Effects of gasifying conditions and bed materials on fluidized bed steam gasification of wood biomass

Abstract

The effect of steam gasification conditions on products properties was investigated in a bubbling fluidized bed reactor, using larch wood as the starting material. For bed material effect, calcined limestone and calcined waste concrete gave high content of H₂ and CO₂, while silica sand provided the high content of CO. At 650 °C, calcined limestone proved to be most effective for tar adsorption and showed high ability to adsorb CO₂ in bed. At 750 °C it could not capture CO₂ but still gave the highest cold gas efficiency (% LHV) of 79.61%. Steam gasification gave higher amount of gas product and higher H₂/CO ratio than those obtained with N₂ pyrolysis. The combined use of calcined limestone and calcined waste concrete with equal proportion contributed relatively the same gas composition, gas yield and cold gas efficiency as those of calcined limestone, but showed less attrition, sintering, and agglomeration propensities similar to the use of calcined waste concrete alone.

Introduction

It is well recognized that gasification is a viable route for converting biomass to simple fuel gases. The gases produced can be utilized directly as fuels for electricity and power generation or as chemical feedstocks for manufacturing methanol, dimethyl ether, Fischer–Tropsch oils, etc. (Han and Kim, 2008, McKendry, 2002, Wei et al., 2007). Compositions of gases produced from the gasification process are governed by operating conditions including reaction temperature, pressure, gasifying medium, types and amount of catalyst (McKendry, 2002).

Steam gasification of biomass is an attractive process for producing H₂-rich gas (Baratieri et al., 2008, Franco et al., 2003, Rapagná et al., 2002, Ross et al., 2007). It has been addressed to decrease effectively the amount of undesirable products including tar and char and the rate of coke formation on catalysts (Franco et al., 2003, Rapagná et al., 2002, Ross et al., 2007, Taralas and Kontominas, 2006). Furthermore, in steam gasification excess steam can be easily separated by condensation.

The significant parameter that affects the gas quality from gasification processes is indeed the catalyst type. Sutton et al. (2001) have summarized the following criteria for assessing the effectiveness of catalysts: effective tar removal, capability of generating a suitable syngas ratio, resistance to deactivation as a result of carbon fouling and sintering, simple regeneration, and low price. Catalysts used in gasification processes are divided into two classes including mineral and synthetic catalysts (El-Rub et al., 2004).

Calcined rocks are the mineral catalysts that contain alkaline earth metal oxides. Simell and Kurkela (1997) have classified them based on CaO/MgO ratio as: limestone (>50), dolomitic limestone (4–50), calcite dolomite (1.5–4) and dolomite (1.5). They found that improvement of the activity of these rocks was achieved by increasing the Ca/Mg ratio, decreasing the grain size, and increasing the active metal content such as iron. The most commonly used calcined rock catalyst is dolomites (El-Rub et al., 2004, Devi et al., 2003). Dolomites are most active in acting as a primary catalytic bed for the removal of heavy hydrocarbons prior to the reforming of the lighter hydrocarbons to produce syngas (Devi et al., 2003, Hu et al., 2006). Olivine was reported to eliminate tar as effectively as dolomite but its attrition resistance is higher than that of dolomite (Rapagná et al., 2000). Clays minerals can reduce tar quite effectively as they contain strong acid sites and have suitable pore diameters (El-Rub et al., 2004). In addition, it was noted that the reduction of iron oxide (Fe₂O₃) in clay to the more reactive form of magnetite (Fe₃O₄) can reduce C₂–C₃ content emissions during air/steam gasification of biomasses (Ross et al., 2007). For synthetic catalysts, alkali catalysts (Li, Na, K, Rb, Cs and Fr) can increase the rate of gasification dramatically and also reduce tar content in the gas product. However, the disadvantages associated with this type of catalyst are the difficulty in recovery, high cost and agglomeration problem at high temperatures (Sutton et al., 2001, El-Rub et al., 2004). For transition metal-based catalysts, nickel catalyst is sufficiently active, less expensive than noble metal catalysts such as Ru, Pt, and Rh and considered to be the most important catalyst in hot gas cleaning processes. They are highly effective to remove tar and help improve the content of syngas. Deactivation of the nickel catalysts are mainly due to carbon deposition and nickel particle growth. Char is an attractive synthetic catalyst due to its low price, being the direct by-product from gasification processes. Char has ample catalytic activity to eliminate tar but it can be deactivated by coke formation and loss of char mass in steam and dry reformation reactions (El-Rub et al., 2004).

