20.5.1 Adding reform catalysts to biomass gasifiers

Catalytic biomass gasification research18 has focused on tar removal and hydrocarbon conversion to CO/H2. Dedicated efforts to develop catalysts for biomass gasification is in its infant stages, and the strategy till now has been to use catalysts (i) off the shelf, commercial, not so cheap, methane steam reforming catalysts, (ii) cheaper materials, dolomite based clays, alkalis salts (Na, K, chlorides). Catalysts are used mostly in fluidized bed reactors, such as bubbling or circulating fluidized beds. Because both the feedstock and the catalyst are solids, the catalyst acts only on the gases and vapors produced. When using a fluidized bed reactor the mechanical strength of the catalyst will be very important, besides activity. Unlike Fluid Catalytic Cracking (FCC) catalysts (also a fluid bed process) where the catalyst is a single structure, steam reforming catalysts have a metal supported on a carrier with promoters which makes the catalyst much more vulnerable for attrition. Tests in a fluidized bed with crushed commercial steam reforming catalysts (Sud Chemie C11-NK and ICI 46-1 S) showed a weight loss due to attrition of 28-33% after 48 hours of testing.19 Stronger fluidizable steam reforming catalysts have been developed. NREL19 developed pure (99.5 wt%) alumina and alumina based (>90 wt%, rest being MgO, SiO2 and K2O) fluidizable supports which had a lower surface area than commercial ones (1.4-2.7 m2/g versus 9.7 m2/g commercial) but a very low attrition rate (0.01 wt%/h versus 0.41­0.69 wt%/h commercial). Dolomite CaMg(CO3)2 has gained the most attention as it is very cheap and easy to apply.20,21 Although its calcined form can convert tars to a large extent it is more often used as a tar-reducer, a guard material, allowing the usage of more active but also more sensitive catalysts downstream.22 Dolomite is not able to effectively convert methane and suffers from attrition.21,23 Olivine23,24 is much more resistant to attrition than dolomite with a somewhat lower activity for tar destruction.

Nickel on alumina based catalysts have been used in the industry for naphtha and natural gas reforming for many years and it was therefore also logical to test them for biomass gasification applications. Baker et al.25 employed several Ni-based catalysts in a fluidized bed. They observed rapid deactivation which was ascribed to carbon fouling. The mineral olivine, which mostly contains SiO4, Mg and Fe with trace elements of Ni, Ca, Al, and Cr, has been proposed as support for nickel based steam reforming catalysts by Courson et al.26,21 The mineral has superior strength and a mild catalytic activity of its own. When the calcination temperature for NiO on olivine is varied three different connections can be made: (i) the Ni is freely deposited onto the support (~900°C) (ii) the Ni is strongly linked to the olivine (~1100°C) and (iii) the Ni is integrated in the olivine structure (~1400°C). The Ni-olivine which was calcined at 1100°C was found to be the most active for dry reforming of methane.21 The catalyst was also tested in a fluidized bed gasifier where it showed a higher tar conversion relative to normal olivine as shown in Table 20.1.28 However, especially the methane was still present in high amounts. The attrition rate of the Ni-olivine was around 0.025 kg/kg of dry fuel. Glass-ceramic catalysts have been proposed by Felix et al.29 Via controlled crystallization of a mixed melt (in the case for steam reforming Li2O-Al2O3-SiO2 with 15 wt% NiO and traces of MgO) a very strong material is produced which is claimed to be more resistant to attrition than olivine. Steam reforming of an artificial syngas (vol%: 16 H2, 8 CO, 12 CO2, 4 CH4, 16 H2O, 44 N2 and 600-700 ppmv of naphthalene) resulted in a

‘steady-state’ relative conversion of ~70-80% naphthalene and 5-10% methane at 800°C.

To the best of our knowledge, no fluid bed catalyst has been developed which has a similar activity and stability compared to fixed bed catalysts.

The current status of catalysts (natural materials or Ni-based) in biomass gasifiers is that they can lower the tar and the higher hydrocarbon content of the gas, which lowers the load on downstream tar removal and upgrading units. There are, however, still operational problems. Catalyst deactivation, catalyst make-up and fluidization problems still need research attention before these dolomite and olivine catalysts could be effectively employed. In practice, tars and hydrocarbons are actually dealt with predominantly downstream of the gasifier.