Assessing the environmental benefits and impacts of exploiting any source of energy requires consideration of the complete supply system for the fuel, its treatment and conversion, and delivery of usable energy, i.e., it requires Life Cycle Assessment (LCA) (Hammond et al., 2015). Because the focus is on assessing possible uses for a limited resource, the functional unit for the LCA (i.e., the basis on which alternatives are compared) is a mass of biomaterial.
For bioenergy crops, LCA is complicated by the need to consider changes in land use; this has generated heated discussion in LCA circles over two types of analysis termed Attributional and Consequential (e.g., Yang, 2016). However, the forms of biomass considered here are currently waste materials. This renders the assessment much more straightforward: the LCA can be earned out using an approach that is well established and incorporated in a number of software packages for assessing the environmental effects of alternative strategies for waste management allowing for recovery of energy and materials. The basic approach is shown in Figure 3. The waste to be used or treated exists regardless of the activities to manage it, so the processes generating the waste are common to all systems to use or manage the material and can, therefore, be omitted from comparative assessment. The starting point for the system is, therefore, the available and unused “waste” biomass. Even with this simplification, there is a methodological issue that affects the outcome of the assessment; it is discussed in section 4.5 below in connection with anaerobic digestion of food waste and animal manure.
Figure 3 shows the distinction between the Foreground System, comprising the operations whose selection or mode of operation is affected directly by decisions based on the study, and the Background System, i.e., other processes connected to the Foreground by exchange of energy and/or materials. The distinction is pragmatic, but a sufficient (but not necessary) condition for processes to be in the Background is that the exchange with the Foreground takes place through a homogeneous market. Ideally, the Foreground activities should be described by primary data, but the background can be described by generic data, such as those available in databases; in this work, GHGenius ((S&T)2 Consultants Inc., 2013) was used. The LCAmust allow for the exchanges between the Foreground and Background, including energy recovery from the biomass “waste”. It is assumed that the other functional outputs from the Background system are unchanged, so that the recovered energy displaces energy from other sources. The total inventory—i.e., resource inputs and environmental emissions—comprises the direct burdens associated with the Foreground plus the indirect burdens associated with Background production of materials and energy used in the Foreground minus the avoided burdens displaced from the Background by ener gy (and, where relevant, materials) recovered from the waste and from “slash-burning” of waste that would otherwise not be put to beneficial use. This modelling approach enables the assessment to account for the efficiency of the technology used in the Foreground to process the biomass and also to describe different uses for the energy recovered from the biomass.
In this work, the Foreground processes for forestry wastes include activities at the source (felling, skidding, etc.), pelletisation where relevant, and transport by rail or heavy diesel truck to the bioenergy conversion plant. Further details are given by Wang (2019). Transport distances have been based on the averages for BC but the overall results are not sensitive to this detail. Inputs to and emissions and transport from the conversion plant are specific to each technology. The focus here is primarily on carbon management, i.e., on life cycle GHG emissions, but human health impacts were also assessed and are a significant consideration in the hierarchy of preferred uses; see section 4.4 below.
Figure 3. Life cycle assessment of waste management systems (Clift et al., 2000).
The climate-forcing impacts of emissions are evaluated in terms of CO,-equivalents based on the values for Greenhouse Wanning Potential (GWP) given in the IPCC Fifth Assessment Report using a time horizon of 100 years (IPCC, 2013). Following common convention, GWP values for a time horizon of 100 years are used, although the increasing urgency of the climate crisis argues for using a shorter horizon. Because carbon in the biomass fonns part of the renewable carbon cycle, removed from the atmosphere by incorporation into biomass grown to replace that used, carbon dioxide emitted from combustion of biomass is conventionally treated as climate-neutral; i.e., it does not contribute to climate change (IPCC, 2011). This is common practice in the LCA of waste management (Astrup et al., 2015), and is further justified in the present study because the biomass will be burned anyway (see Figure 2) so that use as a traded fuel does not lead to additional carbon emissions. For carbon emitted in fonns other than carbon dioxide, notably methane, the impact assessment allows for the different GWP. Emissions from other energy sources supplied fr om the Background and used in the Foreground are treated as nonrenewable.
Uses of Biomass
Technologies and efficiencies
As noted above, in view of the urgency of the climate crisis, only technologies already deployable or considered close to deployable har e been assessed. The overall energy conversion efficiencies cited in this section refer to net conversion of the higher heating value of the feedstock into that of the product in the conversion process, and are representative median values based on available literature. Conversion efficiency is the dominant parameter determining the ranking of technological alternatives (Wang, 2019; Wang et al., 2020), as expected from Figure 1. The LCA also allows for other processes in the supply chain, including transport; again, as expected fr om Figure 1. the results are sensitive to transport distances for forestry waste materials. The avoided burdens arising from the use of bioenergy are assessed for the specific conventional energy source replaced, mainly using the GHGeuius database ((S&T)2 Consultants Inc., 2013).
