Elsevier

Biomass and Bioenergy

Volume 26, Issue 3, March 2004, Pages 269-279
Biomass and Bioenergy

A comparison of Rh/CeO2/SiO2 catalysts with steam reforming catalysts, dolomite and inert materials as bed materials in low throughput fluidized bed gasification systems

https://doi.org/10.1016/S0961-9534(03)00105-3Get rights and content

Abstract

The gasification of cedar wood in the presence of Rh/CeO2/SiO2 has been conducted in the laboratory scale fluidized bed reactor using air as a gasifying agent at low temperatures (823–973K) in order to produce high-quality fuel gas for gas turbine for power generation. The performance of the Rh/CeO2/SiO2 catalyst has been compared with conventional catalysts such as commercial steam reforming catalyst G-91, dolomite and noncatalyst systems by measurements of the cold gas efficiency, tar concentration, carbon conversion to gas and gas composition. The tar concentration was completely negligible in the Rh/CeO2/SiO2-catalyzed product gas whereas it was about 30, 113, and 139g/m3 in G-91, dolomite and noncatalyzed product gas, respectively. Since the carbon conversion to useful gas such as CO, H2, and CH4 are much higher on Rh/CeO2/SiO2 catalyst than others at 873K, the cold gas efficiency is much higher (71%) in this case than others. The hydrogen content in the product gas is much higher (>24vol%) than the specified level (>10vol%) for efficient combustion in the gas turbine engine. The char and coke formation is also very low on Rh/CeO2/SiO2 catalyst than on the conventional catalysts. Although the catalyst surface area was slightly decreased after using the same catalyst in at least 20 experiments, the deactivation problem was not severe.

Introduction

The efficiency of the biomass to electricity apparently depends on the overall efficiency of the gas engine [1], [2], [3], [4], [5], [6]. The limitation of the gasification and gas-engine efficiency makes the difficult to attain an overall efficiency higher than 30% with biomass gasification and power generation technology [7], [8], [9]. The cold gas efficiency of the product gas from gasifier plays a major role for overall power generation efficiency. Furthermore, the cold gas composition from the biomass gasification is directly related to the cold gas efficiency for heat production by combustion. Biomass gasification-based commercial power plants of different sizes in China use various conventional gasifiers at high temperatures; however, the cold gas efficiency does not exceed 70% [10]. For example, the cold gas efficiency is 70% for stratified down-draft gasifier at 1373K, 50% for down-draft gasifier at 1073K, and 67–75% for circulating fluidized bed gasifier at 923–1123K. The conventional gasification processes provides 50–80% carbon in the biomass to gas depending on the temperature at around 973–1173K and equivalence ratio at around 0.2–0.35 [11], [12], [13], [14], [15], [16] and it would be increased to about 80–90% if a catalyst is used to convert the tar and char to gas [17], [18], [19]. The rest of the carbons related to the tar and char. As a result, the product gas composition does not meet the expectation level of 100% carbon conversion (C-conv.).

Furthermore, the tar and char create some extra problems in the operation of gas engine such as corrosion, erosion, burning difficulties, internal deposition, etc. Therefore, for the satisfactory IC engine operation, an acceptable particle content <50mg/Nm3 and a tar content <100mg/Nm3 is postulated; however, in the case of gas turbine the fuel gas should be completely free of tar unless if remains in the vapor phase. The other requirements are the gas heating value 4–6MJ/ms3 (LHV) and the minimum gas hydrogen content >10vol% [8], [20]. These targets could be met only by developing new gasification technology. The tar and char can be destroyed by breaking them down into permanent gas that do not condense at low temperatures. It can be carried out in two ways; thermally at above 1473K or catalytically by passing the gas over catalysts at a lower temperature, typically around 1073–1123K [17], [18], [19]. In fact, the catalytic cleaning of hot gas of biomass gasification resulted in prescribed limit; however, the conventional catalytic converter is a two-stage system and the temperature is still high. The optimized gasifier (design and operation variable) followed by a catalytic reactor has nowadays been widely used. Dolomites [21], [22] and steam reforming nickel-based catalysts [23], [24], [25], [26], [27] are the most common catalysts for tar cracking in the secondary reactor at 1073–1173K for dolomite and 973–1073K for nickel-based catalysts. However, this type of catalyst is deactivated during reaction by carbon deposition on the surface. And the deactivation problem is more severe when the catalyst is used in the primary reactor [28], [29].

We have developed an Rh/CeO2 catalyst [30], [31] to convert total carbon and to produce high-quality gas from biomass gasification in a single reactor at low temperature in order to improve the overall efficiency. However, the catalyst still has deactivation problems due to sintering of CeO2 under the reaction conditions and the cost is a bit higher than conventional catalyst [32]. The problem motivated us to further develop the catalyst in order to prevent the CeO2 sintering. In this regard, we have prepared Rh/CeO2/SiO2 catalyst with 35 mass% CeO2 and 1% Rh by loading first the CeO2 on high surface area SiO2(380m2/g) by incipient wetness method followed by the Rh loading by a simple impregnation method. Interestingly enough, the Rh/CeO2/SiO2 catalyst showed the excellent performance in cellulose gasification at very low temperature and the TEM images of the used catalyst showed no sintering of the CeO2 and Rh [33], [34]. The Rh/CeO2/SiO2(35) catalyst was also used in a real biomass (cedar wood) gasification system; however, the result was not promising. Thus, we have prepared the catalyst with higher loading of CeO2 and tested in the cedar wood gasification, where about 98% carbon in the wood converted to the product gas at 923K [35]. This paper describes the continuation of the previous work. The biomass feeding rate in the previous work was 60mg/min, however, most of the experiments in this paper were carried out using 150mg/min and higher of biomass feeding rate. The reaction conditions were optimized and the cold gas efficiency was calculated and reported in this paper.

Section snippets

Fuel preparation

The feedstock considered in this study is cedar wood, which is available in Japan. The raw saw dust of this wood collected is rarely being directly fed into the gasifier. To ensure the consistent feeding and optimized gasification products, the feedstock was dried to about 10% moisture content and ground with a ball mill to about 0.1–0.3mm. The properties as well as ultimate and proximate analyses of the cedar wood were carried out and summarized in Table 1.

Catalyst preparation

In this gasification system, the

Results and discussion

The efficient production of electricity from biomass mainly depends on the availability and cost of the biomass resources. Therefore, if we cannot reduce the biomass consumption per unit electricity production, it would be challenging to meet the cost effective utilization of biomass for energy production. Although, some advanced technology for biomass gasification power generation such as IGCC is considered to be an effective process to improve the overall efficiency, the gasification section

Summary and conclusions

The cold gas efficiency and the tar and dust content in the product gas from the biomass gasification are the crucial factors to be improved for efficient use of biomass for electricity generation by gasification technology. The gasification of cedar wood by Rh/CeO2/SiO2(60) catalyst in a fluidized bed reactor produced the high-quality gas for power generation engine. The hydrogen content in the fuel gas produced in this system for engine use is much higher (>24vol%) than the specified range

Acknowledgements

This research was supported by the Future Program of Japan Society for the Promotion of Sciences under the Project “Synthesis of Ecological High Quality of Transportation Fuels” (JSPS-RFTF98P01001).

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