Article pubs.acs.org/EF
Release of Inorganic Elements from Gasification and Co-Gasification of Coal with Miscanthus, Straw, and Wood at High Temperature Marc Blas̈ ing,* Nurul Baity Ahmad Hasir, and Michael Müller Institute of Energy and Climate Research (IEK-2), Leo-Brandt-Str. 1, 52425 Jülich, Germany ABSTRACT: Three biomass energy feedstocks (wood, straw, and miscanthus) are co-gasified at 1400 °C with two lignites and two hard coals. The objective of this work was to determine the release of alkali metal, chlorine, and sulfur species and to correlate the release with the share of the fuel in the blends. The release of the inorganics is detected using a molecular beam mass spectrometer. Significant species are 34H2S+, 36HCl+, 58NaCl+, 60COS+, and 74KCl+. By comparison of the experimental data with calculations, the influence of the inorganic content of the fuel blends on important release mechanisms and trends is determined. It is shown that the Al/Si ratio and the chlorine content have a major influence on the release of alkali metal species in general. Additionally, it was found that the Al/Si ratio has a strong influence on the release of sulfur species through changes of the availability of Ca.
1. INTRODUCTION According to the BP Energy Outlook 2035, fossil fuels will remain the primary source of energy until the year 2035.1 Despite the economic benefits of coal for the generation of electrical power, the environmental impact of the utilization of coal cannot be neglected as coal contributes to about 44% of the CO2 emissions worldwide.2 Environmental concerns have grown over the years because of concerns about global warming which has been linked to the anthropogenic release of greenhouse gases, such as CO2, into the environment. Therefore, many countries have taken the initiative via the development of so-called clean coal technology, e.g. integrated gasification combined cycle (IGCC) power plants with CO2 capture. Power plants with IGCC have proven to be highly efficient and also offer the chance to sequester CO2.3 An advantage of the entrained flow gasifier unit of the IGCC plant is the fuel flexibility where the feedstock covers a broad range from coal, petrol coke, and secondary fuel to renewable biomass. The operating temperature of this gasifier is usually between 1400 and 1600 °C, which leads to the destruction of tars and other residues. This also leads to a higher carbon conversion, up to 99%, and the production of cleaner product gas.4 Coal-based IGCC technology has already been employed in test facilities in Buggenum, The Netherlands and Puertollano, Spain.3 Co-gasification of biomass and coal has not been demonstrated in large scale in the Polk IGCC plant and in the NUON Power Buggenum IGCC power plant.5 Cocombustion studies with straw and coal have been conducted in Denmark.4 The integration of biomass in the fuel mix of IGCC power plants can lower the emission of CO2. In comparison with coal, biomass has often less sulfur, which is one of the main harmful components released in the environment. Additionally, blending of biomass with coals for energy purposes reduces the intermittence of biomass, a problem that is faced by biomass-fed power plants. However, at the high operating temperatures of entrained flow gasifiers (about 1400 °C), inorganic species tend to volatilize more easily and cause problems such as slagging, fouling, and corrosion of the downstream parts of the power plant. Chloride species like HCl and alkali metal chlorides, NaCl and KCl, are known to be aggressively corrosive.6 This affects the © 2015 American Chemical Society
overall efficiency of the plants. Herbaceous biomass, which is rich in alkali metals, is a major source of such problems. Blending coal with straw can reduce the emission of alkali metal species because alkali metals can be captured by inherent coal minerals, especially aluminosilicates. The latter can form nonvolatile alkali metal aluminosilicates as shown for co-combustion of biomass.7−9 Long and Wang10 studied the co-gasification of coals and biomass at different ratios under reaction conditions similar to that in entrained-flow gasification. They found a significant reduction in SO2 release with higher portion of biomass. Their experiment also suggests that there was an interaction between coals and biomass that leads to such effects. Additionally, it has been shown recently that secondary reactions play a major role in the release of sulfur species during co-gasification of coal with petroleum-based fuels, whereas the release of chlorine species was correlated to the dilution of the fuels through fuel blending.11,12 Although there have been recent studies13−24 that focus on the mentioned effects and the release behavior of inorganics in general, there is still a lack of knowledge especially regarding fuel blends, and the release behavior is not satisfactorily understood. Therefore, the objective of this work was to determine the release of alkali metal, chlorine, and sulfur species and to put the release into correlation with share of the fuel in the fuel blends. The release of the inorganics in the hot gas phase is detected using molecular beam mass spectrometry (MBMS).
2. EXPERIMENTAL SECTION 2.1. Fuel Preparation. Two lignites from the Hambach mine in the Rhineland (HKN, HKT), one world market hard coal (Colombia, Col), a German hard coal (STD-3), and three biomasses (straw, miscanthus, wood) were collected and prepared for analysis. The biomasses were delivered pelletized. The wood was made mainly from clean sawdust. The straw and miscanthus came from The Netherlands. The fuels were crushed in a mill and sieved. Samples with a particle size of
