Site Variation in Life Cycle Energy and Carbon Footprints of Mallee

May 7, 2015 - This study reports the site variations in life cycle energy and carbon footprints of mallee biomass from nine sites in Western Australia...
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Site Variation in Life Cycle Energy and Carbon Footprints of Mallee Biomass Production in Western Australia Yun Yu,† John Bartle,‡ Daniel Mendham,§ and Hongwei Wu*,† †

School of Chemical and Petroleum Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia Department of Parks and Wildlife, Government of Western Australia, Locked Bag 104, Bentley Delivery Centre, Western Australia 6983, Australia § Commonwealth Scientific and Industrial Research Organisation (CSIRO) Land and Water, Private Bag 12, Hobart, Tasmania 7001, Australia ‡

S Supporting Information *

ABSTRACT: This study reports the site variations in life cycle energy and carbon footprints of mallee biomass from nine sites in Western Australia based on the latest field data and additional fertilizer application to compensate for nutrients exported from the sites. Across the sites, the energy and carbon footprints of mallee biomass range from 299 to 451 MJ/dry tonne and from −2.0 to 31.5 kg of CO2 equivalent (CO2-e)/dry tonne, equivalent to 5.8−8.7% of energy and −0.3−5.5% of carbon embedded in the biomass product at farm gate, respectively. Compensating for nutrient export clearly increases biomass energy and carbon footprints because of the increased fertilizer applications. Overall, the results show that mallee biomass production is close to being renewable and carbon-neutral. Selecting sites with a low initial soil carbon and achieving a high biomass yield can increase the soil carbon during biomass production, greatly reducing the carbon footprint of the produced biomass.

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INTRODUCTION Biomass resources, such as forest residue, agricultural residue, annual or perennial energy crops, and algal biomass, have been considered for the production of renewable energy and fuels.1−4 In Western Australia, mallee planting is being developed as an important strategy for managing dryland salinity, water quality, and restoration of biodiversity in the low to medium rainfall (300−600 mm mean annual rainfall) wheatbelt areas.5−7 Such plantings are integrated with existing agricultural activities (e.g., cropping or grazing), mostly in wide-spaced narrow belt configurations, which impose limited competition with adjacent crop or pasture.8 Mallees are multibranched short trees, which can be harvested on a short cycle (e.g., 3−7 years9 depending upon the yield) and then rapidly regenerate as coppice. Since the 1990s, over 12 000 ha of mallee belt have been established by farmers in Western Australia. If the cultivation and processing of mallee biomass becomes commercially viable, there is a potential to supply up to ∼10 million tonnes of dry biomass annually5,10 as a second-generation feedstock for various bioenergy applications.11−13 The successful development of a mallee-based biomass industry depends upon not only the economic performance of mallee production systems13−16 but also the energy and environmental performance.10,17 Life cycle assessment (LCA) is an important method to assess the energy and environmental impacts of various bioenergy and biofuels.18−20 Particularly, if a carbon-trading scheme is adopted in Australia, carbon credits associated with mallee biomass will provide additional income to farmers, further improving the economic performance of mallee biomass production. Mallee biomass production inevitably consumes various non-renewable energy and resources (e.g., fertilizers, mineral fuels, etc.10,17). Therefore, it is important to © 2015 American Chemical Society

have an accurate estimation of the life cycle energy and carbon footprints of mallee biomass. A methodological framework10,17 was established previously for estimating the life cycle energy and carbon footprints of mallee biomass based on average biomass yields from typical planting sites without considering the compensation of nutrients exported from the soil. However, subsequent field data showed that the annual biomass yield could vary with soil conditions (e.g., water holding capacity and fertility), climate (e.g., temperature, rainfall, radiation, and frost), mallee species, and harvest management, ranging from 2.8−18.2 dry tonnes ha−1 year−1 for coppice biomass across the range of species and sites.21 Such variation may strongly influence the energy and carbon footprints of mallee biomass produced at different sites. Subsequent field data also showed that mallee biomass production leads to substantial export of nutrients (i.e., N, P, and K) from the soil.22 Obviously, this nutrient export must be compensated (e.g., via fertilizer application) to make biomass production sustainable. Consequently, it is the objective of this study to carry out a detailed analysis on the site variations in life cycle energy and carbon footprints of mallee biomass based on the comprehensive field data on biomass yield, nutrient export data, and fertilizer use from nine sites across the wheatbelt areas of Western Australia.

