• Ei tuloksia

A TMOSPHERIC IMPACT OF BIOENERGY PRODUCTION

Life cycle assessment (LCA) approach was used to estimate the climate impacts of the bioenergy production based on TIM and RCG biomasses. This assessment was done for biogas-based bioenergy production. For RCG the LCA was done also for the combustion option. The LCA for energy based on combustion of RCG biomass cultivated on mineral soil (present study) is compared with the published LCA for energy gained from RCG biomass cultivated on organic soil.

6.2.1 Atmospheric impact of biogas production based on TIM and RCG Management-related GHG emissions were three times higher from TIM system than those from the RCG system during the study period (Table 6). The differences arise from the agricultural practices (Table 3) of the systems, as TIM requires more field operations and transportation of the yield than RCG. For TIM, the fresh yield was harvested once in 2009 and twice in 2010 and 2011. Fertilizers were also applied with the same frequency (one to two times per year). For RCG, spring harvesting was done and fertilizers were applied only once during the entire growing season. When the net emissions (balance in Table 5 and management-related emissions in Table 6) were calculated per harvested yields (Table 3) , the net emissions were generally lower for RCG (720 and 570 g CO2-eq per kg DW in 2010 and 2011,respectively) than for TIM (310, 1500 and 930 g CO2-eq per kg DW in 2009, 2010 and 2011 respectively).

Table 6 Management-related greenhouse gas emissions (kg CO2-eq ha-1) for mixture of timothy and meadow fescue (TIM) and reed canary grass (RCG).

2009 2010 2011

To estimate the net emissions per MWh of produced energy from biogas, CH4 production potentials of 350 and 260 m3 CH4 per ton of dry biomass were used for the TIM and RCG biomass, respectively. Energy content of 10 kWh m-3 was used for CH4. The net emissions for energy produced from biogas were higher for TIM than

132

6.1.4 Net GHG balance as CO2 equivalents

Annual values of NEE and fluxes of CH4 and N2O expressed as CO2 equivalents were summed to obtain the overall annual GHG balances (Table 5):

GWPtotal (kg CO2- eq ha-1) = NEE (kg CO2 ha-1) + CH4 (kg CO2-eq ha-1) + N2O (kg CO2-eq ha-1).

The key component of the annual GWPtotal was NEE for all treatments. The additional C uptake as CH4 accounted for less than 1% of the total net uptake of C.

The effect of N2O emissions on the annual GHG sink potential was highest on TIM system in 2010, when 30% of the sink potential was reduced by N2O emissions. On RCG system, during the initial year, N2O emissions increased GWP by 40%. On both cropping systems, the effect of N2O emissions was lowest in 2011 when 8% of the annual GHG sink potential was reduced by N2O emissions. In 2011, the plants were thriving and the N2O emissions were low.

To determine the net GHG balance as CO2 equivalents, the content as CO2 of the yield (Table 3), representing a loss of CO2 from the ecosystem, was added to the

GWPtotal. Based on the net GHG balance, all treatments acted as net CO2 sources

(Table 5). For RCG, the net GHG balance was fairly similar during studied years.

The net GHG balance of TIM varied from 1300 up to 18 000 kg CO2-eq ha-1 yr-1. The lowest value occurred during the first year resulting from the low crop yield. It was the highest in 2010 and 2011 when the yields doubled compared to the yield in 2009. (TIM), reed canary grass (RCG) and soil without vegetation (BARE) from 2009 to 2011. Superscript refers to the chapters in this summary.

NEE CH4 N2O(5 GWPtotal Yield Balance (* Data for RCG have not been published previously. The methods for flux measurements and processing of the data were done as described in Chapter 4.

133

6.2 ATMOSPHERIC IMPACT OF BIOENERGY PRODUCTION

Life cycle assessment (LCA) approach was used to estimate the climate impacts of the bioenergy production based on TIM and RCG biomasses. This assessment was done for biogas-based bioenergy production. For RCG the LCA was done also for the combustion option. The LCA for energy based on combustion of RCG biomass cultivated on mineral soil (present study) is compared with the published LCA for energy gained from RCG biomass cultivated on organic soil.

6.2.1 Atmospheric impact of biogas production based on TIM and RCG Management-related GHG emissions were three times higher from TIM system than those from the RCG system during the study period (Table 6). The differences arise from the agricultural practices (Table 3) of the systems, as TIM requires more field operations and transportation of the yield than RCG. For TIM, the fresh yield was harvested once in 2009 and twice in 2010 and 2011. Fertilizers were also applied with the same frequency (one to two times per year). For RCG, spring harvesting was done and fertilizers were applied only once during the entire growing season. When the net emissions (balance in Table 5 and management-related emissions in Table 6) were calculated per harvested yields (Table 3) , the net emissions were generally lower for RCG (720 and 570 g CO2-eq per kg DW in 2010 and 2011,respectively) than for TIM (310, 1500 and 930 g CO2-eq per kg DW in 2009, 2010 and 2011 respectively).

