Mid-Century Advanced Biofuel Potential for the US: A Thought Experiment

Principal Investigator

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At a Glance

The US transportation sector emits about a quarter of total US greenhouse gases. It may be the most challenging sector to decarbonize, given its heavy reliance on petroleum and millions of small emission sources. Biofuels are one of the few decarbonization options, especially for difficult-to-electrify modes. Moreover, deployment of biofuel production systems that incorporate CO₂ capture and storage may be essential for achieving mid-century greenhouse gas emission reductions that limit global warming to 2^oC. The required speed and scale of deployment of biomass supply infrastructure and conversion facilities to meet future biofuels targets that could mitigate significant transportation sector emissions have no historical precedents, as illustrated here. Incentives stronger than those that drove the expansion of the US corn-ethanol industry will be needed for an advanced biofuel industry to contribute significant carbon mitigation by mid-century.

 

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Research Highlight

Co-funded by CMI and Stanford’s Global Climate and Energy Project, the Energy Systems Analysis Group (ESAG) at Princeton University continued a collaboration with the University of Minnesota (UMN) and Colorado State University (CSU) to assess potential mid-century contributions from negative-emissions biomass-based transportation fuels. ESAG’s focus has been on understanding the prospective performance and economics of a range of conversion processes for making transportation fuels from lignocellulosic (plant dry matter) biomass while capturing CO₂ for geologic storage. UMN and CSU are focusing on understanding biomass production for energy on abandoned croplands, where soil organic carbon storage provides a negative emissions opportunity.

By ESAG’s estimates, gasoline- and diesel-like fuels made from seven exajoules of lignocellulosic biomass grown sustainably in the US without displacing land for food production could make a substantial contribution to reducing transportation-sector emissions in the future. This would take on transport modes (air and long-haul trucks, trains, and ships) that are particularly resistant to electrification. As a thought experiment, we examine the plausibility of different scale-up rates for this industry and the resulting contributions that advanced biofuels might make by mid-century.

Prospective scale-up rates are compared with the most relevant historical precedent, the expansion of the US corn-ethanol industry. That industry grew slowly for about the first three decades, but more rapidly once significant incentives were introduced beginning in 1999 when California banned MTBE as a gasoline oxygenate, spurring increased demand for ethanol as a substitute (Figure 2.3.1a). Additional incentives followed, further accelerating growth, but subsequently slowed as total output approached the corn-ethanol supply limit under the RFS-2 legislation. The curve fit to the data in Figure 2.3.1a derives from the following equation:

Where P_t is petajoules of feedstock processed in year t. Time zero (t₀) is 1981, representing the start of the corn-ethanol industry. C₁, C₂, b, and t_infl are constants that have been tuned to achieve a visual best fit. In Figure 2.3.1a, C₁ = 12.6/year, C₂ = 1680 PJ/year, t_infl = 2009.5, and b = 0.6/year.

Eqn. 1 is a modified logistics function. (A pure logistics function includes only the second term on the right.) Logistics functions are used to describe growth processes (e.g., population expansion or infectious disease spread) that begin slowly, then accelerate exponentially before decelerating and eventually reach a saturation level.

A logistics function of the same form as Eqn. 1 is developed to represent the scale-up trajectory of a future lignocellulosic biofuels industry. It is assumed that commercial biofuel production would begin in 2025 with enough biomass processed to produce 500 million gallons of biofuel, or 65 PJ on a higher heating value basis (65 PJ_HHV), assuming a biofuel energy content similar to gasoline. For comparison, the average US corn-ethanol facility has a production capacity of 78 million gallons per year, or 7 PJ_HHV, of ethanol, and the largest one has a capacity of 375 million gallons per year, or 33 PJ_HHV.

If growth of an advanced biofuel industry from 2025 follows a trajectory like that seen for corn ethanol, i.e., slow linear growth for about 20 years before accelerating, the output of a lignocellulosic bioconversion industry by 2050 would still be only a fraction of that of the current corn-ethanol industry. 2050 is chosen as a nominal target date for discussion because deep reductions in carbon emissions economy-wide would be needed by then if global warming is to be limited to less than 2°C.

Alternatively, if sufficient incentives were in place by 2025 so that investment in the industry accelerates without the slow initial phase, growth trajectories like those in Figure 2.3.1b could result. The solid lines in Figure 2.3.1b follow Eqn. 1, but without the linear term (i.e., C₁ = 0). The value of C₂ is 7000 PJ/y, the projected future sustainable biomass feedstock supply. The value of t_infl varies in 2.5-year increments from one line to the next, and for each value of t_infl, b is set such that the amount of biomass processed in 2025 corresponds to the production of 500 million gallons of biofuel. The dashed lines plot the slopes of the solid lines, i.e., the dashed lines show annual growth rates.

Figure 2.3.2 compares metrics derived from Figure 2.3.1 for the US corn-ethanol and prospective advanced biofuel industries. The target feedstock-energy input, which reflects the scale of the bioconversion industry, for the advanced biofuel industry is more than triple that for the corn-ethanol industry. For the fastest assumed growth, the advanced biofuel industry would essentially reach the target level by 2050, but doing so would require an average annual growth rate (from 10% to 90% of the target) nearly quadruple that observed for the corn-ethanol industry during its most rapid expansion phase. With the slowest assumed growth rate, the advanced biofuel industry reaches only about half of the target by 2050, but still must grow nearly twice as fast as the corn-ethanol industry did in order to reach this modest level.

 

	Corn ethanol	Advanced (lignocellulosic) biofuel industry
TARGET total feedstock input, PJ/y	2,150	7,000
Date when 90% of TARGET is reached		2043	2047	2051	2055	2058	2062
Years required from 10% to 90%	17	14	17	20	23	25	27
Average feedstock-energy growth, PJ/y/y	111	417	343	292	254	234	205
Average feedstock-volume growth, Mm3/y/y	10	187	154	131	114	105	92

_{Figure 2.3.2. Comparison of historical US corn-ethanol industry and a prospective advanced biofuel industry.}

 

Also shown in Figure 2.3.2 are average growth rates expressed in terms of biomass feedstock volumes, which reflect the scale of the biomass feedstock supply industry (as distinct from the biomass conversion industry). For the advanced biofuel industry to achieve 90% of the target level by 2043, the average growth in volume of biomass handled is 187 million m³/y/y. This is 19 times the average annual growth seen for the corn ethanol industry. It is so much larger both because of the larger target scale for the bioconversion industry and because the volumetric energy density of baled crop residues or grasses, which constitute the lignocellulosic biomass supply, is only about one-fifth of that for corn grain. At the target lignocellulosic biomass supply level, the biomass collection and transport infrastructure would need to handle 17 times as much volume as managed today by the corn-handling infrastructure for the ethanol industry.

Advanced lignocellulosic biofuel conversion technologies are not yet commercial today. In practice, they would need to be commercially ready within the next three or four years for industrial production to start in 2025 at the scale envisioned in Figure 2.3.1.

 

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* This work was done with Hans Meerman, who completed his post-doctoral appointment at Princeton in 2017 and is now with the Center for Energy and Environmental Sciences, University of Groningen, Netherlands.