• high-efficiency biological methane activation;
• high-efficiency biological synthesis of liquid fuels; and
• process intensification approaches for biological methane conversion.

The benefits of converting natural gas to liquid fuels for use in transportation have long been recognized. First, the existing transportation infrastructure is based on liquids, and such fuels can be conveniently “dropped in” without substantial changes in vehicles. Second, liquid fuels from methane have lower emissions than petroleum-based fuels. Liquid fuel produced from methane decreases emissions by up to 50%, compared to unconventional petroleum, and decreases particulate matter by up to 40%, compared to combustion of conventional diesel. Further, methane is responsible for 10% of the nation’s greenhouse gas emissions (on a CO₂ equivalent basis), in part because its global-warming potential is twenty times greater than that of CO₂ over a 100-year period. Technologies capable of capture and conversion of methane will help mitigate the global-warming potential of these emissions.

—DE-FOA-0000881

ARPA-E notes in the FOA that the combination of horizontal drilling technology with hydraulic fracturing has led to significant increases in proven US natural gas reserves. On an energy basis, this amount of natural gas could fully satisfy the nation’s demand for transportation energy, without compromising its use in all other sectors, for approximately 50 years, based on current rates of consumption.

In addition to the long-term projected supply of natural gas, the price spread between natural gas and wholesale gasoline also encourages natural gas use for transportation, the agency says.

However, the direct use of natural gas in transportation is limited due to the inherent low energy density of natural gas and infrastructure changes that are required, which leads to reduced vehicle range and high storage cost. While chemically converting natural gas to liquid fuels (GTL) is a proven technology that increases volumetric energy density, the current conversion approach through Fischer-Tropsch (FT-GTL) is challenged by both high capital costs and low conversion efficiencies.

In principle, bioconversion of methane, the main component of natural gas, to fuels with high specificity and high process energy efficiency can be achieved under a single set of mild conditions. Moreover, this direct route to conversion, without relying on upstream unit operations for syngas production, has the potential to reduce capital expenses (CapEx) by more than 50%. In combination, these factors might lead to a significantly smaller capital investment for bioconversion than for current commercial GTL processes.

—DE-FOA-0000881

However, microbial synthesis of fuels or fuel precursors from methane using aerobic methanotrophs that activate methane with methane monooxygenase (MMO) leads to an energy efficiency less than 51%. Even if an organism fully leveraged the most recent developments in synthetic biology and industrial biotechnology, bioconversion through MMO will have difficulty being cost effective or disruptive to the fuel market.

The REMOTE program envisions the development of transformative bioconversion technologies that are capable of producing liquid fuels economically from natural gas at less than $2 per gallon of gasoline equivalent and at levels sufficient to meet US demand for transportation fuels.

Such technologies would support natural gas bioconversion facilities with a lower capital cost and at smaller scales than current GTL facilities. Such small-scale deployments will enable the use of natural gas resources that are currently flared, vented, or emitted, not only recovering wasted resources but also significantly mitigating greenhouse gas emissions.

The three primary challenges addressed by this program are the low carbon yield, low energy efficiency and slow kinetics in the process of bioconversion of methane to liquid fuels. As a result, two key components of REMOTE are biocatalyst engineering and bioprocess intensification.

• Biocatalyst Engineering. The low energy efficiency for the bioconversion of methane to liquid fuels by engineered aerobic methanotrophs provides a significant opportunity for improvement, both in activation and in fuel synthesis. For example, ARPA-E notes, the standard Gibbs free energy, ΔG°, of the conversion of methane to n-butanol is 10 times larger than the comparable ΔG° of conversion of glucose to ethanol during yeast fermentation.

  Most of this energy loss is released in the form of heat, and cannot be recovered in practice. More efficient metabolic pathways can be envisioned.

• Bioprocess Intensification. A practical process for bioconversion of methane to fuels needs to address a kinetic challenge as well. Several limitations are apparent:

  1. the rate of mass transfer of methane to the liquid phase due to low gas solubility;
  2. the low rate of product synthesis inherent to slow enzyme kinetics from methane activation and fuel synthesis; and
  3. the low catalyst loading in traditional bioreactors.

  Such a process could also run into unreasonable energy requirements for product separation.

