duction from biomass competitive in such regions in the short term. However, such operations would be restricted to selected regions in the United States, and, in a long-term sustainable scenario, would require biomass production at the same rate as its consumption. The committee considered it to be unlikely that such localized operations would contribute significantly to the nation’s H2 supply. Therefore, such cases were not considered further in this analysis, nor were fertilizer costs and the energy required to produce, harvest, and transport biomass.

Environmental Impact of Biomass Used for Hydrogen Production

In the overall process of biomass production and gasification, no net CO2 is generated, except for the CO2 released from fossil fuels used for (1) harvesting and transportation of biomass, (2) operation of the gasification systems, and (3) electricity, as well as for (4) production and delivery of fertilizers in an advanced biomass system. Biomass handling alone is estimated to consume about 25 percent of the total capital costs of operation of a midsize biomass gasification plant. Furthermore, biomass production requires, in addition to land (see above), about 1000 to 3000 t of water per ton of biomass, as well as nutrients in the form of nitrogen (ammonia), phosphorus (phosphate), sulfur, and trace metals. Profitable future hydrogen production from biomass will require energy crops with increased growth yields, which translates into increased need for fertilizers, energy for production of fertilizers, and potentially water. As is the case with the production of food crops, erosion, nutrient depletion of the soil, and altered water use practices could result in potentially significant environmental impacts as a consequence of farming activities. These effects need to be carefully considered.

Technologies for Hydrogen Production from Biomass
Current Technologies

Current technologies for converting biomass into molecular hydrogen include gasification/pyrolysis of biomass coupled to subsequent steam reformation24 (Milne et al., 2002; Spath et al., 2000). The main conversion processes are (1) indirectly heated gasification, (2) oxygen-blown gasification, and (3) pyrolysis, as well as (4) biological gasification (anaerobic fermentation). Biomass gasification has been demonstrated at a scale of 100 tons of biomass per day.25 Only a small, 10 kg/day of H2 pilot biomass plant is in operation, and no empirical data on the operation, performance, and economics of a full-scale biomass-to-hydrogen plant are available.26 The thermodynamic efficiencies of these processes are currently around 50 percent. Considering the low energy content of biomass, between 0.2 percent and 0.4 percent of the total available solar energy is converted to molecular hydrogen.

Biomass gasifiers are designed to operate at low pressure and are limited to midsize-scale operations, owing to the heterogeneity of biomass, its localized production, and the relatively high costs of gathering and transporting biomass. Therefore, current biomass gasification plants are associated inherently with unit capital costs that are at least five times as high as those for coal gasification (see Figure 5-2 in Chapter 5) and operate at lower efficiency.

Coproduction (biorefinery) of, for example, phenolic adhesives, polymers, waxes, and other products with hydrogen production from biomass, is being discussed in the context of plant designs to improve the overall economics of biomass-to-hydrogen conversion27 (Milne et al., 2002). The technical and economic viability of such coproduction plants is unproven and was not considered in this analysis.

Several major technical challenges of biomass gasification/pyrolysis exist and include variable efficiencies, tar production, and catalyst attrition28 (Milne et al., 2002). Moisture content as well as the relative composition and heterogeneity of biomass can result in significant deactivation of the catalyst. Recent fundamental research has identified a new, potentially inexpensive class of catalysts for aqueous-phase reforming of biomass-derived polyalcohols (Huber et al., 2003). In contrast to residue biomass, the use of bioenergy crops as biomass for gasification is advantageous, as its composition and moisture content are predictable, and the gasification process can be optimized for the corresponding crop.

Using anaerobic fermentation to convert biomass into hydrogen, a maximum of about 67 percent of the energy content (e.g., of glucose) can be recovered in hydrogen theoretically (calculated after Thauer et al., 1977). Considering the currently known fermentation pathways, a practical efficiency of biomass conversion to hydrogen by fermentation is between 15 and 33 percent (4 mol H2/mol glucose), although this is only possible at low hydrogen partial pressure. However, more efficient fermentation pathways could be conceived and would require significant bioengineering ef-

24  

M.K. Mann and R.P. Overend, National Renewable Energy Laboratory, “Hydrogen from Biomass: Prospective Resources, Technologies, and Economics,” presentation to the committee, January 22, 2003.

25  

Roxanne Danz, Department of Energy, Office of Energy Efficiency and Renewable Energy, “Hydrogen from Biomass,” presentation to the committee, December 2, 2002.

26  

M.K. Mann and R.P. Overend, National Renewable Energy Laboratory, “Hydrogen from Biomass: Prospective Resources, Technologies, and Economics,” presentation to the committee, January 22, 2003.

27  

M.K. Mann and R.P. Overend, National Renewable Energy Laboratory, “Hydrogen from Biomass: Prospective Resources, Technologies, and Economics,” presentation to the committee, January 22, 2003.

28  

M.K. Mann and R.P. Overend, National Renewable Energy Laboratory, “Hydrogen from Biomass: Prospective Resources, Technologies, and Economics,” presentation to the committee, January 22, 2003.



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