As mentioned above, steam gasification can provide high H₂ content, but the undesirable products such as CO₂ and tars are also simultaneously generated. To enhance the efficiency of steam gasification, considerable efforts have been devoted to produce high yield of H₂ with simultaneous capture of CO₂. It has been reported (Florin and Harris, 2006) that the CaO introduced could capture CO₂ via Eq. (1) during gasification reaction, thus shifting the equilibrium reactions of the water–gas reaction by Eq. (2), methane reforming by Eq. (3) and water–gas shift reaction by Eq. (4) to promote a H₂-rich gas product.

$$\text{CaO}+{\text{CO}}_2\to {\text{CaCO}}_3\text{,}\mathrm{\Delta }{\text{H}}_{298}=-170.5\text{kJ/mol}$$

$$\text{C}+2{\text{H}}_2\text{O}\to {\text{CO}}_2+2{\text{H}}_2\text{,}\mathrm{\Delta }{\text{H}}_{298}=+100\text{kJ/mol}$$

$${\text{CH}}_4+2{\text{H}}_2\text{O}\to {\text{CO}}_2+4{\text{H}}_2\text{,}\mathrm{\Delta }{\text{H}}_{298}=+165\text{kJ/mol}$$

$$\text{CO}+{\text{H}}_2\text{O}\to {\text{CO}}_2+{\text{H}}_2\text{,}\mathrm{\Delta }{\text{H}}_{298}=-41\text{kJ/mol}$$

Lin et al., 2002, Lin et al., 2003, Lin et al., 2006 have developed HyPr-RING (hydrogen production by reaction-integrated novel gasification) which used calcium oxide (CaO) and/or calcium hydroxide (Ca(OH)₂) as the CO₂ adsorbent. However, it was found that this process had to be operated at a high steam pressure. From the limitations of capital and operating costs for high pressure processes, hydrogen production with CO₂ capture at atmospheric pressure should be more desirable. Hughes et al. (2005) have shown that at atmospheric pressure, the optimum temperature for carbonation of CaO should be in the range 650–750 °C. Furthermore, the capability of CaCO₃ formation at the respective temperature depended on the CO₂ partial pressure; CO₂ can be best adsorbed at a suitable partial pressure for a given temperature (Xu et al., 2005).

It should be noted that CaO is an attractive material that can play the combined roles of catalyst and adsorbent in a steam gasification process. Limestone is a commonly used material in the gasification process and CO₂ adsorption. However, it was reported that there were particle attrition and deactivation problems for long operation time and at temperatures higher than 650 °C (Florin and Harris, 2008). In this study, we propose to use waste concrete as another potential bed material in a steam gasification process. It is a material left for disposal after the concrete structures such as buildings, bridges, dams, road surfaces are demolished. Generally, the concrete contains CaO in the forms of tricalcium silicate (3CaO · SiO₂), dicalcium silicate (2CaO · SiO₂), tricalcium aluminate (3CaO · Al₂O₃) and tetracalcium aluminoferrite (4CaO · Al₂O₃ · Fe₂O₃) (Mindess et al., 2003).

In this work, the steam gasification of biomass (larch) was performed in a batch bubbling fluidized bed reactor. The effects of gasification conditions including types of bed material (silica sand, calcined lime stone and calcined waste concrete), temperature (650 and 750 °C) and gasifying agents (steam and N₂) on gasification products were investigated. Analytical techniques of thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to investigate CO₂ adsorption and physical deterioration of bed material at the end of gasification operation. In addition, the combined use of calcined limestone and calcined waste concrete as a new bed material in the steam gasification process was also studied.