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MATERIALS AND METHODS

The LCA is conducted to compare the energy and carbon footprints of mallee biomass production from nine sites across the wheatbelt areas of Western Australia, as shown in Figure S1 of the Supporting Information. Received: March 24, 2015 Revised: May 7, 2015 Published: May 7, 2015 3748

DOI: 10.1021/acs.energyfuels.5b00618 Energy Fuels 2015, 29, 3748−3752

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Energy & Fuels

Figure 1. LCA of energy inputs and GHG emissions of mallee biomass production at various sites in Western Australia. (a) Total energy input during mallee biomass production for a duration of 67 years. (b) Energy input per dry tonne of mallee biomass. (c) Total GHG emissions during mallee biomass production for a duration of 67 years. (d) GHG emissions per dry tonne of mallee biomass. Site 1, Alexander; site 2, Bird; site 3, Fuchbichler; site 4, Morrell; site 5, Quicke; site 6, Stanley; site 7, Strahan; site 8, Sullivan 1; and site 9, Sullivan 2. The error bars show the ranges of energy inputs and GHG emissions, which are calculated on the basis of the ranges of biomass yield and fertilizer use for each site. The site details, including rainfall, evaporation, mallee species, and planting year are presented in Table S1 of the Supporting Information. The system boundary of this LCA is from cradle to farm gate, including biomass establishment (seed, seedling, planning, site preparation, planting, and post-planting), management (post-harvest), harvest, and on-farm haulage (see Figure S2 of the Supporting Information), following a similar methodology as for our previous work.10,17 For the comparison of mallee biomass across sites, a mallee production system was considered to have a 67 year duration, including an initial 7 years from mallee establishment to first harvest, followed by multiple coppice harvest cycles of 4 or 5 years. The life cycle inventory (LCI) of mallee biomass for a typical site can be found in Table S2 of the Supporting Information. The variations in the LCI at various sites are summarized in Table S3 of the Supporting Information. The exports of key nutrients (i.e., N, P, and K) from various sites during biomass production are plotted in Figure S3 of the Supporting Information. During each harvest, all aboveground biomass is harvested and cut into biomass chips, which are then transferred to the farm gate. The biomass loss during mallee harvest and on-farm haulage is assumed to be minimal. The LCA in this study follows the ISO 14040 series guidelines,23 and the detailed approaches for assessing life cycle energy and carbon footprints can be found in the Supporting Information. The functional units of energy and carbon footprints are MJ/dry tonne and kg of carbon dioxide equivalent (CO2-e)/dry tonne, respectively. The dynamic carbon sequestration in both aboveground biomass (i.e., wood, bark, twig, and leaf) and belowground biomass (i.e., root) is also simulated. Furthermore, land use change has the potential to either release or sequester carbon.24 Clearing of native vegetation for agricultural use often reduces soil carbon stocks, while there is a potential to increase soil carbon stocks through re-establishment of perennial vegetation,24 e.g., mallee planting in wheatbelt areas. In this study, the dynamic change of soil carbon (0−30 cm) is simulated using the forest carbon accounting model (FullCAM) of the Australian Government,25−27 which has been well-calibrated and widely used to predict carbon sequestration in

woody products (above- and belowground biomasses), debris (litter and dead roots), and soil of terrestrial ecosystems in Australia.

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RESULTS AND DISCUSSION Site Variation in Life Cycle Energy and Carbon Footprints of Biomass. Figure 1 presents the LCA of energy input during mallee biomass production at various sites in Western Australia. There is a significant variation in the total energy input of mallee biomass production, ranging from ∼335 to ∼1087 GJ/ha for the whole 67 year production period. Among all of the activities for biomass production, the contribution of mallee establishment is small. The energy input mainly results from harvest, on-farm haulage, and post-harvest. The total energy input strongly depends upon the biomass yield (see Figure 1a). This is because biomass harvest and on-farm haulage are directly related to the biomass yield; post-harvest is mainly contributed by fertilizer application that is also dependent upon biomass yield because fertilizers are applied at replacement rates in this study to compensate for nutrients exported from the soil. Thus, the total life cycle energy input appears to have a linear relationship with the biomass yield (see Figure S4 of the Supporting Information). In comparison to the total energy input, there is a small variation (i.e., 299−451 MJ/dry tonne) in the energy input per dry tonne of mallee biomass (i.e., energy footprint) for all sites. The energy footprint is higher than that reported in our previous work,10 in which the application of fertilizers to compensate for nutrients exported from the soil was not considered but is required for sustainable biomass production. Among the nine sites, the highest energy footprint of mallee biomass is found at site 5 (see Figure 1b). This is mainly due to the high fertilizer input at site 5 (see Figure S3 of the Supporting Information), 3749

