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Densified Biomass Can Cost-Effectively Mitigate Greenhouse Gas Emissions and Address Energy Security in Thermal Applications Thomas O. Wilson,† Frederick M. McNeal,‡ Sabrina Spatari,§ David G. Abler,† and Paul R. Adler*,‡ †
Department of Agricultural Economics and Rural Sociology, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ USDA-ARS Pasture Systems and Watershed Management Research Unit, Building 3702, Curtin Rd., University Park, Pennsylvania 16802, United States § Civil, Architectural, and Environmental Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States
bS Supporting Information ABSTRACT: Regional supplies of biomass are currently being evaluated as feedstocks in energy applications to meet renewable portfolio (RPS) and low carbon fuel standards. We investigate the life cycle greenhouse gas (GHG) emissions and associated abatement costs resulting from using densified switchgrass for thermal and electrical energy. In contrast to the large and positive abatement costs for using biomass in electricity generation ($149/Mg CO2e) due to the low cost of coal and high feedstock and power plant operation costs, abatement costs for replacing fuel oil with biomass in thermal applications are large and negative ( $52 to $92/Mg CO2e), resulting in cost savings. Replacing fuel oil with biomass in thermal applications results in least cost reductions compared to replacing coal in electricity generation, an alternative that has gained attention due to RPS legislation and the centralized production model most often considered in U.S. policy. Our estimates indicate a more than doubling of liquid fuel displacement when switchgrass is substituted for fuel oil as opposed to gasoline, suggesting that, in certain U.S. locations, such as the northeast, densified biomass would help to significantly decarbonize energy supply with regionally sourced feedstock, while also reducing imported oil. On the basis of supply projections from the recently released Billion Ton Report, there will be enough sustainably harvested biomass available in the northeast by 2022 to offset the entirety of heating oil demand in the same region. This will save NE consumers between $2.3 and $3.9 billion annually. Diverting the same resource to electricity generation would cost the region $7.7 billion per year. While there is great need for finding low carbon substitutes for coal power and liquid transportation fuels in the U.S., we argue that in certain regions it makes cost- (and GHG mitigation-) effective sense to phase out liquid heating fuels with locally produced biomass first.
1. INTRODUCTION Governments around the world are investigating ways to incorporate biomass into the transportation,1 heat, and power sectors to meet ambitious climate change mitigation targets.2 Despite the presence of large subsidies, efforts to incorporate cellulosic biomass into the transportation sector are slow to diffuse into commercial scale production. In the United States, renewable electricity mandates have come in the form of renewable portfolio standards (RPS). To date, 29 states have enacted binding RPS legislation. Together, these states account for more than half of all U.S. electricity sales.3 In addition, there have been recent discussions on Capitol Hill surrounding the enactment of a federal RPS. As a result, utilities are investigating the retrofit of coal generating stations to partially or entirely generate electricity from biomass.4,5 One source of biomass that has gained particular attention in the agricultural community is switchgrass. Switchgrass is attractive as a dedicated energy crop since it can be grown on marginal soils, limiting competition with food production, and requires relatively few inputs to achieve attractive yields.6 Biomass is more costly than fossil fuels because of the following: (1) Fossil fuels are generally found in large deposits r 2011 American Chemical Society
in one location, whereas biomass is geographically dispersed. (2) In its natural form, biomass lacks energy density when compared with fossil fuels, which increases transportation and storage costs per unit of energy. (3) Biomass does not lend itself well to materials handling. Switchgrass, in particular, is commonly packaged in bale form, and bales must be handled one to three at a time, resulting in compounding labor and operating expenses during each transfer. These factors create logistical challenges along the biomass feedstock supply chain.7,8 Biomass densification offers a means to overcome many of these logistical challenges.9 Densified products not only have greater energy density than raw biomass, but can be handled using bins, hoppers, and conveyance equipment developed for bulk materials such as coal and grains.10 Densified products include pellets, cubes, and briquettes. Switchgrass can be densified and employed in several thermal and electrical energy applications. Received: August 7, 2011 Accepted: November 22, 2011 Revised: November 17, 2011 Published: November 22, 2011 1270
dx.doi.org/10.1021/es202752b | Environ. Sci. Technol. 2012, 46, 1270–1277
Environmental Science & Technology Pellet appliances that are capable of utilizing switchgrass pellets to replace fossil fuels in home heating are commercially available. Cubes and briquettes can replace fossil fuels in larger, commercial scale boilers or be employed in electric power production. Moreover, briquettes and cubes achieve the desired logistical features of weight-limiting a semitrailer while retaining the favorable materials handling characteristics associated with pellets at a significantly lower cost. While biomass combustion itself is carbon neutral,11 fossil energy related CO2 and other greenhouse gas (GHG) sources associated with biomass production, transportation, and conversion must be properly inventoried to determine the net impact of biomass energy compared to fossil alternatives. Several authors have estimated the emissions associated with switchgrass establishment, production, and delivery to final consumption;12,13 however, no study has integrated production and densification costs of switchgrass with life-cycle emissions to arrive at abatement costs for heat and power applications. We use data from a large switchgrass producer in the rural northeastern U.S. to estimate production costs specific to the region. Farms in this region are characterized by relatively small fields within a fragmented landscape. We examine the GHG reductions and associated abatement costs of replacing fossil fuels with densified switchgrass products in both thermal and electric applications and compare these options.
