|Current case design block flow diagram of thermochemical gasoline from biomass-derived methanol and the methanol-to-gasoline process. Source: NREL. Click to enlarge.|
A new report from the US Department of Energy’s National Renewable Energy Laboratory (NREL) concludes that gasoline produced via the methanol-to-gasoline (MTG) route (earlier post) using syngas from a 2,000 dry metric tonne/day (2,205 US ton/day) biomass-fed facility could have a plant gate price (PGP) of $1.95/gallon US ($0.52/liter).
This is a gallon ethanol equivalent on an energy basis (gee) price of $1.39/gallon ($0.37/liter). (Gasoline has a higher energy content than ethanol.) In comparison, based on analysis work completed at NREL, the predicted PGP for ethanol produced via the thermochemical and biochemical pathways are $1.57 per gallon ($0.41 per liter) and $1.49 per gallon ($0.39 per liter), respectively.
...the results from this preliminary evaluation indicate great potential for producing gasoline from biomass via thermochemical biomass conversion to syngas and the MTG process, and thus warrant a more detailed study. Future work areas of interest include obtaining better process information on the MTG section of the plant, especially equipment and operating costs; increasing the heat integration throughout the process; scale-up of the MTG fluidized bed reactor; testing the MTG reactor and catalyst with methanol from biomass-derived syngas; testing of the MTG fluidized bed reactor at higher pressure; and evaluating the possibility of selling raw MTG gasoline and refining it in an existing refinery.—Phillips et al.
NREL conducted the analysis to investigate one of several possible biofuels that can be produced using the thermochemical route of gasification and synthesis. The basis for this study was a stand-alone gasification/synthesis process including sub-processes or unit operations for integrated tar reforming, acid gas scrubbing, and synthesis to methanol followed by conversion to gasoline.
The report uses a new technoeconomic model developed in Aspen Plus to look at the future potential of the described biomass-to-gasoline (BTG) process, based on calculations for a mature plant (the nth plant) and 2012 technology targets as established in the Multi-Year Technical Plan of the US Department of Energy (DOE) Office of the Biomass Program.
Very broadly, the biomass-to-gasoline (BTG) process assessed in the NREL report gasifies biomass to produce a syngas rich in hydrogen and carbon monoxide. This syngas is then converted into methanol, and the methanol is converted to gasoline using the methanol-to-gasoline (MTG) process first developed by Exxon Mobil. More specific steps include:
Feed Handling & Preparation. The biomass feedstock (2,000 dry metric tonne/day [2,205 dry US ton/day]) is dried from the as-received moisture content to that required for proper feeding into the gasifier using flue gases from the char combustor and tar reformer catalyst regenerator. Prior to drying, wood chips with a diameter larger than 2 inches are sent to the hammer-mill for further size reduction.
Gasification. This report presumes indirect gasification. Heat for the endothermic gasification reactions is supplied by circulating hot synthetic olivine “sand” between the gasifier and the char combustor. Conveyors and hoppers are used to feed the biomass to the low-pressure indirectly-heated entrained flow gasifier.
Steam is injected into the gasifier to aid in stabilizing the entrained flow of biomass and sand through the gasifier. The biomass is chemically converted to a mixture of syngas components (CO, H2, CO2, CH4, etc.), tars, and a solid char that is mainly the fixed carbon residual from the biomass plus carbon (coke) deposited on the sand. Cyclones at the exit of the gasifier separate the char and sand from the syngas. These solids flow by gravity from the cyclones into the char combustor.
Air is introduced to the bottom of the combustor reactor and serves as a carrier gas for the fluidized bed plus as the oxidant for burning the char and coke. The heat of combustion heats the sand to more than 1,800°F (982 °C). The hot sand and residual ash from the char is carried out of the combustor by the combustion gases and separated from the hot gases using another pair of cyclones.
The first cyclone is designed to capture mostly sand while the smaller ash particles remain entrained in the gas exiting the cyclone. The second cyclone is designed to capture the ash and any sand passing through the first cyclone. The hot sand captured by the first cyclone flows by gravity back into the gasifier to provide the heat for the gasification reaction. Ash and sand particles captured in the second cyclone are cooled, moistened to minimize dust, and sent to a landfill for disposal.
