The Pyrolysis-Bioenergy-Biochar Pathway to Carbon-Negative Energy

Avoiding irreversible climate change requires >50% reduction in anthropogenic greenhouse gas (GHG) emissions by the year 2050 and the net removal of GHGs from the atmosphere by the end of the 21st century. This challenge is particularly daunting given that energy derived from fossil fuels is at the core of all modern economies and some sectors of the economy, such as transportation, will be almost impossible to completely decarbonize. To address this challenge, we are investigating the integrated pyrolysis-bioenergy-biochar platform (PBBP) to determine whether this system can produce economically viable carbon-negative energy products. Use of the biochar coproduct as a soil amendment is the key to transforming bio-oil and other energy products produced by fast-pyrolysis of biomass from near carbon neutral to carbon negative. This is so because the half-life of biochar C in soil environments is approximately 1000 years. Major accomplishments during the first two years of our project include the design and development of a biochar module within the Agricultural Production Systems sIMulator (APSIM), a widely used and publically available cropping systems model. The APSIM Biochar Model provides for the first time a means of systematically investigating complex soil-biochar-crop-climate-management interactions, and critically a means of estimating the agronomic and environmental impacts of soil biochar applications. Much effort during the second year of the project was focused on quantifying and developing a mechanistic understanding of how biochar influences nitrogen dynamics and bioavailability in soil environments. Sorption of nitrate and ammonium by biochar were shown to be strongly dependent on pH and peak pyrolysis production temperature. We also focused on developing a rapid method for determining the size and properties of the labile and recalcitrant biochar fractions, as these are needed to accurately parametrize “biochar quality” within the APSIM Biochar Model. And, we made substantial progress in calibrating and validating the APSIM Biochar Model using literature data from globally diverse field and laboratory experiments. Techno-economic analysis (TEA) of the PBBP was used to determine the minimum fuel selling price (MFSP) and lifecycle GHG emissions for a 1000 dry ton per day fast pyrolysis plant. Both MFSP and GHG emissions were shown to be strongly dependent on the ash content and O/C ratio of the biomass feedstock. The results indicate a tradeoff between economic and environmental benefits based on feedstock selection. The MFSP for 346 different feedstocks range from $2.3/gal to $4.8/gal of liquid fuels in the diesel/gasoline range. The TEA demonstrated that the PBBP has the potential to produce carbon negative energy products even when indirect land use and synergistic agronomic and environmental effects of soil biochar applications are discounted. During the third year of the project we will focus on completing on-going laboratory and field studies designed to improve and validate the APSIM Biochar Model and on integrating output from the APSIM Biochar Model with econometric, TEA, and lifecycle assessments of the economic viability and environmental impact of the PBBP for three case studies; the upper Mississippi River basin, California, and the U.S. Southeast. Introduction Avoiding irreversible climate change requires >50% reduction in anthropogenic greenhouse gas (GHG) emissions by the year 2050 and the net removal of GHGs from the atmosphere by the end of the 21st century [1]. To address the need to remove GHG from the atmosphere, we are investigating the integrated pyrolysis-bioenergy-biochar platform (PBBP) for the production of carbon-negative energy (Figure 1). Bio-oil and non-condensable gases produced by fast pyrolysis of biomass are sources of potentially viable liquid transportation fuels, heat, power, bio-asphalt, and other products that can offset fossil fuels [2]. Biochar, the condensed aromatic carbon-rich solid co-product of biomass pyrolysis, is a soil amendment that is effective for sequestering carbon for centuries, if not millennia, while improving soil quality and reducing leaching of nutrients [3, 4]. The key challenge for scaling up the PBBP industry is the identification of economically-viable markets for both the bioenergy and biochar co-products. The overall goals of our project are: 1) To advance basic understanding of the impacts of biochar on agroecosystems; 2) to assess the technical and economic viability of an integrated pyrolysis-bioenergy-biochar industry in the Upper Mississippi River Basin (UMRB), California, and U.S. Southeast; 3) to assess regional and global impacts of an integrated pyrolysis-bioenergy-biochar industry on indirect land use and net GHG emissions; and 4) to build a foundation for the development of a vanguard economicallyviable carbon-negative integrated pyrolysis-bioenergy-biochar industry. The proposed research systematically builds capacity to achieve these goals through the following specific objectives: Cellulosic biomass Figure 1: The integrated pyrolysis-bioenergy-biochar platform has the potential to provide carbon negative energy products and multiple ecosystem services. 1) Develop, parameterize, and validate a biochar module for the Agricultural Production Systems sIMulator (APSIM). 