Our bioenergy activities include: (i) completion of studies on production of fuels and electricity from switchgrass and from switchgrass + coal, (ii) launching an activity to explore system implications of using mixed prairie grasses instead of switchgrass in making energy with coal, and (iii) launching a new activity exploring prospects for co-processing with coal for energy crop residues and mixed prairie grasses on degraded grasslands in China.
Making Fuels and Electricity from Switchgrass and from Switchgrass + Coal
Major findings of studies modeling polygeneration systems based on switchgrass (led by Larson) and coal + biomass (led by Williams) are:
Systems using only switchgrass with either CO2 vented or CCS would not be economically interesting if the value of GHG emissions were zero but would often be economically attractive if GHG emissions were valued at $100/tCequiv. At $100/tCequiv the CCS options would often be more economically attractive than the systems that vent CO2.
Systems with CCS using coal + modest amounts of switchgrass (separately gasified): (i) exploit simultaneously the low cost of coal, scale economies of coal conversion, and the negative emissions potential of photosynthetic CO2 storage; (ii) could be characterized by GHG emission rates for synfuels that are significantly less than either for coal synfuels with CCS or hydrocarbon fuels derived from crude oil (see, for example, option 5 in Figure 1); (iii) would not be economically interesting if the value of GHG emissions were zero; (iv) would often be more economically attractive than coal-based synfuels with either CO2 vented or CO2 stored if GHG emissions were valued at $100/tCequiv.
In comparing F-T liquids to crude oilderived fuels (first two options), the third and fourth options use coal, with CO2 vented and captured/stored, respectively. The sixth option adds enough biomass to realize net zero GHG-emissions (coal emissions are offset by negative CO2 emissions both from underground storage of photosynthetic CO2 and soil/root carbon buildup from planting mixed prairies grasses on carbon-depleted soils). The fifth option has the same biomass input fraction as the sixth but uses switchgrass (with no soil/root carbon buildup).
All technological elements for energy conversion are ready for deployment except the biomass gasifier—which could become commercial during 2010-2015. Early applications of these concepts are likely to be based on agriculture or forest industry residues and use of a single gasifier for coal plus a modest amount of biomass. There has already been considerable experience at one commercial facility co-gasifying biomass with coal.
Systems Implications of Using Coal/Mixed Prairie Grasses Rather Than
Coal/Switchgrass
A new activity (led by Williams and Larson) has begun exploring the implications of producing synfuels + electricity from mixed prairie grasses grown via low-intensity techniques on carbon-depleted soils (as proposed in a 2006 Science article by David Tilman and collaborators) rather from a monoculture crop (e.g., switchgrass). The prairie grass strategy would help address biodiversity loss concerns while significantly increasing the negative emissions potential of biomass by complementing geological storage of photosynthetic CO2 (described above) with root and soil carbon buildup during grass growing. Some preliminary findings (not yet published) are: (i) for a system fired with coal and mixed prairie grasses with the same biomass fraction of total fuel input and producing the same outputs as the switchgrass case discussed above, the net lifecycle GHG emission for F-T liquids would be reduced to zero (see Figure 1), (ii) the biomass required to produce a net zero GHG-emitting liquid fuel in this manner is ~ 1/3 of that required for making cellulosic ethanol (see Figure 2), and (iii) assuming a crude oil price of $50 a barrel and a $100/tCequiv value of GHG emissions, it would be economically worthwhile for an Iowa corn grower to instead grow mixed prairie grasses for making F-T liquids in combination with coal.
The bar on the left is for the zero net GHG emissions coal/biomass F-T liquids case described in the note to Figure 1. (The coal input is also indicated.) The other cases are estimates for cellulosic ethanol with three alternative vintages of the conversion technology.
Prospects for Co-processing Coal and Biomass for Energy in China
Plans have been announced for building many coal synfuels plants in China. Although it seems that there are no plans for including CCS at these plants, the capture group intends to explore CCS prospects at coal synfuels plants as well as at coal power plants in China over the remaining period of the CMI project. The group also intends to explore prospects for coprocessing biomass with coal in making synfuels in China—considering as feedstocks both crop residues and mixed prairie grasses. Although China is generally not considered rich with biomass resources, the coal/biomass strategy (discussed above) enables biomass resources to be used far more effectively for low-carbon fuels production than with conventional biofuels strategies (see Figure 2). Articulating the overall strategy and policy issues related to its implementation will be the focus of the PhD thesis of Yuan Xu, a Woodrow Wilson School graduate student being co-supervised by Socolow and Williams.
Cathy Kunkel (a 2006 Princeton physics grad) is spending this academic year in Li Zheng’s group at Tsinghua University (Beijing) doing exploratory work on the prospects for coprocessing both crop residues (for all of China) and prairie grasses (in Inner Mongolia) with coal in the making of synfuels. What she has learned so far, considered in the context of the coal/biomass co-processing models the capture group has developed, indicates that neither of these options for China should be dismissed out-of-hand. China produces substantial amounts of crop residues (of which some 360 million tonnes per year seem to be available for energy). Moreover, grasslands account for about 40% of China’s land area—much of which is heavily degraded and the restoration of which is a high political priority in China. Moreover, although yields on restored grasslands in Inner Mongolia (the focus of Kunkel’s research) are low (~ 1.5 tonnes per hectare per year) our preliminary calculations suggest attractive economics for farmers growing grasses for energy relative to what they are doing with the land at present—if the oil price is ~ $50 a barrel and the value of GHG emissions is ~ $100/tCequiv.