The synfuels systems analysis discussed in the previous section was not location specific. To illustrate possibilities more concretely, a case study for Illinois was carried out led by Eric Larson of the Williams group in collaboration with Giulia Fiorese and Stefano Consonni at the Politecnico di Milano (Figure 5). The analysis was for coal/biomass-fueled coal minemouth plants with CO2 storage in deep saline formations 100 km away. The analysis includes detailed estimates of energy and carbon balances and costs for the biomass supply—assumed to be either corn stover or low-input, high-diversity, perennial grasses (also known as “mixed prairie grasses” or MPGs) grown on degraded lands that are not suitable for food production and have carbon-depleted soils. These feedstocks were selected for focused attention because of growing concerns about biomass grown for energy on lands that could alternatively be used for growing food—concerns about higher food prices and about indirect land use impacts. Growing MPGs in C-depleted soils leads to a build-up of soil and root carbon, which is ecologically desirable and also implies that less biomass input is required at the polygeneration plant to realize zero net GHG emissions for the liquid fuels than would be required for a system that does not benefit from soil/root carbon buildup. Both plants were designed to provide liquid fuels with a zero net GHG emission rate (GHGI = 0).
The MPG-fueled plants have net output capacities of 13,000 B/D and 410 MWe. Delivered MPG costs and associated energy use and greenhouse gas (GHG) emissions were estimated as levelized values over a 30-yr period, assuming the field is annually harvested. Fuel-cycle-wide energy requirements associated with MPG production and delivery total 1.0 GJ per dry tonne (dt)—equivalent to 6% of the energy in the delivered biomass. The corresponding GHG emissions amount to 77 kgCO2eq/dt— equivalent to 4.5% of the CO2 that would be released if the MPGs were burned. In both instances MPG transport is the dominant contributor (Figure 5). The average cost of delivered MPGs is $132/dt ($7.1/GJHHV)—about 5 times the assumed delivered coal price on an energy basis. For the plant design using corn stover, net plant output capacities are 7,700 B/D and 290 MWe,—much less than for MPGs because there is no credit for soil/root C buildup. The estimated delivered yield is 3.8 dt/ha/yr. The average delivered cost is $63/dt ($3.6/GJHHV)—about half of that for MPGs.
There are many possibilities for plant sites considering that coal underlies 65% of Illinois, the Mt. Simon aquifer underlies much of Illinois, and the fact that MPG and corn stover are abundant (with estimated state-wide supplies of ~ 2 and ~ 20 million dt/yr, respectively) and relatively uniformly distributed (Figure 6).
Using the above discussed biomass cost estimates and the synfuel economic analysis methodology developed in support of the NRC study discussed earlier, the overall economics for these polygeneration plants were explored in relation to the economics for four large coal-only systems. One measure of economic performance, the breakeven crude oil price (BECOP), is shown as a function of the GHG emissions price in Figure 7 for these alternative systems. The BECOPs are essentially the same for MPGs and corn stover even though the delivered biomass cost is about twice as large for MPGs both because the biomass feed rate required to realize a zero net F-T liquids GHG rate is much less for MPGs and because of scale economies. At $0/t CO2eq the least costly F-T liquids option is a coal once-through plant that vents its CO2; with no cost for GHG emissions, the BECOP for the coal/biomass polygeneration plants with CCS is 2.5 X as large. But at $60/t CO2eq , the coal/biomass polygeneration plants with CCS are the most cost-competitive synfuel options—with a BECOP below $40 a barrel and essentially at the same level as for the much larger coal once-through system that vents CO2 when the GHG emissions price is zero.
The same approach to analyzing supply availability in Illinois was used to estimate corn stover and MPG availability and cost in a swath of 23 central US states that in 2007 accounted for 94% of US corn production and ~12 million ha of CRP enrollments (86% of US total). The 23-state potential energy production from coal plus MPG and corn stover is 1.4 million B/D of zero-GHG emitting FTL fuels plus 400 TWh of decarbonized electricity (equivalent to 20% of US coal power generation in 2007).
The biomass potential for co-processing at coal/biomass polygeneration plants might well be greater than these calculations suggest—even in these 23 states. These states contain a USDA-estimated 135 million hectares (~70% of U.S. total) of abandoned or degraded agricultural land that might be considered for growing MPGs. Other prospectively important biomass supplies include other crop residues and woody biomass supplies such as urban wood wastes and forestry residues: mill residues, logging residues, diseased tree kills, fuel treatment thinnings, and productivity enhancement thinnings.
To summarize, coal/biomass polygeneration with CCS systems supplied with biomass that is not grown on cropland offer both the potential for providing synthetic fuels that would be highly competitive under a serious carbon mitigation policy and for making significant contributions to overall supplies of low-carbon liquid fuels.