2) Quantify the public and private benefits accrued from integrating biochar into pyrolysis-based bioenergy production systems for three case studies. 3) Use techno-economic analysis to assess the economic performance of pyrolysis plants producing bioenergy and biochar co-products and use life cycle assessments to determine the net GHG emissions from an integrated pyrolysis-bioenergy-biochar production facility. 4) Estimate carbon credits for indirect land use avoidance and compare system production costs. Background Political: The outcome of the 2016 US presidential and congressional elections have resulted in a shift in US political priorities, which have dimed the prospects for large scale commercialization of the PBBP in the US in the near future. As long as the environmental costs associated with fossil fuels are externalized and the environmental benefits associated with the PBBP are discounted, it will be difficult for the PBBP to compete with fossil fuels on the large scale. However, the small scale biochar industry is still growing rapidly in the US; entrepreneurs are primarily targeting niche high-value markets for biochar in horticulture, land reclamation, and organic agriculture. Most of the small biochar production facilities use slow pyrolysis or gasification and are either not capturing the bioenergy co-products or are producing process heat and/or electricity rather than liquid biofuel products. The industry is also targeting feedstocks which can be obtained for little or no cost, such as orchard trimmings and bark beetle killed tress harvested to suppress fire in the intermountain west. On a global scale, the Paris Climate Agreement, which went into effect in October 2016, highlighted the need for carbon negative systems to offset GHG emissions from sectors of the global economy that will be extremely difficult to de-carbonize. While there are many opportunities to reduce GHG emissions through improved energy efficiency, there are relatively few identified opportunities for industrial scale systems that on net remove CO2 from the atmosphere. The two most prominent carbon negative systems are Bioenergy Carbon Capture and Sequestration (BECCS), where biomass is burned directly or co-fired with fossil fuel to generate electricity and the emitted CO2 is captured and geologically sequestered, and PBBP (also called bioenergy-biochar systems; BEBCS). A recent paper published in Science [5] compared the economic prospects for PBBP with BECCS and concluded that PBBP is economically more viable for carbon prices below $1000 Mg C. Hence, when policy makers finally choose to seriously address climate change, the PBBP is poised to assume a prominent role in that effort. China is emerging as a global leading for biochar research (Figure 2) and is likely to be the first country with wide-spread commercialization of the PBBP. China has severe air quality problems stemming in part from the widespread practice of burning crop residues. China also has severe soil contamination problems, a critical need to improve nutrient and water use efficiency in agricultural production, and a rapidly growing demand for both food and energy. All of these issues can be partially addressed through PBBP [6]. Technology: Various biorenewable energy production technologies have been investigated in order to reduce the country’s dependence on foreign energy, enhance energy security, utilize excess agricultural resources and mitigate environmental concerns [7]. Biorenewable resources can be converted into bioenergy, transportation fuels, chemicals and fibers through biochemical and thermochemical conversion technologies. Here we focus on pyrolysis, the thermochemical conversion of biomass feedstocks in the absence of oxygen into various gas (non-condensable gases), liquid (bio-oil), and solid (biochar) co-products. Although it has a relatively low energy density, the noncondensable gas can be combusted to generate heat and/or power. Bio-oil, a viscous and dark-brown fluid with water content of 15-20%, can be refined to produce liquid fuels and other products [9]. Biochar is primarily used as a soil amendment because it has the potential to sequester carbon, improve soil quality, and increase plant productivity. Pyrolysis technologies are broadly categorized, based on different reaction conditions, as fast pyrolysis, slow pyrolysis, catalytic pyrolysis, and autothermal fast pyrolysis. Fast pyrolysis is a rapid (~2s) thermochemical conversion technology operating at moderate temperatures (typically ~ 500°C) [8]. Fast pyrolysis systems are optimized for the production of bio-oil. Production of bio-oil is maximized (up to 75 wt%) under fast pyrolysis temperatures of around 500°C. Slow Pyrolysis has a lower heating rate of 0.11°C/s compared to fast pyrolysis (10-200°C/s) [8], and higher yield of biochar around 2535 wt % [10]. The autothermal fast pyrolysis operates by inserting oxygen into the pyrolysis reactor at equivalence ratios of about 0.05 to provide heat for the endothermic pyrolysis decomposition reactions by partially oxidizing the biomass. Autothermal fast pyrolysis could reduce operating cost and increase commercialization feasibility because it does not require external heat supply [11–13]. Raw bio-oil has several undesirable properties, such as high water content, high viscosity, high ash content, high oxygen content and high corrosiveness [8], which make it less 0 200 400 600 80