Climate effects of woody biomass and fossil fuel use in stand-alone and integrated energy systems

Biomass is a key resource in a society based on renewable energy, but is a limited resource and the use of biomass in one sector will influence its availability for other sectors. The global energy system is heavily dependent on fossil fuels, and the climate impacts of CO2 occur regardless of the source of emissions. As a result, the climatic effects of biomass use in an energy system depend largely on which biomass feedstock and bioenergy pathway is being used, and what type of fossil fuel pathway is being replaced. In this study, we evaluate the CO2 emissions and climate effects of woody biomass and fossil fuel use. We analyse the potential production of electricity, heat or transport distance when using one kWh of woody biomass and fossil energy system designed to provide the same service to society as the most energy efficient bioenergy systems. The fuel cycle inputs are included in the analyses and are based on different state-of-the art as well as emerging technologies for energy conversion. We quantify the primary energy use and annual CO2 emission of different bioenergy and fossil alternatives. We then calculate the cumulative CO2 emission and climate effects in terms of cumulative radiative forcing for the fossil and bioenergy systems. The results show that primary energy use, CO2 emission, and cumulative radiative forcing vary strongly between the studied alternatives. The use of biomass should be considered in the context of the overall energy system, and in relation to the development of energy conversion technologies and potential integration between different energy sectors. This may identify pathways that are primary energy efficient and that give climate benefits in both the short and the long term. The use of bioelectricity and electric vehicles instead of biomotor fuel-based vehicles gives about twice the transport distance per unit of consumed woody biomass. Integrated energy systems that supply a package of energy services including electricity, heat and transport distance reduce the primary energy use and increase the climate benefits of woody biomass. The replacement of coal for heat and electricity production by the here studied woody biomass could give climate benefits immediately. Introduction Global energy supply depends heavily on fossil fuels. In 2014, fossil coal, oil and gas provided 81% of global primary energy use, and the use of fossil fuels is projected to increase even though the part of renewable energy use may also increase in global energy mix [1] [2]. However, efficient use of energy and switching to energy efficient supply chains based on renewable energy resources are key elements to mitigate climate change and improve energy security [3]. Of the renewable energy resources, biomass is a key resource with large potential for expansion [2] and unlike other renewable resources, biomass can be stored and converted to different energy carriers. The conversion from the existing fossil fuel systems to more sectors integrating renewable-based systems will take a long time and energy services are also likely to be supplied the coming 30-40 years from standalone fossil fuel plants [1]. Electricity, heat, and motor fuels are the major energy carriers to supply different energy services [4]. Woody biomass can 4-358-17 TRUONG, GUSTAVSSON 912 ECEEE 2017 SUMMER STUDY – CONSUMPTION, EFFICIENCY & LIMITS 4. MOBILITY, TRANSPORT, AND SMART AND SUSTAINABLE CITIES be used directly for electricity and heat production. Woody biomass can also be used in conventional fuel-based vehicles (FV) after conversion to suitable motor fuels by using different stateof-the-art and emerging technologies for biomotor fuel production. Furthermore, the recent development of technologies shows that electric vehicles (EV) could be an efficient means for transportation, also helping to integrate the transport sector with heat and electricity production systems [5]. However, biomass is a limited resource and the use of biomass in one sector will influence its availability for other sectors, stressing the importance of using highly energy-efficient bioenergy systems to supply energy services. In Sweden, biomass is a key resource in the existing national energy system. In 2015, 134 TWh of biomass was used in Sweden and that covered 25.4 % (31.3 % excluding losses in nuclear power plants) of total energy supply [6]. However, a large portion of forest residues such as forest slash (branches, foliage and tops) and stumps are left on the forest floor and decay naturally. The potential production and harvest of biomass in Sweden is large and increasing due to factors such as improved forest management and recovery practices of forest slash [7]. Various studies (e.g. [8], [9]) suggest that biomass production could be doubled, while some recent studies have estimated the potential recovery to be higher due to improved forest management and harvest strategies [10] [11]. Development of energy conversion technologies and sectoral integration of energy systems could improve the primary energy efficiency of energy systems. Heat and electricity can be coproduced from woody biomass and used interchangeably in several end-use applications. Biomotor fuel production may have different coproducts [12] [13] which could be integrated in energy systems in other sectors that may improve their system efficiency. In this study, we evaluate the climate effects of using forest slash for energy in heat, electricity and transportation sectors. We consider different pathways for using bioenergy to replace fossil fuels to provide an equivalent service to society including district heat, electricity and transport distance based on different state-of-the art and emerging technologies for energy conversion. We quantify the primary energy use and annual CO2 emissions of