aSchool of Environmental Engineering, University Malaysia Perlis UniMAP, Arau 02600, Perlis, Malaysia Abstract
MSW pyrolysis and gasification are the possible alternative to the direct use of fossil fuel energy. MSW, a CO2 neutral source of renewable fuel, can contribute to the demand for heat, electricity and synthesis gas (syngas). Nevertheless, there are inefficiencies in the technology, which at present render MSW pyrolysis and gasification economically unviable. The presence of condensable organic compounds and methane in the product gas renders the gas unsuitable for specific applications. Elimination of the condensable organic compounds and methane by a suitably cheap technology will enrich the economic viability of MSW pyrolysis and gasification. This paper contains an extensive review of the three main groups of catalysts, which have been estimated for the elimination of these hydrocarbons. These three groups of catalysts are dolomite, alkali metals and nickel. Keywords. MSW pyrolysis and gasification; Tar; Synthetic gas (syngas); Catalysts; Reforming; Dolomite; Alkali metals and nickel 1. Introduction
Notible progress has been achieved in recent years in the design of gasifiers. Nevertheless, gas cleaning is still the bottleneck in advanced gas utilization that limits the deployment of the use of MSW for electricity generation . The continual build-up of condensable organic compounds (often called to as tars) present in the produce gas can cause blockages and corrosion and also reduce overall efficiency. In addition, the presence of impurities (such as methane) can affect the end usage of the synthetic gas (syngas) and the techniques involved in the removal of the impurities in such processes are costly. Nitrogen and sulphur are present in many of the by-products and the corresponding oxides are produced during combustion of the fuel gas; these oxides (NOx and SOx) can have a negative environmental impact. Since the mid-1980s, interest has expanded on the subject of catalysis for MSW pyrolysis and gasification. The advances in this area have been driven by the need to produce a tar-free product gas from the pyrolysis and gasification of MSW, since the removal of tars and the reduction of the methane content increases the economic viability of the MSW pyrolysis and gasification process. The literature in this field ranges from papers on bench-scale reactors to those on the use of plant-scale gasifiers. Research on catalysts for use in the process is often carried out specifically in relation to gasifier design or MSW feed type. Nevertheless, the criteria for the catalyst are fundamentally the same and may be summarized as follows: 1. The catalysts should provide a suitable synthetic gas (syngas) ratio for the intended process. 2. The catalysts must be effective in the removal of tars.
3. If the desired product is synthetic gas (syngas), the catalysts must be capable of reforming methane. 4. The catalysts should be resistant to deactivation as a result of carbon fouling and sintering. 5. The catalysts should be easily regenerated.
6. The catalysts should be inexpensive.
Catalytic decomposition of the unwanted hydrocarbons is also known as hot gas cleaning. The catalysts employed in this process are responsible both for bringing and purification about compositional adjustment of the product gas. Hot gas conditioning is achieved by passing the raw gasifier product gas over a firm catalyst in a fluidized-bed (or a fixed-bed) under temperature and pressure conditions that essentially match those of the gasifier. As the raw gas passes over the catalyst, the hydrocarbons may be reformed on a catalyst surface with either steam (Eq. (1) or carbon dioxide or both (Eq. (2) to produce additional carbon monoxide and hydrogen:
(1) CnHm+nH2O ↔ nCO+(n+m/2)H2
(2) CnHm+nCO2 ↔ 2CO+(m/2)H2...