Liquid Fuels from Coal and Biomass with Low GHG Emissions

The car accounts for 12% of GHG emissions and is the major oil consumer—requiring the equivalent of most Persian Gulf oil. Climate and oil supply concerns make it likely that low GHG-emitting fuels from secure primary energy sources will be needed for cars and other transport uses. Although introducing hydrogen (H2) could accomplish this, the hydrogen economy is a long way off. The Capture Group is exploring a way to get much more quickly the climate benefits of H2 without abandoning liquid fuels. It involves making liquid fuels from coal and biomass with CO2 capture and storage (CCS) in both instances.

Interest in coal synfuels stems from coal’s abundance and low cost, but GHG emissions are much higher than for crude oil-derived hydrocarbon (HC) fuels. However, in making a synfuel from coal, only about 40% of the carbon ends up in the fuel. Most of the rest can be captured as CO2 and stored—thereby reducing GHG emissions to levels slightly less than for today’s HC fuels.

Eric Larson has been exploring production of liquid fuels and electricity from biomass via gasification (see previous section). Bob Williams is extending Larson’s work to include CCS and to show how a global system of making synfuels from coal and biomass with CCS might be designed with near-zero GHG emissions at attractive costs, while greatly reducing land requirements.

In gasification-based systems ½ of the carbon in the biomass can be captured and stored when only a liquid fuel is produced and ¾ can be captured and stored with the coproduction of liquid fuel and electricity. Because the carbon in biomass is pulled from the atmosphere via photosynthesis, CCS for biomass implies negative CO2 emissions that “make room in the atmosphere” for CO2 released when coal-derived synfuels are burned, thereby reducing net emissions to near zero.

Williams has shown that a system might be designed to provide clean liquid fuel from coal and biomass with the same fuel cycle GHG emission rate as for H2 derived from coal with CCS, with 1/3 less land than would be required with conventional biofuels. But without valuing CO2 emissions, CCS makes economic sense for neither coal nor biomass. At zero carbon price, an oil price of $80 a barrel is needed to make biofuel with CCS cost-competitive. But at $100/tC the price needed to make profitable CCS for coal gasification combined cycle power plants with today’s technology (equivalent to a $0.30 a gallon gasoline tax), the breakeven oil price is less than $10 a barrel. Williams estimates that at such a carbon price, a clean liquid fuel with ultra-low GHG emissions could be made from biomass plus coal at an average plant-gate cost of $1 a gallon of gasoline equivalent and a breakeven crude oil price under $30 a barrel.

Figure 2. Carbon contents of fuels produced from both fossil sources and biomass, with and without carbon capture and storage. Combining about ¾ coal synfuel (here DME) produced with CCS with roughly ¼ CCS biofuel (DME) gives fuel-cycle emissions comparable to hydrogen produced from coal with CCS.

A previous study showed that energy crops would be produced on an area equal to 12% of US cropland (4.5% of total area in farms) if there were a market for biomass produced at today’s costs. With this land conventional biofuels could support 20% of U.S. cars with reduced GHG emissions equal to 25-30% of current car emissions. But with the biomass/coal CCS strategy (assuming the liquid fuels are used in fuel-efficient hybrid-electric cars to replace today’s gasoline cars and the power displaces today’s coal steam-electric power), 65% of cars could be supported with the same land while reducing emissions by an amount equal to that for all cars.

No technological breakthroughs are required to implement this strategy – the gasification and capture components are already proven. The largest technical uncertainty is whether CO2 storage is viable at “gigascale.” The scenario described would require CO2 storage at a rate of 260 MtC/y (equivalent to 17% of U.S. CO2 emissions) and would not be pursued without a climate policy that gives the incentive needed to promote CCS.

 


Wind CAES

Wind power has the potential to generate large amounts of electricity, but the intermittent nature of wind limits the contribution it can make to meeting energy demands. Jeff Greenblatt and colleagues have been comparing two strategies for ameliorating the intermittency problem by transforming wind into a baseload power source: dedicated backup using natural gas plants, and storage of wind electricity using compressed air energy storage (CAES). In CAES, excess wind power is used to compress air and store it in underground caverns. During times of low wind, air is withdrawn and heated with fuel (usually natural gas) to power a turbine and generate electricity. Overall, fuel consumption is only one-quarter of that in a natural gas combined cycle plant.

The group’s study has found that wind’s intermittency can be overcome using either strategy, though at a price premium. The group’s general methodology enables cost comparisons to be made under a variety of physical and market conditions. They have discovered, for instance, that with a price on carbon dioxide emissions, wind in combination with one of these backup strategies become increasingly attractive relative to conventional generation, such as baseload combined cycle natural gas. Increasing natural gas cost and/or decreasing wind turbine cost further strengthens the competitiveness of baseload wind power.