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Sustainable fuel for the transportation sector : A recent study

hybrid hydrogen-carbon (H2CAR) process for the production of liquid hydrocarbon fuels is proposed wherein biomass is the carbon source and hydrogen is supplied from carbon-free energy. To implement this concept, a process has been designed to co-feed a biomass gasifier with H2 and CO2 recycled from the H2-CO to liquid conversion reactor. Modeling of this biomass to liquids process has identified several major advantages of the H2CAR process. (i) The land area needed to grow the biomass is <40% of that needed by other routes that solely use biomass to support the entire transportation sector. (ii) Whereas the literature estimates known processes to be able to produce 30% of the United States transportation fuel from the annual biomass of 1.366 billion tons, the H2CAR process shows the potential to supply the entire United States transportation sector from that quantity of biomass. (iii) The synthesized liquid provides H2 storage in an open loop system. (iv) Reduction to practice of the H2CAR route has the potential to provide the transportation sector for the foreseeable future, using the existing infrastructure. The rationale of using H2 in the H2CAR process is explained by the significantly higher annualized average solar energy conversion efficiency for hydrogen generation versus that for biomass growth. For coal to liquids, the advantage of H2CAR is that there is no additional CO2 release to the atmosphere due to the replacement of petroleum with coal, thus eliminating the need to sequester CO2.
To overcome the environmental challenges associated with coal and the land limitations with the bioenergy crop, we suggest an alternative pathway where neither coal nor biomass is treated as a sole source of energy to produce liquid hydrocarbon fuel. In our proposal, the primary purpose of either coal or biomass is to provide carbon atoms needed for the production of liquid hydrocarbons. Thus, the goal is to accomplish the complete transformation of every carbon atom contained in either of the feed stocks to liquid fuel by supplementing the conversion process with a carbon-free energy source. We propose to generate H2 from a carbon-free primary energy source such as solar, nuclear, wind, etc. and then use it to supply the hydrogen atoms needed for the chemical transformation. While not necessary, a portion of this H2 could also be used to provide the energy needed for the transformation and, thus, further improve the carbon efficiency. A schematic of the proposed process is depicted in Fig. 1. There are a number of important consequences of Fig. 1. First, there is no CO2 emission from the chemical processing system, and the only CO2 released to the environment is from the transportation engine. Therefore, for coal, it eliminates the need to sequester CO2 produced in the liquefaction process. Second, an associated benefit of the absence of CO2 release from the chemical processing system is that 40% of the amount of coal or biomass is needed to deliver the same quantity of liquid fuel. This is a great advantage in prolonging the life of the known coal reserves as well as in reducing the land area needed for the bioenergy crop. The large reduction in land area provides an opportunity for sustainable production of hydrocarbon fuel for the transportation sector. Third, by providing open-loop H2 storage, this solution addresses one of the grand challenges of the H2 economy. The addition of H2 atoms to carbon atoms from coal or biomass provides a high-density method for storage of massive quantities of H2. Fourth, on a carbon atom basis, the energy content of the liquid fuel is higher than that of coal or biomass. Moreover, conversion of the 60–70% of the carbon atoms normally lost from a given amount of coal or biomass into liquid fuel provides a further means to store large quantities of carbon-free energy in a usable form for the transportation sector. The proposed solution provides an important step toward meeting the goal of generating 10 TW of carbon-free power by 2050 (3).
We recently found two sources that mention the reaction of H2 from renewable sources with biomass to produce liquid fuel (19, 20). However, our proposal is expected to have much broader impact because it is more encompassing due to the judicious inclusion of coal and nuclear energy. More importantly, we suggest a number of processing steps in Fig. 2 that make this processes technically viable and also provide quantitative assessment of the relative benefits.

