Full length articleTemporal and geographic drivers of biomass residues in California
Introduction
Characterizing biomass residue supply is central to understanding the role bioenergy and bioproducts can play in offsetting fossil fuel and petrochemical consumption in the United States (U.S.). Numerous biomass inventory assessments have demonstrated the diversity and long-term abundance of the resource at the national and regional scales (Turhollow et al., 2014; Perlack et al., 2011; Langholtz et al., 2016; Williams et al., 2015). At the same time, data on biomass production, quality, and availability have largely been reported or derived at the county and annual scales, while techno-economic analyses suggest that finer resolution spatial and temporal data are needed to estimate local opportunities, barriers, and costs for facility-level decision-making (Tittmann et al., 2010; Breunig et al., 2017; Jaffe, 2018; Xie et al., 2014). This study seeks to provide the spatial and temporal resolution needed for decision-making at the locality and facility-levels for California, while also providing broader perspectives relevant to state-level bioenergy production strategies.
Biomass residue inventories generally use residue yield factors to approximate waste production from databases reporting harvested acres or harvested produce, food and cotton productivity, forested land acres, and livestock and population head counts. Few studies have contributed to our understanding of the spatial heterogeneity and temporal variation in residue yields. In the absence of survey or other measured data, seasonal variation must be inferred from seasonal and quarterly reports on agriculture and municipal activities. Monthly production of food waste, from farm to plate, was estimated by Breunig et al. using seasonal and quarterly harvest and waste disposal reports (Breunig et al., 2017). Studies of seasonality for agriculture in California dating back to 1976 estimate monthly production of crop residues, and have been used extensively by subsequent resource assessments (Williams et al., 2015; Knutson et al., 1976; Knutson and Miller, 1982; von Bernath et al., 2004; Williams et al., 2008, 2006). Matteson and Jenkins estimated monthly production for several food processing residues in a 2007 analysis of food processor waste in California (Matteson and Jenkins, 2007).
Past projections of biomass residues rely on high-level trends in human population, forest and agriculture land availability, with little variation in the yield factors used. Notable exceptions include a projection of biomass residues developed by the California Biomass Collaborative (CBC) out to 2020 (Williams et al., 2008), and a projection of municipal solid waste (MSW) disposal rates out to 2025 developed by the California Department of Resources Recycling and Recovery (CalRecycle) (Facility Information Toolbox CalRecycle, 2018). Projections of county-level forestry-, municipal-, and agricultural-residue production and consumption out to 2030 were developed as part of the Billion-Ton Report series in the US (Turhollow et al., 2014; Perlack et al., 2011; Langholtz et al., 2016). Published in a publically available database, this study is widely used to estimate biomass resources, despite limited characterization of drivers at local and regional scales.
In California, the management of biomass residues, which consists of the organic fraction of municipal solid waste (MSW), crop residues, food and fiber processing residues, and forestry residues, is evolving as the state aggressively pursues its 2020 goal of 75% diversion of MSW from landfills, pursues tighter restrictions on greenhouse gas (GHG) and criteria air pollutant (CAP) emissions, and seeks ways to reduce fire risks in forests and wildlands. Expanding bioenergy generation and composting of organics can complement source-reduction strategies to reduce landfill methane emissions and avoid the burning and mismanagement of municipal-, agriculture-, and forestry-organic wastes. However, careful estimation of net changes in emissions and other impacts is necessary, as residue collection, conversion, and byproduct management require energy and result in emissions. Consequential life-cycle assessment (LCA) (Finkbeiner et al., 2006) is a useful method for evaluating net changes in environmental impacts as a result of shifting organic residue management. Robust LCA of statewide scenarios requires detailed information on patterns and drivers of waste biomass quantity, quality, and geography that are currently lacking.
Our research builds on past work by deriving sub-annual biomass residue yields and developing new methods for constructing a comprehensive county-level biomass residue inventory for California. We also develop estimation and adjustment methods for missing or outdated information on seasonality, waste volumes, and management practices, and identify key uncertainties that could be reduced with additional survey or measured data (Williams et al., 2015; Breunig et al., 2017). Secondly, we identify socio-economic and environmental trends affecting biomass residue production and quantify expected future changes to develop scenarios for biomass residue availability out to 2050. Technical availability factors, which represent the fraction of residue that is potentially available for bioenergy after accounting for established uses, such as animal feed, and likely limitations to collection, are useful for gauging the impact of logistical challenges and market competition (Williams et al., 2015; Breunig et al., 2017). These factors are challenging to bound, as they are subject to unknown market dynamics and site-specific economics. For this reason, we limit the scope of this paper to the drivers of current and future gross biomass residue availability. Finally, we provide a discussion on the allocation of county-level residue inventory to sub-county locations that can be used to enable bioenergy/bioproduct facility siting research.
Our study ultimately provides a current inventory and set of projections for California with greater temporal, geospatial, and compositional specificity than any previous work, including sub-annual detail that is crucial in estimating energy generation potential and making strategic infrastructure investments. Our scenarios of residue availability for forestry, agri-industry, and municipal sectors out to 2050 provide the first estimate of biomass residues past 2030 that we are aware of. The methods documented here also can be applied more broadly across the U.S. and globally. California is a leader in developing and implementing environmental policy. Therefore, understanding opportunities and barriers to organic residue diversion in California can result in valuable insights for stakeholders across the U.S. and world.
Section snippets
Sub-annual availability factors
When crops are harvested, there are typically three categories of biomass generated: marketable product (produce), culls, and residues. A crop is grown for specific portions of its biomass (fruit, seed, fiber, root etc.), however a plant requires additional biomass to support itself. The above-ground fraction of the plant that remains once the marketable product is harvested is referred to as residue, and part or all of this biomass may remain on the field to ensure soil health (Collins et al.,
Sub-annual variations in wet and dry residues
Seasonality of biomass residue supply when aggregated as collective dry-matter is prominent in the Northern Valley (which includes the agriculturally-productive Central and Sacramento Valleys) and moderate in most other regions (Fig. 1, Fig. 2). Biomass categories with an average moisture content >50% or biomass types typically processed using anaerobic digestion are grouped as high-moisture solids (HMS); this includes row crop residues, agricultural culls, high-moisture food processing wastes,
Conclusions
To develop a comprehensive strategy for improved biomass residue management and diversion from landfills, decision-makers need a complete understanding of the quantities and types of residues being generated across time and space. This information can guide research and technology development, as the specific composition of residue types/blends is important for deciding how best to manage and derive value from them. We have presented the most detailed biomass residue inventory and projection
Acknowledgements
This work was supported by the California Energy Commission. We thank S. Smith, N. Carr, and S. Sherman for their insights and thoughtful advice. This work was part of the DOE Joint BioEnergy Institute (http://www.jbei.org) supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, through contract DE-AC02-05CH11231 between Lawrence Berkeley National Laboratory and the U.S. Department of Energy. The U.S. Government retains, and the publisher,
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