# Steam Gasification of Lignite in a Bench-Scale Fluidized-Bed Gasifier Using Olivine as Bed Material
## 1 Introduction  
The environmental issues associated with global warming have resulted in a strong tendency towards the reduction of greenhouse gas emissions in the field of energy production. This trend has thus drawn attention to clean-coal technologies, such as gasification, which, compared to conventional combustion, can reduce CO\({}_{2}\) emissions by up to 90% [1; 2]. The gasification process consists of partial oxidation of an organic feedstock that takes place at high temperatures (around 700-1200 \({}^{\circ}\)C) in the presence of a gasification agent (air, steam, oxygen, or a combination of these). Gasification consists of a combination of chemical processes, such as drying, pyrolysis, combustion, and partial oxidation. The main product is syngas, which is a mixture of gases with a high calorific value, typically composed of H\({}_{2}\), CO, CO\({}_{2}\), and CH\({}_{4}\). Some undesired products are also generated during the process, such as particulate, acid gases, and tars (condensable heavy hydrocarbons) [3], which have to be removed in order to make the gas usable for energy purposes or the production of base chemicals. Gasification can be a promising option for exploiting abundant carbonaceous solid resources, such as lignite [4], for the production of a versatile fuel gas with a wide range of possible final uses. The product gas can in fact be used for the production of electricity in integrated gasification combinedcycle systems (IGCC) or fuel cells [5], or it can be exploited for the synthesis of liquid fuels [6] and Fischer-Tropsch synthesis [7; 8], thus reducing imports of oil and natural gas from outside Europe and reducing emissions from the transport sector. Lignite, with its high heating value and low volatile content compared to biomass, could be a very suitable feedstock for the gasification process [9; 10]. Olvine, a mineral mainly composed of magnesium, iron oxide, and silica, is used as an inventory of the fluidized-bed reactor, because of its advantageous qualities. As stated in the literature, olivine is recommended as bed material because of its reported activity in tar reduction, which is comparable to that of calcined dolomite. Furthermore, olivine has a stronger attrition resistance, compared to dolomite, which makes it more suitable as bed material [11; 12]. Previous works have already reported lignite gasification in fluidized-bed reactors. [PERSON] et al. studied lignite steam gasification (particles of 500-1000 \(\upmu\)m) in a fluidized-bed reactor with a bed of silica and alumina in the range of temperatures of 850-950 \({}^{\circ}\)C [13]. Additionally, [PERSON] et al. investigated lignite gasification (particles of 2-6 mm) in the dual fluidized-bed gasifier developed by Vienna University with olivine as bed material, at an operating temperature of 850 \({}^{\circ}\)C, with steam/fuel ranging from 0.9 to 1.4 [14]. Furthermore, [PERSON] et al. performed a statistical analysis based on the experimental results of lignite gasification (particles of 70-500 \(\upmu\)m) in a fluidized-bed reactor of silica sand particles with steam and oxygen as oxidizing agents [15]. The novelty of the present work is the assessment of the fluidized-bed technology for the steam gasification of small particle size lignite (around 50 \(\upmu\)m) pre-treated by the WTA process (fluidized-bed drying process with internal waste heat utilization, in german: Wirbelschichttrocknung mit interner Abwarmenutzung), with a bed of olivine particles in a temperature range of 750-850 \({}^{\circ}\)C and a steam/fuel ratio equal to 0.65. Furthermore, in this work, the operating temperature was changed in order to study its effect on the gas quality, and in some of the experimental tests, air injections were added in the freeboard, in order to reproduce, on a smaller scale, the oxygen added in the post gasification zone of the High Temperature Winkler (HTW) gasifier [2; 16], and thus to assess its effectiveness in tar reduction.