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Figure 2. Dynamic changes of carbon in soil (0−30 cm) and above- and belowground biomasses over time during mallee biomass production for a duration of 67 years. (a) Site 1, Alexander; (b) site 2, Bird; (c) site 3, Fuchbichler; (d) site 4, Morrell; (e) site 5, Quicke; (f) site 6, Stanley; (g) site 7, Strahan; (h) site 8, Sullivan 1; and (i) site 9, Sullivan 2. The dashed lines show the range of soil carbon change.

carbon/ha for a production period of 67 years. This amount of carbon sequestered is even lower than that of the aboveground biomass for a harvest cycle. Second, the soil carbon decreases during the first harvest cycle. This is due to the relatively low inputs of carbon from debris and roots by young mallees compared to the decomposition of soil carbon.29,30 In subsequent harvest cycles, the soil carbon may start to increase or decrease, depending upon the trade-off between the input and decomposition of organic material in the soil at various sites. Generally, a low initial soil carbon level with a high biomass yield was predicted to result in an increase in soil carbon, i.e., sites 1 and 6, while a high initial soil carbon level and a low biomass yield may lead to a decrease in soil carbon, i.e., sites 2, 4, and 9. For other sites, there are small changes in the soil carbon after 67 years since planting, probably because the carbon sequestration and carbon release in soil are almost balanced. The GHG emissions and CO2 sequestration of mallee biomass from various sites are compared in Figure 3. Mallee biomass production generally has a positive carbon footprint for most of the sites, because the carbon sequestration in belowground biomass is smaller than the GHG emissions during mallee production. If the soil releases CO2 during mallee production, it leads to a positive carbon footprint. For example, the GHG emissions as a result of biomass production and CO2 release from soil for site 4 are ∼58.0 and ∼42.5 tonnes of CO2-e/ha for a production period of 67 years (equivalent to ∼5.6 and ∼4.1% of carbon embedded in the aboveground biomass), respectively. Such GHG emissions cannot be offset by the carbon sequestration because of belowground biomass (only ∼43.2 tonnes of CO2-e/ha, equivalent to ∼4.2% of carbon embedded in the aboveground biomass), thus resulting in a positive carbon

significantly increasing the post-harvest energy input. In addition, biomass yield also affects the energy input. A low biomass yield leads to increased energy input in harvest and on-farm haulage. Figure 1 also shows that total life cycle greenhouse gas (GHG) emissions during mallee biomass production follow similar trends to total life cycle energy input. A large variation in the total GHG emissions can be found for mallee biomass production from different sites, but this variation is small when normalized to the total biomass yield for the production period. The GHG emissions are estimated to be ∼25.4−39.7 kg of CO2-e/dry tonne for all sites, equivalent to ∼4.4−6.9% of carbon embedded in the aboveground biomass. A high GHG emission is also observed for site 5 obviously because of its high fertilizer use and low biomass yield. During the life cycle of mallee production, carbon can also be dynamically sequestered in both above- and belowground biomasses. Moreover, the soil may sequester additional carbon, depending upon the balance between the addition and decomposition of organic material in soil.28 Therefore, the overall carbon footprint can be negative if the carbon sequestration as a result of belowground biomass and soil carbon change is more than the GHG emissions during biomass production. The dynamic changes of carbon in soil (0−30 cm) and belowground biomass were simulated using the FullCAM model, with the aboveground biomass yield calibrated by the field data. Two important findings can be observed from the simulation results in Figure 2. First, the carbon in the belowground biomass was predicted to increase rapidly in the first 20 years, then start to plateau, and reach equilibrium in the following years. Thus, the carbon sequestration in the belowground biomass is limited, ranging from 4.4 to 17.2 tonnes of 3750

