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Water for Energy and Fuel Production
The major drawback of steam reforming of biomass is the tar formation that is not
easily amenable to steam reforming process. The tar formation can be minimized
by suitable operating conditions (i.e., operating at very high temperature), suitable
gasifier design (i.e., entrained bed reactor), or incorporation of additives or promoters
to the catalysts. At temperatures above around 1000°C, tar can be cracked, and for
temperatures above around 1200°C, pure syngas can be obtained. Higher residence
time can also help cracking of the tar. The additives such as dolomite and olivine to
the nickel catalyst help to reduce the tar formation. Alkaline metal oxides are also
used to reduce the tar formation.
Another important issue with biomass gasification and reforming is the formation
of ash that can cause slagging, fouling, and agglomeration. The inorganic impurities
in biomass can be removed by biomass pretreatment using leaching and extraction
processes. The literature has shown the leaching and subsequent gasification to produce hydrogen as a viable process for olive oil waste [156–171].
Pacific Northwest National Laboratory studied the gasification of biomass to
produce a variety of gaseous fuels using appropriate catalysts. The earlier studies
used a catalytic steam gasification of biomass with concurrent separation of hydrogen in a membrane reactor that employed a permselective membrane to separate
the hydrogen as it is produced [156–171]. The process was particularly well suited
for wet biomass and may be conducted at temperatures as low as 300°C. One
experiment was conducted at 4000 psi pressure and 450°C, although most others
were at 15–30 psi. The process was named SepRx. Optimal gasification conditions were found to be at about 500°C, an atmospheric pressure, and a steam/
biomass ratio of 10/1. In the presence of a nickel catalyst, the product hydrogen
concentration of 65 vol% was generated under these optimal conditions. Rapagna
[168] examined steam gasification of almond shell in the temperature range of
500°C–800°C. Smaller particle size yielded more hydrogen. Rapagna and Foscolo
[169] examined catalytic steam gasification in a fluidized bed reactor followed by
a fixed-bed catalytic reactor. Over a temperature range of 660°C–830°C, the catalytic converter using different steam reforming nickel catalysts and dolomite gave
as high as 60% hydrogen yield.
Steam gasification and steam reforming can be coupled processes. Mckinley
et al. [165] examined various biomass gasification processes for the production of
hydrogen. Turn et al. [166] showed that for a noncatalytic gasification of sawdust, the
highest hydrogen yield was obtained at 825°C and for a steam/biomass ratio of 1.7.
Zhou et al. [167], however, showed that for the production of hydrogen, adding steam
to the gasification process was not as effective as adding steam to downstream nickelcatalyzed steam reforming process.
4.5.2.6 mixed Feedstock
In the recent years, significant efforts have been made to gasify and steam reform
mixed feedstock of coal and waste, coal and biomass, and various types of biomass. These studies are described in a recent publication by Lee and Shah [2] and
others [5,29–32,172]. Gasification and steam reforming of mixed feedstock has a
very bright future. De Ruyck et al. [172] examined the co-utilization of biomass and
natural gas in a combined cycle through primary steam reforming of natural gas.