Review
Research progress of hot gas filtration, desulphurization and HCl removal in coal-derived fuel gas: A review

https://doi.org/10.1016/j.cherd.2012.04.009Get rights and content

Abstract

The present review paper highlighted on the recent progress of hot gas filtration, desulphurization and HCl removal in coal-derived fuel gas for combined cycle power generation (IGCC) or molten carbonate fuel cells (MCFC) technologies. As a critical process in the gasification system, hot gas filtration in the particulate control device (PCD) was introduced with enhanced understanding of equipment and operation, filter element and failsafe material properties, and gasification ash characteristics. The issues associated with the commercialization of hot gas filtration were also addressed, and some novel systems and methods were also discussed. The hot gas desulphurization in coal-derived fuel gas has concentrated on developing regenerable sorbents including the single and composite oxides of Zn, Fe, Cu, Mn and other species, and the reduction of metal oxides in the highly reducing atmosphere followed by vaporization of elements can be a problem for reactivity and regeneration. With regard to the removal of HCl, the studies have indicated sorbents prepared by pelletizing the powders of naturally available alkali metal and alkali earth metal substances can rapidly react with HCl vapor and reduce the HCl vapor concentration to less than 1 ppmv, and some sorbents lab-made have very high chlorine capacity. The sorbents based hot gas cleaning also has some challenges. Kinetics studies showed that unreacted shrinking core (USC) can be applied to the modeling of H2S and HCl removal by sorbents at high temperature, and the surface chemical reaction and reactant diffusion by product layers between solid sorbents and gases were very important mechanisms. The paper also proposed and discussed a rational concept for the simultaneous removal of multiple contaminants including ash, H2S and HCl, which will offer a possible cost reduction by two or more processes in a single vessel for hot gas cleaning.

Highlights

▸ Research progress of hot gas filtration, desulphurization and HCl removal is highlighted. ▸ Hot gas cleaning has concentrated on developing regenerable sorbents. ▸ Shrinking core model can be applied to H2S and HCl removal by sorbents. ▸ Simultaneous removal of multiple contaminants could be achieved by a combined concept.

Introduction

Because of high efficiency and environmental friendship, the high temperature molten carbonate fuel cells (MCFC) and the integrated gasification combined cycle (IGCC) are most promising devices for future power generation plants from coal gasification followed by hot gas cleaning (Müller, 2012). The IGCC or MCFC system with power generation efficiency of 45–55% may lead to 20–30% reduction of CO2 emissions compared with current pulverized coal-fired power plants. Coal is a complex heterogeneous substance that contains several impurities including sulfur, chloride, nitrogen, and alkali metal compounds, etc. The raw gas from gasification of coal is composed dominantly of H2 and CO with small quantities of CO2, CH4 and H2O, and it also contains low concentrations of impurities, such as ash, HCl (plus HF), H2S (plus COS) and others. The impurities in a raw fuel gas must be removed before a gas combustion process with gas turbine or fuel cell to protect the equipment from its corrosive effects and also to meet the strict government regulations for emissions. The quality of fuel gas and levels of impurities significantly varies depending on type of fuel, gasifier and cleaning system. As well-known, wet gas cleaning processes using a series of water scrubbers and a selective amine-based absorption process are well-proven technologies to efficiently remove these solid and gaseous impurities from the fuel gas and thus adopted in many IGCC projects. However, it is thermally inefficient and produces waste water sludge (Zhou et al., 2008, Ghidossi et al., 2009). In addition, the scrubber technologies for removing HCl may cause a side reaction by which polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) can form by de novo synthesis. In addition, it also causes great corrosion of HCl vapor in contact with metal components. The reliability and efficiency of hot gas cleaning processes could be considered as the most important factors for the success of gasification for IGCC and MCFC technologies. In the past few years, significant improvements have been brought to hot gas cleaning technologies. The present review paper focuses mainly on the recent progress of hot gas filtration, desulphurization and HCl removal in coal-derived fuel gas.

Section snippets

Hot gas filtration

One of the most complicated tasks in the field of hot gas cleaning is the removal of ash particles from complex mixture of the products formed during thermal treatment. A robust and completely reliable technology is still to be developed to achieve high efficiencies. Cyclones, electrostatic precipitators, fabric filters, and wet scrubbers are conventionally employed to control the emission of particulates in gas streams with relatively low temperature conditions (Chen et al., 2009).

Hot gas desulphurization

Economic studies have shown that hot gas desulphurization could result in lower capital and operating costs than conventional cold gas desulfurization. The hot gas cleaning system used to remove H2S, COS and CS2 consists of desulfurization and regeneration reactors. The concentration of H2S plus COS and CS2 in raw gas products from typical coal gasification processes is in the range of 1000–14,000 ppm. H2S is the major species and roughly ten times COS and CS2. In the IGCC system, the removal of

Removal of HCl

Chlorine is the primary halogen in coal, and its content is typically in the range of 0.01–0.1 wt% (Davidson, 1996). When coal is injected into a gasifier, a significant amount of the Cl is converted to HCl in the devolatilization process. The concentration of HCl in raw fuel gas from several gasification processes is in the range of 40–700 ppmv. The removal of HCl vapor from the feed-gas can be beneficial in any power plant, especially in the high-temperature molten carbonate fuel cells and the

Simultaneous removal of ash, H2S and HCl

In the efficient removal of contaminants, the consideration has been that the raw syngas from the gasifiers contains many kinds of contaminants including ash, H2S and HCl gases. The simultaneous removal of these contaminants from the hot gas is difficult and needs a complex sequence of selective process steps. The combined concept for the removal of multiple contaminants offers an attractive engineering solution because it has the potential to simplify the complex cleaning sequence and to lower

Conclusions

The recent progress of hot gas filtration and removal of H2S and HCl in coal-derived fuel gas was reviewed for combined cycle power generation (IGCC) or molten carbonate fuel cells technologies. A conceptual design for a commercial large scale pulse less filter system was introduced, and some novel systems and methods were also discussed. The metal oxides of Fe, Zn, Mn, Mo, V, Ca, Sr, Ba, Co, Cu and W are feasible for removal of H2S in hot gas cleaning, and especially Zinc, Copper, Iron, and

Acknowledgments

The work is supported by the Program for New Century Excellent Talents in University (NCET-10-0279), by the 100 Talent Program of Chinese Academy of Sciences and by the Natural Science Foundation of China (50906079, 51006097).

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