Although optimal configurations for the chemical processing system shown in Fig. 1 are yet to be defined, we chose the gasification route to provide guidance for the benefits. In a typical gasifier, oxygen and steam are supplied along with a carbon-containing feed stock. The resulting combustion energy not only provides heat for the endothermic gasification reaction, a majority of which is stored in the CO and H2 exiting the gasifier, but also compensates for the energy losses from the system. CO2 is formed in the gasifier from the combustion reaction and through the water–gas shift (WGS) reaction in post-gasifier processing. Whereas in the past it has been common to talk about the possibility of sequestering the resulting CO2, in the H2CAR process we plan to either suppress the formation of this CO2 or react it with H2 from a carbon-free energy source such as solar, nuclear, etc. to produce liquid fuel. The reverse WGS reaction of CO2 with H2 to form CO and H2O is an endothermic reaction and requires high temperatures to obtain a reasonable conversion. To simplify the overall process, we propose to recycle CO2 from the H2-CO to liquid conversion processes such as an FT process to a suitable location in the gasifier (Fig. 2). Furthermore, to help drive the thermodynamic equilibrium to the favorable H2/CO ratio of near two, the proposed process directly feeds H2 from the carbon-free energy source to the gasifier.
To our knowledge, such a gasifier with a recycle CO2 stream and H2 co-feed has never been built. The advantage of this configuration is that at steady-state operation, there is no CO2 buildup and therefore no net or little CO2 formed in the gasifier. This means that nearly all of the carbon atoms fed to the gasifier from coal or biomass are converted to CO. Of course, CO2 will be present in the gasifier effluent stream. Under typical operating conditions of conventional gasifiers, the gas composition of gaseous effluent stream is found to be close to thermodynamic equilibrium (21, 22). Similarly, for the proposed gasifier, we expect CO2 concentration to be determined by equilibrium considerations at the high temperatures of 800–1,300°C prevalent in the gasifier. Therefore, the formation of CO2 in the presence of added H2 will be greatly reduced. As a result, CO2 acts as an inert that is simply circulated through the overall process. For simplicity, we have named the hybrid H2-carbon process of Fig. 2 as the H2CAR process.

In the H2CAR process, addition of sufficient quantity of H2 along with oxygen to the gasifier may be thought of as providing energy for the gasification of the biomass or coal to CO. The oxidation of H2 is an exothermic reaction, and conversion of some H2 to water results in the net contribution of energy needed for gasification. Alternatively, high-temperature heat from a nuclear reactor or solar concentrators can be used to supply energy for the gasification.

The advantage of feeding H2 from a carbon-free energy source and recycling CO2 to the high-temperature gasification step is that it decouples the reverse WGS reaction requirement from the catalyst in H2-CO to liquid conversion reactor. Generally, the H2-CO to liquid conversion reactors operate at temperatures below 350°C, where the reverse WGS reaction is not favorable. The H2CAR process takes the advantage of the preferable high temperature range prevalent in the gasifier to run reverse WGS reaction. This allows a degree of freedom to tailor the FT synthesis catalyst specifically for the desired liquid hydrocarbon molecule. Another advantage of this process configuration is that net CO2 formation is minimized. Therefore, the cost associated with CO2 handling is reduced.

To quantify the impact of the proposed H2CAR route, we have done order of magnitude calculations for both biomass and coal as the carbon source. In the year 2005, the United States transportation sector alone consumed nearly 13.8 Mbbl/d of the world's total oil consumption of 82.5 Mbbl/d (10). Therefore, calculations were done to displace 13.8 Mbbl/d of oil with a synthetic fuel such as diesel. It is believed that the magnitude of the United States transportation sector is large enough to provide clear insight into the pros and cons of the proposed pathway. H2CAR results for biomass are presented first followed by those for coal.


In Images:
1.Fig. 1. Schematic of the proposed process. Some images courtesy of Department of Energy/National Renewable Energy Laboratory
2.One of the possible configurations of the proposed H2CAR process. Some images courtesy of Department of Energy/National Renewable Energy Laboratory.

Release link: http://www.pnas.org/cgi/content/full/104/12/4828