A project is carried out to develop a process for tar elimination downstream of a gasifier making use of cheap and active materials as catalysts. In the first stage of the project, screening of catalysts was carried out in a fixed-bed tubular reactor. The results of the fixed-bed experiments will be used in the design of the process. This paper presents a review of the various types of catalysts that have been used in several research works to reduce the tars in the producer gas derived from the biomass gasification process. The catalysts are divided into two classes according to their production method:  minerals and synthetic catalysts. A summary of the review and recommendations for good catalyst candidates and future work are also provided.

Review of Catalysts for Tar Elimination in Biomass Gasification Processes

Syngas from gasification of carbonaceous feedstocks is used for power production and synthesis of fuels and commodity chemicals. Impurities in gasification feedstocks, especially sulfur, nitrogen, chlorine, and ash, often find their way into syngas and can interfere with downstream applications. Incomplete gasification can also produce undesirable products in the raw syngas in the form of tar and particulate char. This paper reviews the technologies for removing contaminants from raw syngas. These technologies are classified according to the gas temperature exiting the cleanup device: hot (T > 300 °C), cold (T < ∼100 °C), and warm gas cleaning regimes. Cold gas cleanup uses relatively mature techniques that are highly effective although they often generate waste water streams and may suffer from energy inefficiencies. The majority of these techniques are based on using wet scrubbers. Hot gas cleaning technologies are attractive because they avoid cooling and reheating the gas stream. Many of these are still under development given the technical difficulties caused by extreme environments. Warm gas cleaning technologies include traditional particulate removal devices along with new approaches for removing tar and chlorine.

A review of cleaning technologies for biomass-derived syngas

Gasification of biomass is a promising source of fuels and other chemical products. However, the removal of tars, ammonia, hydrogen sulfide, and other byproducts from the raw gas is required. The gas clean‐up technology that offers more advantages is hot catalytic gas conditioning downstream of the gasifier reactor. Here, we review the applications of basic, acidic, metallic, and redox catalysts for the removal of such by-products and research strategies to prevent coke accumulation on the catalytic surface, to increase the resistance to sulfur poisoning, and to bring the by‐product removal reactions to temperatures below 600°C.

Hot Gas Removal of Tars, Ammonia, and Hydrogen Sulfide from Biomass Gasification Gas

This paper aims to propose an effective tar conversion approach during biomass pyrolysis via in-situ dry reforming over rice husk (RH) char and char-supported Ni-Fe catalysts. Utilizing high pyrolysis temperature, tar from biomass pyrolysis could be removed effectively in the gasifier by mixing with the char-supported catalysts, simplifying the follow-up tar removal process. Under the optimized conditions, the conversion efficiencies of condensable tar can reach about 92.3% and 93% using Ni-Fe char (without calcination) and Ni char (with calcination), respectively. It is noteworthy that the condensable tar could be catalytically transformed into the non-condensable tar or small molecule gases resulting in the heating value increase of gaseous products to benefit of the power generation systems. Compared with the other catalysts preparation methods, Ni–Fe char exhibited more advantages of convenient and energy-saving. In the presence of catalysts, the concentration of CO2 (vol.%) was reduced slightly, while the CO concentration (vol.%) increased greatly because of dry reforming. Due to carbon loss, parts of RH char-supported catalysts (C-SiO2 catalysts) could be converted into SiO2-based catalysts because of high-content amorphous nano-sized SiO2 in RH char. In addition, partial metal oxides or ions via carbon (i.e., biochar) and gas (i.e., H2, CO) in-situ reduction were transformed into metallic states contributing to the enhancement of tar conversion. Therefore, RH char plays two significant roles during the process of biomass pyrolysis. On one hand, it works as an intermediate reductant to reduce the metal oxides and CO2; on the other hand, it can be considered as an adsorptive-support to adsorb metal ions and tar. After that, the char-supported catalysts could be used for tar conversion. In particular, since the metal catalysts still remain in the solid residues, the pyrolysis char could be regenerated via thermal regeneration using waste heat or gasified into syngas directly.

In-situ catalytic conversion of tar using rice husk char-supported nickel-iron catalysts for biomass pyrolysis/gasification

Recent advances in catalysts for hot-gas removal of tar and NH3 from biomass gasification

Catalytic cracking of benzene on iron oxide-silica: catalyst activity and reaction mechanism

Necessary conditions for the optimum linear dynamic model reduction problem subject to a pulse input (Dirac delta function) are derived. These conditions are based on the integral square error and are applicable to multiple input-single output systems. Such a problem can be made computationally feasible for finite and infinite time intervals and for various forms of the low order system. A numerical example based on unsteady state heat conduction is presented

Least squares reduction of linear systems using impulse response

Mathematical modeling of combustion and gasification of a wet coal slab—I: Model development and verification

Mathematical modeling of combustion and gasification of a wet coal slab—II: Mode of combustion, steady state multiplicities and extinction

Nine cavity growth models for underground coal gasification are described and critiqued with respect to the way in which each model considers cavity geometry, cavity growth mechanisms, fluid flow, heat/mass transfer and experimental verification. Finally, the authors indicate what they feel is necessary in order to develop a realistic, reliable UCG cavity growth model that can be used for developing site selection criteria and optimizing the operational parameters of the process. Although UCG cavity growth models have made a significant contribution to the understanding of the cavity growth process, there is not currently a viable UCG cavity growth model that can be used a priori to predict the growth of the cavity. A number of the models that are discussed have been used to predict field test results by using such methods as adjusting parameters or making arbitrary assumptions. As a result, these models are unsuccessful when applied to field test for which they have not been calibrated. The situation has resulted because the UCG cavity growth models to date have not accurately described in their models the factors controlling the growth of the cavity.

James B. Riggs

Papers

Catalytic cracking of benzene on iron oxide-silica: catalyst activity and reaction mechanism

Least squares reduction of linear systems using impulse response

Mathematical modeling of combustion and gasification of a wet coal slab—I: Model development and verification

Mathematical modeling of combustion and gasification of a wet coal slab—II: Mode of combustion, steady state multiplicities and extinction

Sweep efficiency models for underground coal gasification: a critical assessment