Petrographic composition of Polish lignite and its possible use in a fluidized bed gasification process

•Petrographic composition indicates suitability for fluidized bed gasification.•Polish lignite deposits are dominated by detritic and xylo-detritic coal.•Fibrous xylites rarely occur in Polish lignite deposits.•The majority of Polish lignite is suitable for fluidized bed gasification.

The main objectives of the study include the estimation of the proportion of lithotypes and macerals in lignite deposits and the determination of petrographic criteria for the assessment of quality and possible use of lignite in fluidized bed gasification process. Measures taken to meet the objectives included field research and laboratory studies. The study included results from 293 samples of low-rank coal from Polish deposits. The data used in the study included currently exploited, abandoned and prospective lignite deposits. Lignite deposits were divided into seven regions: A—Western, B—Legnica, C—Wielkopolska, D—Konin, E—Łódź–Bełchatów, F—North, and G—Radom. Polish lignite deposits include mainly humic coal, with four basic lithotypes identified: xylitic coal, xylo-detritic coal, detro-xylitic coal and detritic coal. Dominating lithotypes include detritic coal and xylo-detritic coal. Fibrous xylites, present only in minor amounts in Polish lignite, have a negative influence on coal gasification process (grinding). The highest content of fibrous xylites is observed in the Konin region. Petrographic composition of coal was determined on the basis of maceral group and the sum of gelified macerals. It has been found that the influence of macerals from the huminite group on fluidized bed gasification is significant. The average content of this component in tested coal is 89%. Meanwhile, average content of macerals from the liptinite group in Polish coal is 8%, while for macerals from the inertinite group it is 3%. The content of mineral matter in lignite deposits is a highly variable parameter. According to the proposed scheme, in order to be suitable for fluidized bed gasification, the maceral composition of the coal should contain at least 80% huminite, while the sum of the macerals from the inertinite group and mineral matter should be less than 20%. Taking into account the whole region, it should be noted that petrographic composition of lignite in almost all of the deposits allows its use in the process of fluidized bed gasification. However, limited usefulness in this process can be observed in some of the deposits containing bituminiferous lignite.