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(depending upon site location) of total energy input and GHG emissions during the life cycle of mallee biomass production, respectively. Our sensitivity analysis also showed that the life cycle energy and carbon footprints are very sensitive to N fertilizer input and biomass yield but not to P and K fertilizer inputs (see Figure 4). Therefore, several key strategies can be considered to reduce the energy and carbon footprints of mallee biomass. First, selecting sites with a low initial soil carbon level and a high biomass yield is important to produce biomass with a low carbon footprint. A negative carbon footprint is also possible if the GHG emissions during mallee production can be completely offset by the carbon sequestration because of the belowground biomass and the soil carbon increase. Second, considering that biochar from biomass pyrolysis retains the majority of nutrients in biomass,31,32 recycling biochar to the soil could be a good strategy to return the majority of the nutrients in biomass back to the soil, thus greatly reducing the fertilizer use to compensate for nutrient export. Returning biochar to the soil also achieves carbon sequestration,33 further reducing the carbon footprint of pyrolysis products (e.g., bio-oil). Finally, future agricultural knowledge on nutrient responses, nutrient sources, and management options will potentially reduce the fertilizer use, e.g., by capturing nutrient runoff from adjacent crop and pasture land (including the nitrogen sources fixed biologically)34 or accessing large sub-soil storage of nutrients because of the deep root system of mallee biomass.35,36

Figure 3. GHG emissions, CO2 sequestration, and overall carbon footprint of mallee biomass production at various sites in Western Australia. Site 1, Alexander; site 2, Bird; site 3, Fuchbichler; site 4, Morrell; site 5, Quicke; site 6, Stanley; site 7, Strahan; site 8, Sullivan 1; and site 9, Sullivan 2.

footprint of ∼31.5 kg of CO2-e/dry tonne for site 4 (equivalent to ∼5.5% of carbon embedded in the aboveground biomass). Therefore, a positive or negative carbon footprint of mallee biomass is determined by soil carbon change during biomass production. To achieve a low carbon footprint, it is important to choose sites with low initial carbon levels and high biomass yields. Practical Implications. This study has highlighted the important roles of site variation and nutrient export in determining the energy and carbon footprints of mallee biomass production. Considering additional fertilizer applications to compensate for nutrient export, it was shown that the life cycle energy and carbon footprints of mallee production account for 5.8−8.7 and −0.3−5.5% (depending upon site location) of energy and carbon embedded in the aboveground biomass, respectively. Overall, the data showed that mallee biomass is close to being renewable and carbon-neutral. It is noteworthy that fertilizer application accounts for 59−72 and 70−81%

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CONCLUSION A LCA has been carried out to evaluate the site variation in the energy and carbon footprints of mallee biomass production in the wheatbelt area of Western Australia. The highlight of the work is the consideration of additional fertilizer application to compensate for nutrients exported from the site for sustainable mallee biomass production. A small variation of 299−451 MJ/ dry tonne (equivalent to 5.8−8.7% of energy embedded in the biomass product) in the energy footprint of mallee biomass at the farm gate is found for all sites in this study, while for the carbon footprint, a large variation ranging from −2.0 to 31.5 kg of CO2-

Figure 4. Sensitivity analysis of life cycle energy and carbon footprints to the inputs of fertilizers (N, P, and K) and biomass yield for two typical sites: (a) site 1, Alexander; and (b) site 7, Strahan. The results of other sites follow a similar trend (see Figure S5 of the Supporting Information). 3751

DOI: 10.1021/acs.energyfuels.5b00618 Energy Fuels 2015, 29, 3748−3752

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Energy & Fuels e/dry tonne (equivalent to −0.3−5.5% of carbon embedded in the biomass product) exists. A low carbon footprint can be achieved when soil carbon increases during mallee biomass production, i.e., for the site with a low initial soil carbon and a high biomass yield. This study clearly demonstrates that nutrients exported from soil during biomass production need to be considered in life cycle analysis, and even when this is taken into consideration, mallee biomass production is still close to being renewable and carbon-neutral.

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Rural Industries Research and Development Corporation, Australian Government: Barton, Australian Capital Territory, Australia, July 2012. (16) Spinelli, R.; Brown, M.; Giles, R.; Huxtable, D.; Relano, R. L.; Magagnotti, N. Agofor. Syst. 2014, 88, 479−487. (17) Yu, Y.; Wu, H. Energy Fuels 2010, 24, 5660−5668. (18) Tonini, D.; Hamelin, L.; Wenzel, H.; Astrup, T. Environ. Sci. Technol. 2012, 46, 13521−13530. (19) Steubing, B.; Zah, R.; Ludwig, C. Environ. Sci. Technol. 2012, 46, 164−171. (20) Cherubini, F.; Strømman, A. H. Bioresour. Technol. 2011, 102, 437−451. (21) Peck, A.; Sudmeyer, R.; Huxtable, D.; Bartle, J.; Mendham, D. Production of Mallee Agroforestry SystemsThe Effect of Harvest and Competition Management Regimes; Rural Industries Research and Development Corporation, Australian Government: Barton, Australian Capital Territory, Australia, April 2012. (22) Mendham, D.; Bartle, J.; Peck, A.; Bennett, R.; Ogden, G.; McGrath, G.; Abadi, A.; Vogwill, R.; Huxtable, D.; Turnbull, P. Management of Mallee Belts for Profitable and Sustained Production; CSIRO, WA DEC, and the CRC for Future Farm Industries: Perth, Western Australia, Australia, May 2012. (23) International Organization for Standardization (ISO). ISO 14040−14043, Environmental ManagementLife Cycle Assessment. ISO: Geneva, Switzerland; 1997. (24) Paul, K. I.; Polglase, P. J.; Nyakuengama, J. G.; Khanna, P. K. For. Ecol. Manage. 2002, 168, 241−257. (25) Kesteven, J.; Landsberg, J.; URS Australia. Developing a National Forest Productivity Model; Australian Greenhouse Office, Australian Government: Canberra, Australian Capital Territory, Australia, May 2004. (26) Paul, K.; Roxburgg, S.; Raison, J.; Larmour, J.; England, J.; Murphy, S.; Norris, J.; Ritson, P.; Brooksbank, K.; Hobbs, T.; Neumann, C.; Lewis, T.; Read, Z.; Clifford, D.; Kmoch, L.; Rooney, M.; Freudenberger, D.; Jonson, J.; Peck, A.; Giles, R.; Bartle, J.; McAurthur, G.; Wildy, D.; Lindsay, A.; Preece, N.; Cunningham, S.; Powe, T.; Carter, J.; Bennett, R.; Mendham, D.; Sudmeyer, R.; Rose, B.; Butler, B.; Cohen, L.; Fairman, T.; Law, R.; Finn, B.; Brammar, M.; Minchin, G.; van Oosterzee, P.; Lothian, A. Improved Estimation of Biomass Accumulation by Environmental Plantings and Mallee Plantings Using FullCAM; CSIRO Sustainable Agriculture Flagship: Canberra, Australian Capital Territory, Australia, Oct 2013. (27) Richards, G. P. The FullCAM Carbon Accounting Model: Development, Calibration and Implementation for the National Carbon Accounting System; Australian Greenhouse Office, Australian Government: Canberra, Australian Capital Territory, Australia, Feb 2001. (28) Mendham, D. S.; O’Connell, A. M.; Grove, T. S. Agric. Ecosyst. Environ. 2003, 95, 143−156. (29) Paul, K. I.; Polglase, P. J.; Richards, G. P. For. Ecol. Manage. 2003, 177, 485−501. (30) Paul, K.; Polglase, P.; Coops, N.; O’Connell, T.; Grove, T.; Mendham, D.; Carlyle, C.; May, B.; Smethurst, P.; Baillie, C. Modelling Change in Soil Carbon Following Afforestation or Reforestation: Preliminary Simulations Using GRC3 and Sensitivity Analyses; Australian Greenhouse Office, Australian Government: Canberra, Australian Capital Territory, Australia, March 2002. (31) Wu, H.; Yip, K.; Kong, Z.; Chun-Zhu, L.; Liu, D.; Yu, Y.; Gao, X. Ind. Eng. Chem. Res. 2011, 50, 12143−12151. (32) Kong, Z.; Liaw, S. B.; Gao, X.; Yu, Y.; Wu, H. Fuel 2014, 128, 433− 441. (33) Lehmann, J. Nature 2007, 447, 143−144. (34) Bennett, R. G.; Mendham, D.; Ogden, G.; Bartle, J. GCB Bioenergy 2014, DOI: 10.1111/gcbb.12207. (35) Nulsen, R. A.; Blign, K. J.; N, B. I.; Solin, E. J.; Imrie, D. H. Aust. J. Ecol. 1986, 11, 361−371. (36) Robinson, N.; Harper, R. J.; Smettem, K. R. J. Plant Soil 2006, 286, 141−151.

ASSOCIATED CONTENT

S Supporting Information *

Details of nine studied sites (Figure S1 and Table S1), approaches for assessing life cycle energy and carbon footprints for a mallee biomass production system (Figure S2), life cycle inventory of mallee biomass (Table S2), site-specific data for the activities in nine sites (Table S3), site-specific nutrient data (Figure S3), correlation analysis (Figure S4), and sensitivity analysis (Figure S5). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.energyfuels.5b00618.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +61-8-92662681. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by the Australian Government through the Second Generation Biofuels Research and Development Grant Program. Partial support from the Australian Research Council is also acknowledged.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b00618 Energy Fuels 2015, 29, 3748−3752