Updated NETL study provides more detailed economic and environmental assessment of coal-to-liquids and CTL with modest biomass
The National Energy Technology Laboratory (NETL) has released a follow-on study to its 2009 evaluation of the economic and environmental performance of Coal-to-Liquids (CTL) and CTL with modest amounts of biomass mixed in (15% by weight) for the production of zero-sulfure diesel fuel. (Earlier post.)
The new study expands upon the prior by adding new scenarios, including: conversion of additional coal types (sub-bituminous coal) at a facility located in the Western part of the United States; poly-generation of electricity with fuels (up to 12% of the total product slate); and how different cooling technologies can be leveraged to reduce water usage.
The authors, Charles White and David Gray from Noblis, also made several additional refinements to update previous results, including modifications to the plant configuration, based on lessons learned and updated performance/environmental metrics. Performance in the study is measured by such metrics as: (1) required selling price of the fuel; (2) crude oil price when the process will become economically viable; (3) the Well-to-Wheels (WTW) life cycle GHG emissions profile of the diesel fuel; and (4) the water usage associated with the facility.
They selected a low-temperature Fischer-Tropsch synthesis process because of (1) commercial availability and operating experience of the FT process for diesel fuel production, (2) robustness of the supporting technologies (syngas production from coal or coal/biomass); and (3) the ability to produce an ultra-clean diesel fuel which is fungible in today’s fueling infrastructure.
Both bituminous and sub-bituminous coals were evaluated as these coal types represent 90% of the US reserve base (53% and 37%, respectively). Switchgrass was selected as a representative type of biomass for use in evaluating Coal and Biomass to Liquids (CBTL) facilities.
They considered two broad design approaches were considered: one in which the facility is designed to primarily produce liquid fuels (a “recycle” configuration in which unconverted syngas is passed back through the FT reactors, resulting in a greater percentage conversion of the original carbon to fuels ; and a poly-generation plant which is designed to also co-produce electric power for sale into the grid by combusting the unconverted syngas (a “once-through” configuration). Both facilities were designed to produce 50,000 barrels per day (bpd) of FT liquids, comprising 34,000 bpd of FT diesel (or 69% of the product) with the balance consisting of FT naphtha.
Applying the 50,000 bpd design constraint to the once-through configuration results in significantly larger gasification, gas-cleanup, and power island areas of the facility, and consequently, higher capital outlays and operating costs for the poly-generation facility. Put simply: more syngas must be generated in the gasification island for the “once-through” cases to make up for the absence of recycled syngas, increasing the size of the facility in those cases.
Finally, life cycle GHG assessments were performed using the “displacement” methodology of accounting for co-products produced in the facilities, namely FT naphtha and in some cases electrical power. As this methodology can be sensitive to the assumed GHG profiles of the co- products, a second methodology was also utilized wherein the life cycle GHG emissions produced prior to the product transportation and use are divided across all of products based on the usable energy fraction of that product.—White and Gray
Key findings of the new study are that diesel fuel produced from coal:
Is economically viable when crude oil prices reach $95/bbl or $98/bbl for the recycle and poly-generation scenarios, respectively. This equates to diesel prices in the range of $2.70 to $2.80 per gallon of petroleum diesel.
Will produce less life cycle GHG emissions than petroleum fuels if produced from a recycle facility, regardless of the LCA methodology employed, so long as CO2 produced by the facility is sequestered. In the case of particularly high-methane content bituminous coals, methane mitigation may also be required at the site of the mine.
Will, in the case of the poly-generation scenario, produce either significantly less or slightly more life cycle GHG emissions than petroleum fuels depending on the LCA methodology used. Therefore, poly-generation facilities might require the use of modest amounts of biomass (less than 10% by weight) or more aggressive carbon capture strategies if petroleum parity is required.
Will require between 1.6 and 7.4 barrels of water for each barrel of FT product produced, depending on the water management strategy utilized.
If a modest amount of biomass is co-gasified with the coal to produce liquid fuels, the point of economic viability is increased by $9 to $15 per bbl, to between $104/bbl and $115/bbl, representing a $0.26 to $0.46 per gallon increase in fuel price over the coal cases.
However, the resulting fuel will produce less GHG emissions than petroleum-derived fuels, regardless of the configuration choice or LCA methodology, if 15% of the feedstock to the facility is switchgrass. A reduction of up to 34% less life cycle GHG emissions than petroleum-derived diesel is possible at this level of biomass usage.
The overall plant efficiency of the sub-bituminous coal cases is higher than that of the bituminous coal cases. This is due to the increased electrical power produced in the bituminous coal cases, which reduces the efficiency of the facility (as power generation is less efficient than fuel production). Less power is produced in the sub-bituminous cases as some of the steam which would otherwise be used for power production is instead utilized to dry the relatively high-moisture content sub-bituminous coal.
The poly-generation cases are all larger and more expensive than the recycle cases due to the 50,000 bpd design constraint. The facilities would be similar in size and cost if the coal input rate—and therefore syngas production rate—was held constant between the two cases, although the poly-generation cases would then have a lower fuels output.
This report was completed under DOE NETL Contract Number GS-10F-0189T/DE-NT0005816.
Charles White and David Gray (2012) Production of Zero Sulfur Diesel Fuel from Domestic Coal: Configurational Options to Reduce Environmental Impact. NETL/DOE-2012/1542