At present, hydrogen is produced almost entirely from fossil fuels such as natural gas, naphtha, and inexpensive coal. Renewable biomass is an attractive alternative to fossil feedstocks because of an essentially zero net CO2 impact. Unfortunately, the hydrogen content in biomass is only 6-6.5%, compared to almost 25% in natural gas. For this reason, on a cost basis, producing hydrogen by a direct conversion process such as the biomass gasification/water-gas shift cannot economically compete with the well-developed technology for steam reforming of natural gas. However, an integrated process, in which biomass is partly used to produce more valuable materials or chemicals with only residual fractions utilized for generation of hydrogen, may be an economically viable option. The proposed method 1, 2 combines two stages:
This concept has several advantages over the traditional gasification/water-gas shift technology.
Firstly, bio-oil is much easier to transport than solid biomass and, therefore, the location of pyrolysis plants could be optimized considering the availability of low-cost feedstock while reforming would be carried out at a site with an existing hydrogen storage and distribution infrastructure.
The second advantage is the potential production and recovery of higher value added co-products from bio-oil that could significantly impact the economics of the entire process. In this concept, the lignin-derived fraction would be separated from bio-oil and used as a phenol substitute in phenol-formaldehyde adhesives while the carbohydrate-derived fraction would be catalytically steam reformed to produce hydrogen. Assuming that the phenolic fraction could be sold for $0.44/kg (approximately half of the price of phenol), the estimated cost of hydrogen from this conceptual process would be $7.7/GJ3, which is at the low end of the current selling prices. Because biomass fast pyrolysis has almost reached commercial status, our work has focused on the catalytic steam reforming of bio-oil and its fractions. We successfully demonstrated that hydrogen could be efficiently produced by catalytic steam reforming the carbohydrate-derived bio-oil fraction using a commercial nickel-based catalyst in a fluidized bed reactor. The equipment is shown in Figure 1. Greater steam excess than that used for natural gas reforming was necessary to minimize the formation of char and coke (or to gasify these carbonaceous solids) resulting from thermal decomposition of complex carbohydrate-derived compounds.
1. D. Wang, S. Czernik, D. Montan, M. Mann, and E. Chornet, I&EC Research, 36 (1997) 1507.
2. D. Wang, S. Czernik, and E. Chornet, Energy & Fuels, 12 (1998) 19.
3. MK. Mann, P.L. Spath, K. Kadam, in proceedings of the 1996 U.S. DOE Hydrogen Program Review, Miami, FL, May 1-2, 1996, NREL/CP-430-21968; pp. 249-272.
For further information please contact:
Dr Stefan Czernik, NREL, 1617 Cole Boulevard, Golden, Colorado, 80401, USA.
Tel: +1 303 275 3821 Fax: +1 303 275 2905 Email: stefan_czerniks@NREL.GOV