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WMR31410.1177/0734242X12465460Waste Management & ResearchBachmaier et al.
Original Article
Changes in greenhouse gas balance and resource demand of biogas plants in southern Germany after a period of three years
Waste Management & Research 31(4) 368–375 © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X12465460 wmr.sagepub.com
Hans Bachmaier1, Mathias Effenberger2, Andreas Gronauer3 and Josef Boxberger3
Abstract For five agricultural biogas plants with a high share of energy crops in the input material, a detailed balance of greenhouse gas emissions (GHG) and cumulated energy demand (CED) was calculated for the years 2007 and 2010. The results vary considerably between plants and over time. In 2010 compared with 2007, all of the five biogas plants reduced their impact on climate change and four of them also reduced their consumption of fossil energy. The strongest influence was from the enhanced utilization of surplus heat energy, whereas variations of environmental impact due to direct emissions from the biogas plants were less distinctive. Compared with a reference system based on fossil resources, electricity production in the biogas plants avoided GHG emissions of 603 g to 940 g carbon dioxide equivalents (CO2-eq)•kilowatt hours electrical energy (kWhel-1) and saved 2.48 to 3.23 kilowatt hours primary energy from fossil energy carriers (kWhfossil)•kWhel-1 CED (results for 2010). Keywords Biogas, agriculture, Germany, greenhouse gas emissions, cumulated energy demand
Introduction Between 2000 and 2010 the number of biogas plants in Germany increased approximately 6-fold, to 6000 installations. Concurrently, the overall installed electrical capacity from biogas rose by a factor of 39 to a value of 2.28 GW (Fachverband Biogas e.V., 2010). In terms of gross output, this is equal to two typical German nuclear power stations. Within the same time period the share of all renewable energy sources rose from 6.4% to 16.8% in respect to electricity supply and from 2.9% to 9.4% in respect to primary energy consumption (BMU, 2010). The political key to this development is the German Act on Granting Priority to Renewable Energy Sources of April 1st, 2000 (BMU, 2000). It contains detailed specifications of feedin tariffs for electricity from renewable energy sources. Since 2000 these specifications have been changed several times with strong impacts on the structure of biogas plants and their input materials. Currently, three quarters of the energy output of agricultural biogas plants is produced from energy crops. This fact influences greatly the greenhouse gas (GHG) and fossil resource balance of electricity from biogas. The production of crops is energy intensive and even more so results in emissions of GHGs from agricultural soils. Besides these input-related factors, the GHG balance and demand of fossil resources [cumulated energy demand (CED)] of biogas production and utilization is dependent
on technological and organizational factors, such as leakage of biogas, storage of digested residue and heat utilization. Several studies have assessed the environmental effects of electricity production in biogas plants using energy crops and animal waste (e.g. Gärtner et al., 2008; Hartmann, 2007; Scholwin et al., 2006). The authors examined different model-based biogas plants, determined their environmental impact and identified potential for improvements. A direct comparison of these studies is difficult as different system boundaries and functional units are used, and there are variations with respect to the treatment of co-products. All of the afore mentioned articles do not deal with real biogas plants. Hundt (2010) analyzed an existing biogas plant with gas conditioning and feed-in. The present study
1Technology
and Support Centre (TFZ) in the Centre of Excellence for Renewable Resources, Straubing, Germany 2Bavarian Research Center for Agriculture, Institute of Agricultural Engineering and Animal Husbandry, Freising, Germany 3Department of Sustainable Agricultural Systems, Division of Agricultural Engineering, University of Natural Resources and Life Sciences, Vienna, Austria Corresponding author: Hans Bachmaier, Technology and Support Centre (TFZ) in the Centre of Excellence for Renewable Resources, Schulgasse 18, Straubing, 94315, Germany. Email:
[email protected]
Bachmaier et al. extends this approach by analyzing several existing biogas plants and evaluating the changes of environmental impacts over time.
