|Schema of synfuel synthesis through solar-driven biomass gasification. Solar energy produces both heat for gasification and H2 via electrolysis. From Hertwich et al. (2009) Click to enlarge.|
Researchers at the Norwegian University of Science and Technology (NTNU) are proposing a new process for producing synfuel from biomass using concentrating solar energy as its main energy source.
High temperature heat for biomass gasification is obtained from a molten-salt system in a solar concentrating tower. Hydrogen for reverse water gas shift reaction to avoid producing CO2 during the process is produced by electrolyzing water, driven by solar power.
The concept’s key feature is the use of high-temperature heat from a solar concentrating tower to drive the chemical process of converting biomass to a biofuel, obtaining a near-complete utilization of carbon atoms in the biomass.
The purpose of the concept, said Edgar G. Hertwich and Xiangping Zhang, is to obtain an easy to handle fuel with near-zero CO2 emission and reduced land-use requirements compared to first and second generation biofuels. A paper describing their proposal was published online 30 April in the ACS journal Environmental Science & Technology.
The researchers modeled the production of methanol, which can be used directly as transport fuel or as input for DME and FT-diesel synthesis. For comparison, they also modeled the production of methanol using only biomass as a fuel and also using coal as source of both carbon and energy. They assumed CO2 capture and storage of excess carbon produced during the fuel production for both of the alternative scenarios.
Transferring the high temperature heat from the solar concentrator to the gasifier and designing the available configuration of the gasifier are technological challenges. One of the feasible approaches is heating sand (Olivine, also the catalyst of gasification reaction) by hot molten salt from a solar concentrator tower, using a gasifier designed as a fluidized-bed of olivine particles.—Hertwich and Zhang (2009)
Hot syngas from the gasifier is cooled and cleaned, then compressed to the reformer (methane reforming) and the reverse water gas shift (Re-WGS) reactor. Additional hydrogen from water electrolysis driven by solar power is fed into the shift reactor to convert CO2 to CO and adjust H2/CO ratio to satisfy the requirement of the synfuel production.
The steam produced by heat recovery steam generation (HRSG) can be used for gasification.
|Characteristics of Three Scenarios with CO2 Capture and Compression|
|Solar-Driven Biomass Gasification||Biomass-Fired Biomass Gasification with CO2 Capture||Coal Gasification with CO2Capture|
|Energy conversion efficiency (%)||60.9||42.0||36.5|
|Fuel productivity (kg fuel/100 kg resource)||121.0||39.9||62.2|
|Land area for biomass growth (m2/ton fuel/yr)||331||1,003||0|
|Land area for solar energy collection (m2/ton fuel/yr)||51.5||0||0|
|Fuel cycle atmospheric CO2 balance (gC/MJ fuel)||0||-32||25|
|Total cost estimate (2001$/GJ), inc. CO2 charge @ $100/tC||7.5||8.9||10.8|
The solar-driven third generation biofuel requires only 33% of the biomass input and 38% of total land as the second generation biofuel, while still exhibiting a CO2-neutral fuel cycle.
Little CO2 is produced during the process so that CO2 capture is not needed. Ninety percent of total carbon from biomass is converted to biofuel and emitted to the atmosphere after utilization as transportation fuel. Ten percent of carbon is oxidized and released to the environment.
The process is carbon neutral, with the important caveat that emissions due to land use change, harvesting, transport, and production of all required capital is not taken into account in this study.—Hertwich and Zhang (2009)
Under the biomass-fired gasification scenario, 30% of the biomass is used to provide heat for biomass gasification and 20% is used to produced electricity and heat for the process and CO2 capture and compression. Only 30% of total carbon from the biomass feedstock is converted to fuel. With 50% carbon storage, the result is a carbon negative process, removing CO2 from the atmosphere.
Under the coal-to-liquids scenario, 60% of total carbon from the coal resource is captured and stored geologically, and 30% of total carbon is converted to transport fuel and then released to the atmosphere by utilization.
The solar driven process has higher initial capital costs; these, however, are offset by lower fuel costs.
Most of the 77 EJ of direct energy use in transportation in 2000 consisted of liquid fuels. Various scenario analyses assume a 2% growth rate, to 114 EJ in 2020. Producing 10% of this energy from second generation biofuels would require 44-130 million hectare (ha), assuming yields of 1-3 kg/m2y. In comparison, cropland today covers 1.5 billion ha.
Producing the same amount with third generation biofuels [solar-driven biomass gasification] requires 3 million ha for solar power and 22-51 million ha for biomass plantations. Given the concern of the effect of biofuels on land use, food production, and biodiversity impacts, reducing the land use by such a substantial amount offers a significant step forward.—Hertwich and Zhang (2009)
Edgar G. Hertwich and Xiangping Zhang (2009) Concentrating-Solar Biomass Gasification Process for a 3rd Generation Biofuel. Environ. Sci. Technol., Article ASAP doi: 10.1021/es802853g