 /kg). With a general yield of 5,4 tons of grains per year, the income from grains sale is /year. Biofertilizer (see the impact of Artificial fertilizer in the images 2 and 3 at the end of the paper): Biofertilizer market is acomplicate issue to solve in Europe. Even if bio-food and sustainable agriculture strongly require the application of a natural fertilizer with the possibility to recirculate phosphorus in the soil, there are many barriers to overcome in terms of permissions and directive. Currently the utilization of digestate for the production of valuable fertilizer is not permitted in Europe. This create problems to farmers who decide to start with the installation of a biogas plant because often they do not have enough land where to spread the digestate. Digestate treatment and inoculation of bacteria and funghi is a well known technology which allow to increase a lot the nitrogen content of the substrate enhancing the fertilizer properties. Here in this study, the biofertilizer production is just illustrated in terms of technology, but even if the estimation for N percentage in weet Sorhum digestate is considered as 5% (60-80% NH-N), the price is very very low. So, the market price is 0,15 /kg. For a total income of about 29,400 /year. With these three sources of income, the total annual potential is shown in the table below: Table XX. Total estimated incomes INCOMES Electricity Grains Fertilizer 29,400 Total: It is so easy to calculate the return of the investment for this pre-feasibility study:. With an investment of 426,000 and a general annual income of 120,698, the estimated return of the investment for Sweet Soeghum biogas plant is defined as 3 years and 6 months. 8 CONCLUSION This paper refers to a pre-feasibility study for a demonstration plant in Belgium. Many data concerning the costs, the incomes and other activities energy consumption have been calculated in order to increase provide an overestimation for the investment. Next steps of this study will be focused on: Contacting Wallon region and discuss terms of collaboration for a demonstration plant which would be able to sell biofertilizer produced evaluating also a fixed feed in tariff for the electricity for sale. Testing in laboratory scale the yield of Sweet Sorghum anaerobic digestion concept in order to verify the good amount of biogas and biomethane concentration (underestimated for sugar juice) Studying biofertilizer production technique (already used for compost) on anaerobic digestate and estimate the price according to a deep the market survey. EUBIA intends to work next to regional and national authorities in order to define a new biofertilzer market for refined digestate, creating standardization parameters, certification schemes thus establishing a new strategy for a pro-biofertilizer European market. 731

8 Figure 3. Anaerobic digestion of Sweet Sorghum concept Scheme (1) Figure 4. Artificial Fertilizer high environmental Impact (1) 732

9 Figure 5.Artificial Fertilizer High Environmental Impact (2) 733