Performance, cost and emissions of coal-to-liquids (CTLs) plants using low-quality coals under carbon constraints

Abstract

Prior studies of coal-to-liquids (CTLs) processes that produce synthetic transportation fuels from coal have focused mainly on designs using bituminous coal with no or limited constraints on carbon emissions. In this study, plant-level techno-economic models are applied to evaluate the performance, emissions and costs of CTL plants using low quality sub-bituminous coal and lignite as feedstock for both a slurry-feed and dry-feed gasification system. The additional cost of carbon dioxide capture and storage (CCS) is also studied for two plant configurations—a liquids-only plant and a co-production plant that produces both liquids and electricity. The effect of uncertainty and variability of key parameters on the cost of liquids products is also quantified, as well as the effects of a carbon constraint in the form of a price or tax on plant-level CO2 emissions. For liquids-only plants, net plant efficiency is higher and CO2 emissions and costs are lower when sub-bituminous coal is used. For both coals, performance of plants with a dry-feed gasifier is better compared to plants with slurry-feed gasifiers, but the costs are comparable to each other, with slurry-feed plants having a minor advantage. A major concern for CTL plants is the high level of CO2 emissions, the major greenhouse gas linked to global climate change. However, this study shows that for the liquids-only plant most of the CO2 emissions can be avoided using CCS, with only a small (<1%) increase in capital cost. Depending on the coal type, gasifier type and CO2 constraint (up to $25/tonne CO2), the nominal cost of liquid product ranges from $75 to $110/barrel. Parameter uncertainties increase this range to $50–140/barrel (90% confidence interval). With or without CCS, co-production plants are found to have higher capital costs than liquids-only plants, but produce cheaper liquid products when the electricity is sold at a sufficiently high price ($50–120/MWh, depending on plant design and carbon constraint). For co-production plants, net plant efficiency, which depends both on coal consumption as well as electricity generation, is higher for plants with a dry-feed gasifier while CO2 emissions are lower from plants with a slurry-feed gasifier. For both coals, capital cost is lower for plants with dry-feed gasifier, with plants using sub-bituminous coal being cheaper than the ones using lignite. A CO2 tax of $25/tonne is not enough to make CCS more economical when the electricity price exceeds about $80/MWh.