  The kinetic challenges of a biological process are compounded by the slow rates of methane activating enzymes and by the large molecular weight of the enzyme complex. To integrate these enzymes in an industrially relevant process, it is necessary to achieve high energy efficiency and catalyst loading, without impairing the catalytic turnover of biological methane activation. This will require new catalysts capable of high enzyme concentrations within the cell, and high cell densities within the reactor.

In preparing for the FOA, ARPA-E conducted a preliminary techno-economic analysis that shows key components of a biological process for the conversion of methane into a liquid fuel, and that illustrates the sensitivity of each on the final fuel selling price. This analysis suggests that the fuel selling price is most sensitive to system variables such as CapEx and volumetric productivity, while feedstock natural gas price and the energy efficiency of conversion both play significant roles. These results emphasize the need to reduce CapEx while improving energy efficiency, ARPA-E says. These factors are determined by both the efficiency of bioconversion and the productivity of the reactor, and are addressed in the FOA by explicitly considering the tradeoffs between cost and performance.

A system demonstrating low CapEx will require high productivity as well as both high metabolic and process efficiency. To achieve this combination, the FOA establishes both system level and component level targets in three categories. Breakthroughs are needed in all three areas, and are synergistic:

1. high-efficiency biological methane activation;
2. high-efficiency biological synthesis of liquid fuel; and
3. process intensification approaches for biological methane conversion.

Program objectives. The broader vision of REMOTE is the development of bioconversion technologies that have reduced emissions and lower cost than FT GTL at all scales. Specifically, more than 60% energy efficiency conversion of methane to a liquid fuel (more energy dense than n-butanol, ≥ 26.8 MJ/L) in a process that can be deployed across scales (CapEx < $50,000/BPD).

• The first specific objective of this program is to develop new, more efficient, biological routes to activate methane. Enzyme-based technologies capable of activating methane to an intermediate with a feasible pathway to fuel production are of particular interest. 

• The second specific objective of this program is to engineer metabolic pathways for the conversion of activated methane to a liquid fuel with high energy density. Pathways that use both carbon and energy efficiently without sacrificing pathway kinetics are of special interest.

• The third specific objective of this program is to develop process intensification applied to methane bioconversion. Specifically, the process-intensified system should address three aspects, (1) the low solubility of methane in the reaction medium; (2) the low synthesis rate of a biological system; and (3) the flammability of methane in air. High system productivities necessary for eventual technology deployment to remote methane sources should be demonstrated.

Technical targets for the three categories of interests are:

• Category 1: High-Efficiency Biological Activation of Methane: a biological system capable of activation of methane at a sustained rate of 1 gCH₄/L/hr or greater. Category 1 projects should include an enzyme-based technology capable of activating methane to an intermediate with a feasible pathway to fuel production that is capable of meeting the following primary technical targets: energy efficiency of > 66%; turnover frequency of > 10/s; and specific activity of > 5 μmolCH₄/g_{total cell protein}/s.

• Category 2: High-Efficiency Biological Synthesis of Fuel: a fuel with an energy density ≥ 26.8 MJ/L that is produced in a 1 L bioreactor with a titer of 10 g_fuel/L. The choice of input molecules to the fuel synthesis pathways should be capable of satisfying the Category 1 Technical Targets.

  Category 2 projects must include a metabolic system that must meet the following primary technical targets: pathway energy efficiency of > 64%; pathway carbon yield of > 67%; and pathway kinetics of > 1 g_fuel/g_CDW/hr.

• Category 3: Process Intensification Approaches for Biological Methane Conversion: a prototype bioreactor capable of producing 1 liter of fuel per week from methane as a feedstock, producing a fuel with energy density ≥ 26.8 MJ/L with an emphasis on compatibility with existing distribution infrastructure.

  Specifically, the system must demonstrate high reactor and system productivities necessary for eventual technology deployment to remote methane sources and is capable of meeting the following primary technical targets: overall process capex of < $100,000/BPD (when calculated for a 500 BPD scale); process energy efficiency of > 25% (overall) and > 35% (metabolic); and process intensification of > 10 g_fuel/L_system/hr, > 25 g_fuel/L_reactor/hr; > 50 gCH₄/L_reactor/hr; and > 400 kW/m³ heat removal.