Low-carbon synthetic gasoline production
Building on its earlier analyses of coproduction of F-T fuels and electricity from coal and biomass with CCS, the Williams Group investigated another commercially-demonstrated route for converting solids into a “drop-in” replacement fuel for transportation: synthetic gasoline synthesized from syngas-derived methanol. Methanol-to-gasoline (MTG) processes produce primarily a finished-grade gasoline, with a co-product of propane and butane.
Haldor Topsoe and Exxon Mobil offer commercial processes for syngas conversion to gasoline. The Exxon Mobil MTG process operated commercially in New Zealand for a decade, before the facility was converted to methanol production in the mid-1990’s when methanol became more profitable. Recent oil prices are driving renewed interested in MTG plants. Commercial coal-toMTG projects are under construction in China and have been proposed for the U.S.
The Williams Group developed detailed process simulations for a number of different coal- or coal/biomass-to-MTG plant designs, along with fuel-cycle-wide GHG emission balances and prospective capital and operating costs. Systems with substantial co-production of electric power were of particular interest, considering the favorable economics demonstrated for such designs in the group’s earlier work on production of Fischer-Tropsch fuels. Comparisons of MTG results with prior FTL results have been facilitated by the group’s use of a common analytical framework that includes Aspen process simulations and an in-house cost database and system evaluation software tool.
A key finding is that coal/biomass-to-MTG process designs and coal/biomass-to-FTL designs with similar features (separate coal and biomass gasifiers, similar biomass:coal input ratio, same biomass input rate, similar electricity:fuel output ratio – two left-most columns in Table 2) offer comparable economics (Figure 3). A detailed paper reporting on the MTG analysis is in preparation for publication.
Co-gasification approach to coproduction
Co-supported by a grant from the National Energy Technology Laboratory, the Williams Group has been investigating a variety of process designs for coproducing electricity and synthetic gasoline or electricity and chemicals (olefins, ammonia, or hydrogen) from a mixture of coal and biomass with CCS. The work has focused on co-gasification of the feedstocks in commercially established entrained-flow coal gasifier designs, unlike earlier work (see above) that examined systems with separate coal and biomass gasifiers.
Co-gasification is technically feasible, but pretreatment of the biomass is important to facilitate feeding. Torrefaction, a slow cooking process that destroys the fibrous nature of biomass (making it easier to feed the biomass into the gasifier along with coal), was investigated as a promising pretreatment strategy. Surprisingly, for a system using dry-feed gasification (Shell) of coal and torrefied biomass and achieving ~90% reduction in GHG emissions (Table 2, right column), the economics are only slightly worse than for a system using separate coal and biomass gasifiers to achieve a similar GHG reduction (Figure 3). This result is surprising, because the co-gasification system utilizes a more costly gasifier (Shell instead of GE) and a more costly pretreatment process (torrefaction), and a less energy-efficient approach to biomass gasification. But shifting from two gasifiers to one leads to scale economy benefits that go a long way in offsetting the cost penalties.
Another notable finding in Figure 3 is that the GHG emission price for breaking even with petroleum-derived gasoline (16 to 26 $/t CO2eq) is far lower than the GHG emission price needed to induce decarbonization of stand-alone coal-fired power plants, which suggests that coproduction systems such as those modeled here offer an attractive way to decarbonize electric power generation as well as providing low-carbon fuels. The work described here will be submitted for publication in the coming year.