Materials and methods The GHG emissions and the fossil resource consumption of electricity production from biogas were analyzed for five biogas plants located on farms in different regions of Bavaria. The plants reflect some of the diversity of agricultural biogas technology and site conditions in southern Germany (different plant manufacturers and plant sizes, varying input materials, etc.). The GHG emissions and the demand of fossil resources were calculated following the lifecycle assessment (LCA) method (ISO 14040 and ISO 14044) by modeling the biogas plants as material flow networks using the software Umberto (Schmidt & Häuslein, 1996). The balancing period is one calendar year. The analyses were performed based on data collections in the years 2007 and 2010. Factors for the conversion into equivalents of carbon dioxide are 25 for methane and 298 for nitrous oxide (IPCC, 2007). The present study uses a substitution approach, side products are taken into account by credits for avoided burden.
Description of the biogas plants The individual biogas plants feature an overall digester volume between 770 m3 and 3000 m3, divided into 2 or 3 process stages and additional storage tanks for the digestate. The co-generation unit (CGU) transforms the biogas into electricity and heat. The installed electric power varies between 280 kWel and 856 kWel. Excluding plant 5, the CGUs are equipped with gas engines. Plant 5 uses a pilot injection engine. In contrast to the gas engines pilot injection engines need small amounts of ignition oil (diesel) in addition to the biogas. The energy crops are stored in silos. Often nearby stables deliver manure and consume heat from the CGU. The input materials of the investigated biogas installations are mainly energy crops combined with a broad range of animal wastes. The share of animal waste ranges between 1% and 45% of the wet matter input. For organic dry matter the range is 0.5–10% (Table 1). As stated earlier, a high share of energy crops in the input is typical for German agricultural biogas plants. Plant 1 is the only one that operates in a two-phase mode, i.e. hydrolysis and acidification of the input materials occur in a separate tank that is not closed to atmosphere. The characteristics of the biogas plants are summarized in Table 1. Figure 1 displays the system boundary of the investigated biogas plants. To illustrate the material and energetic flows Figure 2 shows exemplarily a schematic drawing of plant 05 in 2007.
Data collection As far as possible, the material and energy flows of the biogas plants were monitored on-site on a continuous basis without interruption (e.g. weight of input substrate, gas volume, methane content of the biogas, electricity consumption and electricity production) or determined from spot measurements (e.g. quality of input materials once per month, emissions from open digestate
369 storage via laboratory testing twice a year). Measures and measuring points at the five biogas plants are listed in Table 2. Measuring data were used to prepare material flow analyses for individual plants. Material flows that could not be measured directly were estimated based on literature data. The scope of this LCA covers the material and energy flows of the cultivation of the energy crops, the transportation and storage of crops and manure, the anaerobic digestion to biogas, the electricity and heat generation from biogas and finally the application of the digestate as fertilizer for crops. The environmental impact data for the operating resources, such as diesel, fertilizer, electricity, pesticides, construction materials, etc., relies on the databases Gemis 4.6 and Ecoinvent 2.2. Production of energy crops. Cultivation, harvest and storage of the respective energy crops were modeled based on data from KTBL (KTBL, 2006, 2008), using intermediate yields. For all crops, tillage soil management was assumed. Losses of ammonia during land application were modeled on a crop-specific basis rather than using an overall estimate. These losses served as basis to calculate the additional demand for mineral fertilizer. According to the assumption in the German National Greenhouse Gas Inventory (UBA, 2009), a soil emission factor for nitrous oxide from mineral and organic fertilizers of 1.0% was used. Indirect emissions of nitrous oxides were not considered. Operation of the biogas plant. At the biogas plants substrate input, energy consumption, biogas production and electricity output were measured on a daily basis. Concerning the electricity consumption of the biogas installations, it was distinguished whether the electricity was supplied from the grid, from own production (plant 3) or from a local small hydropower plant (plant 2). For direct emissions of methane from the biogas plants which could not be measured (e.g. emissions through the overpressure control), a share of 1% of total biogas production was assumed. Clemens et al. (2009) confirm this emission level in field measurements. Exhaust gas emissions from the co-generation units were determined from spot measurements at identical units at other biogas plants. Based on observations at different biogas plants a 1% loss of total electricity output during the transformation and feeding of the electricity into the medium-voltage grid, was assumed. Construction of biogas plant. For the construction of a biogas plant, the material flows of concrete, asphalt, steel and brick are dominant in terms of cumulated energy demand and greenhouse gas emissions. Therefore, the calculation of the environmental impact was restricted to these materials. District heating pipelines were not included in the analysis owing to a lack of information on the quantity and quality of the materials. Storage of digested residue. At two of the experimental sites (1 and 4), the digested residue was stored in open tanks. The methane emissions from these open storage tanks were estimated based on batch tests for residual biogas potential (Lehner et al., 2009). Emissions of nitrous oxide were not taken into account. The digestate tanks of biogas plant 2, 3 and 5 are equipped with a gastight cover and connected to the biogas collection system. The digested residues are used for fertilizing the energy crops.