Gas Cleanup & Conditioning. This consists of multiple operations: reforming of tars and other hydrocarbons to CO and H2; syngas cooling/quench; and acid gas (CO2 and H2S) removal. Tar reforming is envisioned to occur in an isothermal fluidized bed reactor; deactivated reforming catalyst is separated from the effluent syngas and regenerated online.
Methanol Synthesis. The cleaned and conditioned syngas is converted to methanol in a fixed bed reactor containing a copper/zinc oxide/alumina catalyst. The mixture of methanol and unconverted syngas is cooled through heat exchange with the steam cycle and other process streams. The methanol is separated by condensing it away from the unconverted syngas. Unconverted syngas is recycled back to the entrance of the methanol synthesis reactor.
Methanol Conditioning. The methanol leaving the reactor has been condensed at elevated pressure and has absorbed a sizeable quantity of gas. The methanol and gas stream is first heated and sent through a turbo expander generator to recover a portion of the compression energy. Once the stream is at a lower temperature it is sent to a distillation column to degas the methanol. This removal of gases could be done at a later stage in the process.
Methanol-to-Gasoline. In the MTG process, dimethylether (DME), the dehydrated derivative of methanol, is reacted over a ZSM-5 zeolite catalyst, on which the chain growth of molecules is sterically hindered, thus allowing only production of gasoline and lighter material. The gasoline product from the MTG process has more than 51 compounds, similar to straight-run gasoline in a petroleum refinery.
Gasoline Separation. The separation of the gasoline mixture is similar to the process used in a gasoline refinery. The design used in this model came from the New Zealand MTG demonstration process design with a few minor modifications. This design utilizes five distillation columns to separate the remaining gas, LPG, light gasoline, and heavy gasoline. The remaining gas is sent to the fuel combustor. The light gasoline continues without further treatment. The heavy gasoline could proceed through a durene isomerizer in order to eliminate the presence of the 1,2,4,5-tetramethylbenzenes by converting them to 1,2,3,5-tetramethylbenzenes. This stream would then be merged with the light gasoline. The two product streams are LPG and gasoline.
Heat & Power. A conventional steam cycle produces heat (as steam) for the gasifier and reformer operations and electricity for internal power requirements (with the possibility to export excess electricity as a co-product). The steam cycle is integrated with the biomass conversion and MTG processes. Pre-heaters, steam generators, and super-heaters are integrated within the process design to create the steam. The steam will run through turbines to drive compressors, generate electricity, or be withdrawn at various pressure levels for injection into the process. The condensate will be sent back to the steam cycle, de-gassed, and combined with makeup water.
An earlier study by researchers at the Pacific Northwest National Laboratory (PNNL) (Jones and Zhu) predicted a PGP for BTG that is approximately 65% higher than the PGP predicted in the NREL BTG study. A principal difference between the two studies, said the NREL authors, is the state of the various technologies throughout the process.
PNNL’s report utilizes and references proven states of technology. The report by NREL analyzes future states of the technologies and predicts the potential of the process. For the gasification and syngas cleanup sections in the NREL report, the 2012 targets used were set by DOE and are defined in the OBP MYPP (Multi-Year Program Plan). In the MTG section of the NREL report, a fluidized bed reactor was utilized (proven at the pilot scale) instead of the fixed bed reactor utilized by PNNL (proven at the commercial scale). The states of technologies have a large impact on the capital and operating costs required for the process. Other differences include but are not limited to stream factor, year dollars used (2007 vs. 2008), and feedstock cost.
Phillips, S. D.; Tarud, J. K.; Biddy, M. J.; Dutta, A. (2011) Gasoline from Wood via Integrated Gasification, Synthesis, and Methanol-to-Gasoline Technologies (NREL TP-5100-47594)
S. B. Jones and Y. Zhu (2009) Techno-economic Analysis for the Conversion of Lignocellulosic Biomass to Gasoline via the Methanol-to-Gasoline (MTG) Process. (PNNL-18481)