Table 6 Management-related greenhouse gas emissions (kg CO2-eq ha-1) for mixture of timothy and meadow fescue (TIM) and reed canary grass (RCG).

2009 2010 2011

To estimate the net emissions per MWh of produced energy from biogas, CH4 production potentials of 350 and 260 m3 CH4 per ton of dry biomass were used for the TIM and RCG biomass, respectively. Energy content of 10 kWh m-3 was used for CH4. The net emissions for energy produced from biogas were higher for TIM than

134

for RCG (Figure 4). Net GHG emissions of produced energy varied from 31 to 150 kg CO2-eq per MWh with TIM. Using the two years of data for RCG the emissions were 57 and 72 kg CO2-eq per MWh. In addition, the sum of energy production potential during the three year study period was lower with RCG (130 MWh ha-1) than with TIM (330 MWh ha-1). All emissions associated with management, chemicals, cultivation and transportation are included in these analyses.

Figure 4 Net greenhouse gas emission (kg CO2-eq per MWh of energy) for energy produced from biogas of mixture of timothy and meadow fescue (TIM) and reed canary grass (RCG). Net emissions for RCG in 2009 were not determined as biomass was not harvested during the first season.

6.2.2 Atmospheric impact of energy based on combustion of RCG biomass cultivated on mineral or organic soil

Net GHG emission of produced energy from combustion of biomass of RCG cultivated on mineral soil was within the range of the values reported for RCG cultivated on an organic soil site in Finland (Shurpali et al., 2010). With RCG biomass on mineral soil, the net emissions were 129 kg CO2 per MWh in 2010 and also in 2011. At the organic soil site, the net emission ranged from -550 to 340 kg CO2 per MWh of produced energy with a median value of 120 during the five-year study period (Shurpali et al., 2010). The negative value means that the organic site was able to fix enough atmospheric CO2 in some years that the net uptake more than compensated the emissions in the energy production chain. As a median value, the atmospheric impact of energy produced from RCG did not depend on the soil type. Coal is a dominant fossil fuel in energy production. Based on LCA, the CO2 emissions from coal energy are 340 kg CO2 per MWh of produced energy (Kirkinen et al., 2007). When compared with energy produced from coal, both RCG systems were climate-smart options (Figure 5).

n.d.

135 Figure 5 Net greenhouse gas emissions of bioenergy production (combustion) based on reed canary grass cropping systems on organic soil (Linnansuo) and on mineral soil (Maaninka) based on life-cycle assessment. For Linnansuo (Shurpali et al., 2010) the data are shown as a boxplot where line marks the median value.

Above and below the median, is 50% of the data. Further, between the box and the whiskers is 25% of the data. At the organic soil site, there are six measurement years with varying climatic conditions which is why there is variation in the annual net emissions. For mineral soil site, there were data only 2 years and those years had the same net emissions value. Therefore, the line represents the result of 2010 and 2011. The thick line represents the net greenhouse gas emissions from coal.

As the management-related emissions of RCG were similar between the two sites and years, 230 kg CO2 ha-1 at the mineral soil site and 190±35 kg CO2 ha-1 at the organic soil site (Shurpali et al., 2010), the large variation in the net emissions of produced energy at the organic soil site was due to precipitation-dependent variation in the yields and overall GHG balance (Figure 6). During wet years, the yields and the net CO2 uptake (NEE more negative) at the organic soil site were generally higher than during the dry years. This dependence between crop performance and the weather conditions were linked to the soil characteristics of the site. At the organic soil site, the water table remained deep below the peat-sand interface and capillary flow did not transfer water to the upper peat layers of the soil where the plant roots are located (Gong et al., 2013). Therefore, the crop performance at the organic soil was precipitation (soil moisture) dependent. A similar weather impact was not observed at the mineral soil although year 2010 was drier and cooler and year 2011 wetter and warmer than the 30-year mean (Chapter 5). It is worth noting here that this study has data only for two years. For better comparison, more data with varying climatic conditions from the mineral soil site would be needed.

134

for RCG (Figure 4). Net GHG emissions of produced energy varied from 31 to 150 kg CO2-eq per MWh with TIM. Using the two years of data for RCG the emissions were 57 and 72 kg CO2-eq per MWh. In addition, the sum of energy production potential during the three year study period was lower with RCG (130 MWh ha-1) than with TIM (330 MWh ha-1). All emissions associated with management, chemicals, cultivation and transportation are included in these analyses.

Figure 4 Net greenhouse gas emission (kg CO2-eq per MWh of energy) for energy produced from biogas of mixture of timothy and meadow fescue (TIM) and reed canary grass (RCG). Net emissions for RCG in 2009 were not determined as biomass was not harvested during the first season.