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Table 1. Characteristics of the biogas plants. Plant identification
1
2
3
4
5
Year of commissioning Input of animal manurea Input of energy cropsa
2005 CL GN, MK, MS, SB
2005 CL, CS, PS MS, GS, GR, CS
2004 CL MS, GS, CS, CCM
2005 CS CGS, MS, CCM, CS
Share of animal waste (percentage of wet matter/dry matter) Number of process stages Process temperature level Volume of primary digester(s), m3 Volume of secondary digester, m3
2007: 31/4 2010: 44/9 2 Mesb 1095c -
2007: 15/5 2010: 6/2 2 Mesb 1608 1134
2007: 0.75/0.5 2010: 0.0/0.0 2 Mesb 1357 1428
Overall volume of digestersd, m3
1095
2542
2605
1540
Storage of digestate Volume of storage tank, m3 CGU engine type Number of engines
Opene 1791 Gg 1
Collect 1134 G 2
Open 1294 G 1
Collect 3289 PIh 2
Electrical capacity, kW Thermal capacity, kW
324
2007: 4/2 2010: 35/10 2 Mesb 2010 2000 (2007) 3000 (2010) 4010 (2007) 5010 (2010) Collectf 1146 G 1 (2007) 2 (2010) 329 (2007) 856 (2010) 447 (2007) 947 (2010)
2005 HL, CL, CS MS, CS, GS, CCM (2010), SG (2007), RGS 2007: 29/5 2010: 22/5 2 Mesb 770 770
380 (2007) 630 (2010) 486 (2007) 800 (2010)
333
280 (2007) 350 (2010) 300 (2007) 400 (2010)
250
232
aIn
order of average mass percentage: CCM: corn-cob-mix; CS: cereal silage; CGS: clover grass silage; CL: liquid cattle manure; CS: solid cattle manure; GN: grain; GR: green rye crop silage; GS: grass silage; HL: liquid hog manure; MK: maize corn silage; MS: maize crop silage; PS: solid poultry manure; RGS: rye grass silage; SB: sugar beets; SG: Sudan grass silage. bMesophilic (mes) (25–45°C). cIncluding volume of pre-digester (hydrolysis stage): 100 m3. dSum of the usable volume of all digesters. eNot covered, only mixed before pumping into manure trailer. fGastight and connected to the biogas collection system. gGas engine. hPilot-injection engine. CGU: co-generation unit.
Figure 1. System boundary of the material flow network, including the production of the energy crops, the transport and the storage of the organic input materials, the biogas production and the biogas conversion into electricity and heat. Upstream chains of input energy and materials are included.