2. METHODS 2.1. Life Cycle Inventory. We use process-based life cycle assessment (LCA) methods14 to estimate GHG emissions associated with switchgrass production, densification, and conversion to heat and power, and calculate production costs associated with process inputs along the supply chain. Several types of fuels and scales of energy utilization are considered for comparison: (1) residential thermal, 114 GJ/y, average NE residential heating demand [comparing fuel oil (fuel oil and heating oil are used interchangeably throughout the manuscript; both refer to No. 2 fuel oil), natural gas, and switchgrass pellet feedstocks];15 (2) commercial thermal, 8325 GJ/y, average heating demand of commercial facilities greater than 4645 m2 (50 000 ft2) (comparing fuel oil, natural gas, switchgrass cubes, and switchgrass briquette feedstocks); (3) electric power, 350 GWh/y biomass electric, representative of 50 MW repower or cofire scenario (comparing conventional coal, conventional natural gas, natural gas combined cycle (NGCC), and switchgrass cubes and briquettes used in retrofitted coal power plants). Cradle to gate modules were developed for each utilization scenario from crop establishment through end-use. Emissions at each step were estimated from a combination of producer data and existing literature. The LCA is closed once the fuel is combusted, as distribution losses are independent of fuel type and combustion technology. Functional units for each module are dependent on the output of the particular operation. For crop production and densification, the functional unit is one Mg dry matter (DM), and for final consumption in thermal and electricity production, functional units are GJ and MWh, respectively. 100-year global warming potentials16 for selected GHGs (CO2, CH4, N2O) were used to evaluate the contribution of emissions to the reduced transmittance of the atmosphere. As is common in biomass LCA,17 19 we assumed that all biogenic carbon that is released during combustion was absorbed during crop growth. Also, we assume crop establishment on marginal soils that were not previously in
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production and therefore do not consider emissions associated with indirect land-use change. While we concede that the largescale production of energy crops would inevitably induce some level of land-use change given the presence of positive opportunity costs, this is independent of biomass end-use and is therefore considered beyond the scope of the current analysis. 2.1.1. Switchgrass Production. Switchgrass production consists of seed production, establishment, and annual maintenance and harvest. Assumed stand lifetime was 10 years. Seed production emissions were estimated on the basis of field operations,20 seed cleaning motor ratings, and system throughput through interviews with a seed producer in Pennsylvania;21 to our knowledge, this is the first life cycle inventory of seed production from a large switchgrass producer (see Supporting Information (SI), Table S-1). Fuel consumption resulting from field operations (e.g., application of fertilizers and pesticides and tillage) were adapted (see SI) from the literature.12 Emissions associated with chemical and fertilizer inputs were evaluated on the basis of results from West and Marland22 and are discussed in the SI. Direct and indirect N2O emissions from N fertilizer application, residue remaining on the land, and roots were estimated on the basis of IPCC methods.23 Harvesting fuel consumption and baler productivity were based on field measurements (see SI). Switchgrass bales were stored uncovered at satellite locations on-farm. 2.1.2. Densification. Since very few operations consistently process perennial grasses as a feedstock, densification plant energy consumption and emissions were developed from equipment power ratings and switchgrass throughput rates. Electricity inputs to the densification plant were developed on the basis of manufacturer quotes on motor ratings and feedstock throughput, and through interviews with pellet, cube, and briquette producers and manufacturers. Other energy inputs included fuel consumption of loaders and the switchgrass necessary to dry the feedstock to the appropriate moisture. It is common practice in the biomass densification industry to use processed biomass to dry incoming feedstock. Given limited experience with the densification of grasses, the largest source of uncertainty in the densification analysis was due to estimated plant throughput. The effect of this assumption was evaluated via sensitivity analysis (Figure S-5). 2.1.3. Residential Thermal. Residential thermal options for the utilization of densified switchgrass were limited to pellet appliances. Currently, appliances are not available to utilize cubes or briquettes at this scale. We assume that pellet appliances capture 80% of total demand, with fuel oil capturing the peaks and periods of low demand. 2.1.4. Commercial Thermal. In commercial systems, the inability of solid fuel appliances to fully modulate dictates optimal boiler sizing to capture shoulder loads. Thermal mass requirements associated with solid fuel boiler design result in poor performance at low loads (