Heat, power and cogeneration
Gasification of biomass with combustion of the product gas has been assessed as a relatively clean form of the simple combustion route (i.e., the upper route in Figure 1). The energy can be used for space and water heating in commercial, institutional and residential buildings via district heating; the efficiency for this use is taken as 73%. For heat output only, the biomass displaces natural gas, the most widely used fuel for heating in BC. Alternatively, the energy can be used to generate electric power; the results presented here are based on a conversion efficiency of 38%. For the initial assessment, the output is taken to replace the average supply in BC but the significance of this assumption is explored in section 4.5 below. The gasified biomass can also be used for Combined Heat and Power generation (CHP). The efficiency is taken here as 30% for electrical output and 50% for heat; i.e., a total overall efficiency of 80%.
Gasification to synthesis gas
Woody feedstock can also be gasified to produce synthesis gas (a mixture of H,, CH,, CO and CO,), for subsequent conversion to an energy-carrier product. Energy inputs and losses are substantial (cf. Figure 1): The overall energy conversion efficiencies have been taken here as 54% for methanol production. 45% for ethanol and 58% for renewable natural gas (RNG).
Hydrothermal Liquefaction (HTL) converts woody feedstock into a mix of liquids that can be separated into gasoline, diesel, jet fuel and fuel oil. The energy supplied to the process and embodied in the other inputs is significant: The net energy efficiency of the process is less than 60% (Nie and Bi, 2018). HTL also produces a solid co-product containing carbon and nitrogen; the LCA includes avoided burdens, assessed assuming that it is used as a soil improver and displaces nitrogen fertiliser.
Anaerobic Digestion (AD) can be used to convert food waste, animal manure and crop residues into biogas, typically containing 60% methane and 40% CO,, with traces of other gases that impart the characteristic odour. Methane is a strong greenhouse gas, so fugitive emissions are a significant part of the GHG assessment; fugitive emissions are taken here as 2% of the total methane produced (Evangelisti et al. 2015). The indirect burdens (cf. Figure 3) associated with electricity and heat consumption in feed preparation and digestion are estimated as fractions of the energy content of the biogas: 11% for electricity and 13% for heat (Berghmd and Borjesson, 2006). Overall energy efficiency depends on the use of the biogas. If it is used close to the AD unit, for heat, electrical power or CHOP, it may not be necessary to separate the CO, from the methane before combustion. However, using the methane as renewable natural gas, for example for distribution via a gas grid, requires separation and purification processes that lower the net energy efficiency and raise the cost. AD also yields a digestate residue rich in soil nutrients. Avoided burdens have been calculated assuming that the digestate is used to replace synthetic fertilisers (Evangelisti et al. 2014).
Assessment of costs and benefits
To assess the economic viability of different applications, the minimum selling price (MSP) of energy products was estimated as the price at which the Net Present Value of a new plant is zero after 20 years, allowing for a real rate of return of 10% and inflation at 2% pa (Wang, 2019; Wang et al., 2020). Comparison of the MSPs with current commodity prices gives an indication of which technologies may be viable. However, this comparison is specific to the current BC context. It is more informative to show the results of the environmental and economic assessment in terms of possible contribution to GHG abatement and the associated cost.
Figure 4 shows the reduction in GHG emissions estimated as potentially available and the associated extra cost (not allowing for carbon tax), for the different combinations of feedstock and technology. The gradient of the line from the origin to the point representing each waste stream and technology represents the associated marginal GHG abatement cost per tonne of CO,-equivalent. For comparison, lines are shown for the currently projected Canada-wide carbon tax of S50 per tonne and for a tax at double this level, i.e., $100 per tonne. Points with negative costs represent technologies that are economically viable even in the absence of a carbon price. Points below the $50 line identify technologies that would be attractive with carbon price at that level. Technologies represented by points above the $100 line would require measures in addition to the carbon tax before they would be pursued.
The Marginal Abatement Costs (MAC) indicated by Figure 4 are broadly consistent with other studies (Wang, 2019). The minimum values are generally lower than values suggested by other studies, but this is to be expected given that the majority of studies hat e investigated wood chips and whole logs, whereas the waste materials considered here tend to have shorter supply chains. The maximum values obtained in this work, for whole wood logs, are consistent with literature data. A systematic sensitivity analysis shows that wide variations in the most significant variables do not change the ranking of options for use of the available biowastes that emerges from Figure 4. Overall, the priority order appears to be robust.