Modeling of biogas plants The scope of the analysis covered the entire process of energy supply from biogas and its respective material and energy flows. Using the software umberto®, a computer-based model
was developed to calculate the material and energy balances of the biogas plants and the respective upstream processes. A material flow network was generated that allows for the detailed monitoring of the entire process chain of biogas production and utilization. All relevant emissions of GHGs that leave the
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Figure 2. Schematic drawing of biogas plant 5 in the year 2007. Table 2. Measuring points at the biogas plants. Plant identification
1
2
3
4
5
Biogas composition Biogas yield Biogas pressureb Biogas temperatureb Mass input Thermal energy
CGUa
CGU CGU CGU CGU Solids feed unit Digesters Backup cooler District heating
Stage 1 CGU CGU 1/2+3 Solids feed unit District heatingc
CGU CGU CGU CGU Solids feed unit Primary cooling circuitd Digesters Cooler
CGU CGU 1/2 CGU 1/2 CGU 1/2 Wheel loader Digesters Backup cooler District heating
CGU Solids feed unit Digesters Backup coolere District heating
aCo-generating
unit (CGU): gas pipe ahead the co-generation unit. on the measuring method of the gas counter additionally the pressure and temperature of the biogas are needed for calculating the standard volume. cThe points where heat energy from the co-generation unit is supplied to external users. dThe thermal energy of the engine first goes to a primary cooling circuit. A heat exchanger transmits the thermal energy to the heating circuit. eSurplus heat that cannot be used has to be removed by using a backup cooler. bDepending
balance envelope were added up. The CED included the primary energy demand of all fossil energy carriers that were supplied from outside of the balance envelope. As electrical energy was the most important output of the biogas plants, all material and energy flows were specified per kilowatt hour of electrical energy fed into the grid. In the material balance, by-products in the form of heating energy and organic nitrogen fertilizer were accounted for as credits for avoided burden. The credit for the use of waste heat was calculated according to the German fossil heat mix with emissions of 376 g carbon dioxide equivalents (CO2-eq)•kWhtherm-1 and a CED of 1.13 kilowatt hours primary energy from fossil energy carriers (kWhfossil)•kWhtherm-1 (BMU, 2007). It was assumed that the fertilizing effect of nitrogen in an organic fertilizer is 80% of the same amount in a mineral fertilizer. Credits were calculated accordingly. If animal waste is digested, methane
emissions from storage can be prevented. Credits for digesting animal waste were calculated on the basis of UBA (2009). The results for GHG emissions and CED of electricity supply from the biogas plants were compared with a reference system based on fossil resources. In Germany, it can be assumed that electricity from biogas replaces electricity from natural gas-fired power plants at a share of 30% and from coal-fired power plants at a share of 70% (BMU, 2007). On average, the electricity from this reference system has a carbon footprint of 825 g CO2-eq• kilowatt hours electrical energy (kWhel-1) and a CED of 2.55 kWhfossil•kWhel-1.
Results and discussion In order to highlight the differences between the individual biogas plants, the results are presented without including the substitution of electricity in the reference system. The results for the
372 individual biogas plants shown in Figures 3 and 4 are discussed as follows. Table 3 shows the avoided environmental impacts in comparison to the electricity produced by the reference system. Plant 1 showed the highest GHG emissions from electricity production. This plant received only a small credit from digestion of liquid manure. In 2010, the share of heat utilization for a district heating system for the nearby village did not change significantly compared with 2007. The 2010 test for the residual biogas potential of the digestate showed lower methane emissions. Overall, the GHG footprint of electricity from this plant in 2010 compared with 2007 decreased only slightly, and the value is the highest among the five experimental plants.
Waste Management & Research 31(4) While plant 2 exhibited comparatively low direct methane emissions, the credits for digestion of animal manure and utilization of heating energy were almost negligible in 2007. The situation changed in 2010 when a biogas pipeline to an additional CGU at a nearby manufacturing plant was installed. This allows for a high share of heat utilization, which improves the GHG balance of this plant significantly. Plant 3 is the only one that did not receive electricity from the grid during regular operation. Only the surplus electricity that was not consumed by the biogas plant was fed into the grid. Therefore, emissions from upstream chains of electricity supply were not to be accounted for. Consequently, this plant
Figure 3. Greenhouse gas (GHG) balance of electricity production in five agricultural biogas plants each for the year 2007 and 2010. Substitution of electricity from the reference system is not included.
Figure 4. Cumulative energy demand (CED) balance of electricity production in five agricultural biogas plants each for the year 2007 and 2010. Substitution of electricity from the reference system is not included.
373
Bachmaier et al. Table 3. Avoided greenhouse gas (GHG) emissions and avoided consumption of fossil energy of electricity from biogas compared with the reference technology. Unit Avoided GHG emissions Avoided consumption of fossil energy
g CO2eq*kWhel-1 kWhfossil*kWhel-1
Plant identification 1_2007
1_2010
2_2007
2_2010
3_2007
3_2010
4_2007
4_2010
5_2007
-593
-603
-595
-829
-842
-923
-625
-837
-621
-2.48
-2.48
-2.25
-3.04
-2.85
-3.23
-2.14
-2.68
-2.28
CO2-eq: carbon dioxide equivalents; kWhel: kilowatt hours electrical energy; kWhfossil: kilowatt hours primary energy from fossil energy carriers.
already showed the lowest level of direct GHG emissions and a very favorable CED in 2007. The plant is equipped with a flare, direct methane emissions in the exhaust gas of the CGU were at a very low level and it received a large credit for heat utilization. Between 2007 and 2010 an increase in heat utilization due to the installation of a drying facility for firewood could even improve the results further. In 2007, plant 4 was marked by high emissions from operation owing to high electricity consumption and emissions from open storage of the digested residue. In the input material, the share of animal manure was low and there was basically no external heat utilization. On the positive side, the energy crops were grown organically without additional mineral fertilization. Owing to a high share of clover grass, atmospheric nitrogen was fixated in the system. Therefore, digested residue could be supplied to other ecological farms, and the plant received a credit for this. Nevertheless, the carbon footprint of electricity from this plant in 2007 was still relatively high. In addition, owing to the high electricity consumption during plant operation, the CED for plant 4 is the largest among the experimental plants. In 2010 the situation had changed as the biogas process had turned more effective resulting in reduced methane emissions from the uncovered storage tank. Also, a considerable share of the surplus heat energy was utilized in a drying facility. Plant 5 is connected to a pigsty and receives pig slurry on a regular basis. Some of the available thermal energy was utilized for district heating. The methane levels in the exhaust gas of the pilot-injection engines were comparably high, and the ignition oil consumption of the pilot-injection engines adds to the fossil energy demand during operation of the biogas plant. In 2010 heat utilization was significantly increased due to the installation of a 1.5-km district heating pipeline to a hospital.
Conclusions Comparing the five biogas plants it is evident that the GHG emissions (Figure 3) and the demand for fossil resources (Figure 4) vary within a wide range. Without looking at the changes between 2007 and 2010 several factors can be identified, which contribute to a low balance of GHG emissions:
• effective digestion leads to a lower need of energy crops per kWhel. As the emission of nitrous oxide correlates with the energy crop production high methane yields also lead to lower nitrous oxide emissions. Plants 3 and 5 digest very efficiently; • feeding energy crops that need reduced fertilizing is also an effective way of reducing the GHG emissions of crop production. Plant 4 feeds a high share of clover grass, a leguminous plant that can fix atmospheric nitrogen and therefore only needs small amounts of fertilizer; • the measures for reducing the methane emissions include covered storage tanks for the digestate (plants 2, 3, 5), gas flares to avoid emissions in case of overpressure in the gas storage (plant 3) and low emission cogeneration units (plants 1–4). • the credits for avoided burden can significantly lower the level of the emissions. The credits for digesting animal waste do not exceed 50 g CO2-eq*kWhel-1, whereas the credits for heat use get close to 300 g CO2-eq*kWhel-1 (plant 2 and 3). All measures but the reduction of nitrous oxide and methane emissions help to reduce the demand for fossil energy. Figures 3 and 4 show, for the five biogas plants, a reduction of their effect on climate change and on their consumption of fossil energy carriers between the year 2007 and 2010. • The strongest influence occurred from the enhanced utilization of surplus heat energy, whereas variations of the environmental impact due to direct emissions from the biogas plants were less distinctive. • Owing to deterioration of the engines, direct GHG emissions from the CGUs increased. Maintenance measures can reverse this development (Aschmann et al., 2011). • Changes in emissions from crop production were mixed and difficult to interpret. This is a critical point as, for electricity production from biogas, GHG emissions from agricultural soils can be very significant. • Investment in additional equipment for reducing methane emissions were not made at any of the five plants. The reason might be that in contrast to increased heat utilization, reduced methane emissions currently have little positive economic effect.
