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Ind. Eng. Chem. Res. 2005, 44, 5864-5868
Distinctive Effects of CaO Additive on Atmospheric Gasification of Biomass at Different Temperatures Guangwen Xu,* Takahiro Murakami, Toshiyuki Suda, Shigeru Kusama, and Toshiro Fujimori Ishikawajima-Harima Heavy Industries (IHI) Corporation, Ltd., Sin-Nakahara-Cho 1, Isogo-ku, Yokohama 235-8501, Japan
Calcium oxide has long been recognized as an effective in-bed catalyst for reforming or cracking tars generated in thermally decomposing hydrocarbon fuels such as biomass. Under pressures as high as 20 atm, the oxide was also widely tested as a good CO2 acceptor to capture CO2 on-site to produce high-caloric pipeline gas from gasifying coal and coal-coke. By using CaO as an additive of the fuel and bed material, the research detailed in this research note demonstrated that CaO could also be a substantially good CO2 acceptor for the atmospheric gasification of biomass, provided the reaction temperature is appropriately low, such as not much over 973 K. At temperatures >1073 K, the additive exhibited basically the catalytic effect only, which led the H2 content of the product gas to increase and the tar release with the gas to decrease. Introduction Gasification of various solid hydrocarbon fuels, such as coal, biomass, and combustible wastes, is becoming increasingly important to future energy generation and to the production of petroleum-alternative liquid fuels. The basic requirements for a high-efficient gasification technology consist of a low tar generation and a high fuel conversion expressed with the cold gas efficiency. In addition, it also requires the syngas produced to have a high caloric value when the gas is adopted as a pipeline gas and household fuel or to have a reasonable H2/CO molar ratio, say, between 1.0 and 3.0, for application of the gas to various synthetic processes producing chemicals and liquid fuels, such as CH3OH, C2H5OH, DME, and GTL oils. The use of calcium-based oxides (sometimes containing Mg, Fe, etc.) derived from calcining carbonate rocks (e.g., limestone and dolomite) as an additive reagent has been widely tested to meet such requirements.1-8 Calcium oxide improves the gasification efficiency and the quality of the product gas through the effects highlighted below. First, it can be a catalyst for the reactions of CO shift and tar reforming/ cracking, leading to CaO-based material
CO + H2O 98 H2 + CO2 ∆H0 < 0
(1)
and CaO-based material
Tars 98 H2 + CO + CO2 + ... ∆H0 > 0 (2) This effect has been extensively tested for atmospheric biomass gasification. The results have shown that the pass of the product gas through a secondary bed packed with CaO-based particles (such as calcined CaCO3, Dolomite, etc.)1,2 and the use of the particles as an additive of the bed material3-5 for the gasifier can both considerably reduce the amount of tars in the resulting syngas, while they simultaneously increase the gas’s H2 * Corresponding author. Fax: 0081-45-759-2210. E-mail:
[email protected] or
[email protected].
content and gas yield (i.e., gas volume). Another effect of CaO on gasification is the so-called CO2-acceptor effect.6,7 In this case CaO particles are generally used as the gasifier’s bed-material to induce an on-site CO2 capture via
CaO + CO2 f CaCO3, ∆H0 < 0
(3)
so that the product gas has a low CO2 content and, in turn, a high caloric value. In a calcinator coupling the gasifier, the formed CaCO3 was regenerated into CaO and reused through recirculation. Because eq 3 is a volume-reducing reaction, high pressure was commonly employed to facilitate its occurrence and progress. Furthermore, reasonably low reaction temperatures are needed to prevent the CaCO3 calcination, i.e.,
CaCO3 f CaO + CO2, ∆H0 > 0
(4)
from taking place inside the gasifier. Because of all of these, the gasification process involving an on-site CO2 acceptor ran generally at 1000-1100 K and g20 atm to produce a high-caloric pipeline gas from coal or coalcoke.6,7 By adopting rather higher pressures (>30 atm) and lower reaction temperatures (∼873 K), Lin et al. realized a nearly complete on-site capture (absorption) of CO2 in a gasifier of coal, resulting in the direct production of H2 from gasifying coal.8 A consequent question is whether the preceding CO2acceptor effect of CaO is also possible for the atmospheric gasification of hydrocarbon fuels and what are the necessary conditions for it. Because of its lower capital and operational costs, the atmospheric gasifier may be much more widely adopted in practice for various isolated power/syngas production systems using, for example, biomass as the original fuel.9 Figure 1 shows the thermodynamic equilibrium data for eqs 3 and 4. The occurrence of eq 3 or 4 completely depends on the combination of the reaction temperature (ordinate) and the partial CO2 pressure (primary abscissa). When the latter is 0.2 atm, which is typical for the product gases from atmospheric gasification processes (indicated by 1.0 atm on the auxiliary abscissa), the
10.1021/ie050432o CCC: $30.25 © 2005 American Chemical Society Published on Web 06/24/2005
Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5865 Table 1. Properties of the Tested Coffee Grounds approximate analysis [wt %] moisture volatile matter fixed carbon ash
size
3.1 70.4 24.3 2.2