6.2.2 Atmospheric impact of energy based on combustion of RCG biomass cultivated on mineral or organic soil

Net GHG emission of produced energy from combustion of biomass of RCG cultivated on mineral soil was within the range of the values reported for RCG cultivated on an organic soil site in Finland (Shurpali et al., 2010). With RCG biomass on mineral soil, the net emissions were 129 kg CO2 per MWh in 2010 and also in 2011. At the organic soil site, the net emission ranged from -550 to 340 kg CO2 per MWh of produced energy with a median value of 120 during the five-year study period (Shurpali et al., 2010). The negative value means that the organic site was able to fix enough atmospheric CO2 in some years that the net uptake more than compensated the emissions in the energy production chain. As a median value, the atmospheric impact of energy produced from RCG did not depend on the soil type. Coal is a dominant fossil fuel in energy production. Based on LCA, the CO2 emissions from coal energy are 340 kg CO2 per MWh of produced energy (Kirkinen et al., 2007). When compared with energy produced from coal, both RCG systems were climate-smart options (Figure 5).

n.d.

135 Figure 5 Net greenhouse gas emissions of bioenergy production (combustion) based on reed canary grass cropping systems on organic soil (Linnansuo) and on mineral soil (Maaninka) based on life-cycle assessment. For Linnansuo (Shurpali et al., 2010) the data are shown as a boxplot where line marks the median value.

Above and below the median, is 50% of the data. Further, between the box and the whiskers is 25% of the data. At the organic soil site, there are six measurement years with varying climatic conditions which is why there is variation in the annual net emissions. For mineral soil site, there were data only 2 years and those years had the same net emissions value. Therefore, the line represents the result of 2010 and 2011. The thick line represents the net greenhouse gas emissions from coal.

As the management-related emissions of RCG were similar between the two sites and years, 230 kg CO2 ha-1 at the mineral soil site and 190±35 kg CO2 ha-1 at the organic soil site (Shurpali et al., 2010), the large variation in the net emissions of produced energy at the organic soil site was due to precipitation-dependent variation in the yields and overall GHG balance (Figure 6). During wet years, the yields and the net CO2 uptake (NEE more negative) at the organic soil site were generally higher than during the dry years. This dependence between crop performance and the weather conditions were linked to the soil characteristics of the site. At the organic soil site, the water table remained deep below the peat-sand interface and capillary flow did not transfer water to the upper peat layers of the soil where the plant roots are located (Gong et al., 2013). Therefore, the crop performance at the organic soil was precipitation (soil moisture) dependent. A similar weather impact was not observed at the mineral soil although year 2010 was drier and cooler and year 2011 wetter and warmer than the 30-year mean (Chapter 5). It is worth noting here that this study has data only for two years. For better comparison, more data with varying climatic conditions from the mineral soil site would be needed.

136

Figure 6 Relationship between annual precipitation and net greenhouse gas (GHG) emissions per MWH of produced energy (by combustion) at organic soil site (closed circle, 2004-2007, Shurpali et al., 2010) and mineral soil site (open triangle, 2010 and 2011, present study) cultivated with reed canary grass.

6.2.3 Sources of uncertainty in LCA calculations for bioenergy production There are potential sources of uncertainties in the LCA estimations of the atmospheric impact of energy based on biomass. As indicated earlier, we have employed both EC and static chamber techniques to measure exchange of NEE, CH4 and N2O in situ. For EC based fluxes, we have performed an analysis to estimate the random errors of 30 min averaged and quality-controlled CO2 fluxes following Vickers and Mahrt (1997). The random error in CO2 fluxes was estimated to be 13 % during the measurement campaign (Lind et al., 2016). Errors in chamber-based data depend on several factors such as the type of the chamber and sampling method, the precision of the instrument, chamber dimensions and operation time. It is worth noting in the context of this study that the uncertainty in LCA of biomass production for energy arises from uncertainties especially in the estimation of NEE and crop yield. These were the two most important components of LCA, as has been reported for RCG on a drained organic soil in Eastern Finland (Shurpali et al., 2010). Although LCA includes other components such as crop management and biomass handling related emissions, these components are minor ones in the overall LCA (Shurpali et al., 2010). For example, for the RCG cultivated on a drained organic soil, the average crop management related CO2 emissions were within 1% of the CO2 emissions associated with biomass removal (crop yield).

We have used values published in previous studies for the estimation of CO2 emissions associated with the field machinery use and transportation, and for energy costs associated with fertilizer production. Thus, the major factors that influence the environmental impact of biomass used for energy production include interannual variation in climate and the type of soil used for cultivation (nutrient

137 poor vs rich, mineral vs organic soils), while the random and systematic errors of actual GHG measurements had lower role.

6.3 SPECIAL NOTES ON THE ESTIMATION OF THE ANNUAL