374 In the future, the validity of GHG balances should be enhanced by providing reliable data on nitrous oxide emissions from energy crop cultivation, methane leakage from biogas plants and emissions from uncovered storage tanks in comparison with conventional manure management. For a holistic assessment of the environmental effects of the biogas technology other environmental impacts, such as acidification, eutrophication or land use, should also be considered.
Perspectives In the next stage of the project it is intended to increase the number of the observed biogas plants. A Monte Carlo simulation will help to reveal the critical range of the results.
Summary For five experimental farm biogas plants GHG balance and CED of biogas production and utilization were calculated using a material flow network. The GHG balance differs considerably between plants and also changes over time. In comparison to the fossil reference system, electricity production in the biogas plants saves GHG emissions from 592 to 842 g CO2-eq•kWhel-1 in the year 2007 and from 603 to 923 g CO2-eq•kWhel-1 in the year 2010. Without accounting for the substitution of electricity from the fossil-based reference system, this is equivalent to a carbon footprint for electricity from biogas of -18 to 232 g CO2-eq•kWhel-1 in 2007 and -98 to 221 g CO2-eq•kWhel-1 in 2010. The main factors for a low carbon footprint of electricity from biogas are: •• effective digestion that leads to high methane yields from the input material; •• downsizing the nitrogen flows by using energy crops that need only little nitrogen fertilizing (e.g. leguminous plants); •• minimization of direct methane emissions by collecting the biogas from the digestate storage tank and by installing biogas flares and low-emission CGUs; •• maximizing the credits for avoided burden for using animal manure as input material, efficient utilization of heat energy and production of surplus organic fertilizer by digesting leguminous crops. Electricity supply from biogas also contributes to a considerable reduction of the use of fossil energy carriers. For the five biogas plants, the savings in CED ranged from 2.14 to 2.85 kWhfossil•kWhel-1 in 2007 and from 2.48 to 3.23 kWhfossil•kWhel-1 in 2010 compared with the fossil-based reference system. In absolute figures the fossil energy consumption of the biogas plants is between -0.30 and 0.41 kWhfossil•kWhel-1 for the year 2007. For 2010 these values change to a range of -0.68 to 0.07 kWhfossil•kWhel-1. The CED of biogas production is dominated by the energy demand for plant production (diesel fuel and
Waste Management & Research 31(4) mineral fertilizer) and for operation of the biogas plant (grid electricity). Again, the results for CED differ substantially from plant to plant. It has to be noted that for biogas plants the CED is not necessarily correlated with GHG emissions. This is owing to the fact that methane and nitrous oxide emissions have a high global warming potential, but hardly any influence on the energy balance of the biogas process chain. All measures but the reduction of nitrous oxide and methane emissions also help to reduce the demand for fossil energy. It has to be noted that the operation of the biogas plant is for most of the biogas plants the dominant sink for fossil energy consumption. Using electricity from biogas instead of grid electricity (plant 3) lowers the consumption of fossil energy.
Funding The work was funded by the Bavarian State Ministry for Nutrition, Agriculture and Forestry.
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