different bioenergy and for corresponding fossil fuel alternatives. Thereafter, we calculate the cumulative CO2 emissions and climate change effects in term of cumulative radiative forcing (CRF) for the bioenergy and corresponding fossil fuel system, per unit of consumed biomass. Finally, we analyse the primary energy savings of some integration of electricity, heat and transport systems and calculate the cumulative CO2 emissions and CRF for such bioenergy systems and corresponding fossil fuel systems. Method and Assumptions We compare bioenergy alternatives with fossil fuel alternatives, to provide the same energy services. The fossil alternatives include electricity and heat production by fossil coal or fossil gas in standalone plants and fossil motor fuels used in light-duty vehicles as well as EV using fossil electricity. Woody biomass is used to produce the corresponding energy services. The potential production of electricity, heat or transport distance when using one kWh of biomass, including the biomass used in the fuel cycle, is analysed for different stand-alone production systems. The use of fossil energy for the different fossil systems is then analysed when the same amount of electricity, heat and transport distance is produced as in the best stand-alone bioenergy system. The woody biomass considered here is forest slash, which has a large potential expansion in Sweden [14] [15]. The fuel cycle of biomass includes harvest of slash, chipping of slash, transport 100 km by truck to a terminal, then 250 km by train to the coast, and then 1,100 km by ship to an international enduser. Dry matter loss during forwarding, chipping roadside and transport is not considered. The lifecycle fossil CO2 emission for the different fossil systems is calculated. The fuel-cycle CO2 emission includes all emission from full lifecycle fuel use for the collection, processing, transportation and delivery of fuel to the conversion plants based on data from [16]. All calculations are based on the lower heating value (LHV) of fuels. If forest slash is not collected and used for bioenergy it is left and decays on the forest floor and releases biogenic carbon as CO2 to the atmosphere. These decaying biogenic CO2 emissions are calculated for a 100-year time period, beginning from year 0 when the energy services are produced. This biogenic CO2 emission is added to the fossil energy system. For all the bioenergy systems the use of one kWh of slash emits 403 g biogenic CO2 emission in year 0 when the forest slash is used for energy [17]. The cumulative biogenic and fossil CO2 emission is then calculated for a 100-year time period for the bioenergy and fossil energy systems. Hence, we consider annual emissions of biogenic and fossil CO2 over a 100-year time period for the whole energy chains. Based on annual biogenic and fossil CO2 emissions, we then calculate the CRF. For the different integrated systems, we assume that the same amount of electricity, heat and transport distance should be produced as in the stand-alone systems. However, we show only results for the most energy efficient integrated systems using woody biomass, coal or fossil gas. We calculate primary energy use, cumulative biogenic and fossil CO2 emission, as well as CRF over a 100-year time period for these integrated systems. BIOENERGY AND FOSSIL SYSTEMS The characteristic of selected energy conversion systems for district heat, electricity and motor fuels based on state-of-theart and emerging technologies for fossil fuel and biomass-based system are shown in Table 1. Light-duty vehicles exist with FVs, both diesel and gasoline versions, and as EVs. Different biomotor fuels can replace fossil motor fuels. Dimethyl ether (DME) and methanol (MeOH) is suitable to replace diesel in compression-ignition engines [23] and gasoline in spark-ignition engines [24], respectively. We assume that DME and MeOH replace diesel and gasoline, respectively. Furthermore, we assume that the use of a fossil motor fuel or a corresponding biomotor fuel will not to change the conversion efficiency of the motor. The energy performance of vehicles varies with manufacturers and vehicle models. In this study, we consider the light-duty vehicle model B-class from the European car manufacturer Mercedes-Benz. This vehicle model exists in electric, gasoline and diesel versions (Table 2). In Table 2, the electricity consumption includes wall-to-vehicle charging losses while the motor fuel consumption is dispensed fuel to vehicles. 4. MOBILITY, TRANSPORT, AND SMART AND SUSTAINABLE CITIES ECEEE SUMMER STUDY PROCEEDINGS 913 4-358-17 TRUONG, GUSTAVSSON For the delivered electricity to vehicles, we consider electricity transmission and distribution losses of 7.4 % which is based on the average losses during 2003–2012 in Sweden [27]. The distribution, storage and dispensing from the refineries to the final distribution point of motor fuels are assumed to be 2 % of final used fuel [5] [28]. DECAY OF FOREST SLASH If the forest slash is left in the forest, it will be decomposed by macroand micro-organisms and release biogenic carbon as CO2 to the atmosphere. The decay rate of the slash varies in time because the initial quality changes to lower qualities that decompose more slowly. Here, CO2 emission from the decay of forest slash is estimated using the process based Q-model [29] with the parameters given in Table 3. The model has been described in several papers, and has proven capability to estimate soil organic carbon changes at stand level and regional and national scales in Sweden [29] [30]. CUMULATIVE RADIATIVE FORCING Based on the annual CO2 emissions for each year, we calculate the cumulative radiative forcing as described by Zetterberg [13] using updated parameter values from IPCC [14] [15] [16]. We use Equation 1 to estimate the removal of CO2 from the atmosphere by natural processes at varying time rates [14] [15] [16]: