Wt. % Oxygen in Feedstock
FIGURE 8.9 Theoretical yield of H2 as a function of the oxygen content in the feed.
Anaerobic Fermentation Metabolic digestion processing
Gasification High pressure Pyrolysis aqueous
Photo-Reforming biology shift
CH4/CO. Bio-shift Synthesis
Shift Reforming shift
H2/CO2 H2/C H2/CO2
H2/CO2 H2/CO2 H2/CO2 H2/C H2/CO2
FIGURE 8.10 Pathways from biomass to hydrogen. Storable intermediates are shown in boxes.
Biomass conversion technologies can be divided into two categories: (a) direct production routes and (b) conversion of storable intermediates. Direct routes have the advantage of simplicity. Indirect routes have additional production steps, but have an advantage in that there can be distributed production of the intermediates, minimizing the transportation costs of the biomass. The intermediates can then be shipped to a central, larger-scale hydrogen production facility. Both classes have thermochemical and biological routes. Figure 8.10 shows a "tree" of possible technologies.
A third area of hydrogen from biomass is metabolic processing to split water via photosynthesis or to perform the shift reaction by photobiological organisms. The photobiological production of hydrogen is an area of long-term research and is not discussed here. The use of microorganisms to perform the shift reaction is of great relevance to hydrogen production because of the potential to reduce carbon monoxide levels in the product gas far below the level attained using water-gas-shift catalysts and, hence, eliminate final CO scrubbing for fuel-cell applications. However, this is also a long-term research goal and is not covered in this chapter.
Among the direct processes, we shall mention here only gasification of biomass. It is a two-step process in which a solid fuel (biomass or coal) is thermochemically converted to a low- or medium-energy-content gas. Natural gas contains 35 MJ/Nm3. Air-blown biomass gasification results in approximately 5 MJ/m3; oxygen-blown results in 15 MJ/m3. In the first reaction, pyrolysis, the dissociated and volatile components of the fuel are vaporized at temperatures as low as 600°C. Included in the volatile vapors are hydrocarbon gases, hydrogen, carbon monoxide, carbon dioxide, tar, and water vapor. Because biomass fuels tend to have more volatile components (70 to 86% on a dry basis) than coal (30%), pyrolysis plays a larger role in biomass gasification than in coal gasification. Gas-phase thermal cracking of the volatiles occurs, reducing the levels of tar. Char (fixed carbon) and ash are the pyrolysis by-products that are not vaporized. In the second step, the char is gasified through reactions with oxygen, steam, and hydrogen. Some of the unburned char can be combusted to release the heat needed for the endothermic pyrolysis reactions.
Gasification coupled with water-gas shift is the most widely practiced process route for biomass to hydrogen. Thermal, steam, and partial-oxidation gasification technologies are under development around the world. Feedstocks include both dedicated crops and agricultural and forest-product residues of hardwood, softwood, and herbaceous species.
Thermal gasification is essentially high-severity pyrolysis, although steam is generally present:
By including oxygen in the reaction gas, the separate supply of energy is not required, but the product gas is diluted with carbon dioxide and, if air is used to provide the oxygen, then nitrogen is also present:
Other relevant gasifying processes are bubbling fluid beds and the high-pressure, high-temperature slurry-fed process.
All of these gasifier examples will need to include significant gas conditioning, including the removal of tars and inorganic impurities and the conversion of CO to H2 by the water-gas-shift reaction:
Significant attention has been given to the conversion of wet feedstocks by high-pressure aqueous systems. This includes the super critical gasification-in-water approach as well as the super critical partial-oxidation approach.
Among the processes for conversion of biomass via storable intermediates, two recently developed processes deserve special attention. The first one is based on pyrolysis with a special temperature profile that results in a carbon char with an affinity for capturing CO2 through gas-phase reaction with mixed nitrogen-carrying nutrient compounds within the pore structures of the carbon char. This provides a high-added-value by-product that significantly increases the economics of the whole process. The patent-pending process is particularly applicable to fossil-fuel power plants as it also removes SOX and NO-,, does not require energy-intensive carbon dioxide separation, and operates at ambient temperature and pressure. The method of sequestration uses existing farm-fertilizer-distribution infrastructure to deliver a carbon that is highly resistant to microbiological decomposition. The physical structure of carbon material provides a framework for building an NPK fertilizer inside the pore structure and creating a physical slow-release mechanism of these nutrients.
The complete process produces three times as much hydrogen as it consumes, making it a net energy producer for the affiliated power plant .
The second process is autothermal reforming of ethanol to produce hydrogen. Ethanol is a well-known gasoline fuel additive. However, a significant fraction of its production cost comes from the need to remove all water. When converting ethanol to hydrogen by autothermal reforming, it is possible in principle to consume less than 20% of the energy content of the sugar, first forming ethanol by fermentation and then reacting it to H2. The efficiency of these processes for a fuel cell suggests that it may be possible to capture over 50% of the energy from photosynthesis as electricity in an economical chemical process that can be operated at large or small scales .
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