Εξαερίωση Ελληνικών Λιγνιτών Και Εξανθρακωμάτων Με Διοξείδιο Του Άνθρακα Και Υδρογόνο

Abstract

Gasification of Greek lignites and chars with carton dioxide and hydrogen was investigated by two parallel and complementary systems: non-isothermal thermogravimetry coupled with gas chromatography and infra -red analyser (TGA-GC-NDIR) and isothermal gasification in turbular reactor. The main issues studied within this work were: (a) non-isothermal C0 2 gasification of lignites and chars (b) catalytic and non-catalytic isothermal lignite -C0 2 gasification (c) two stage gasification (lignite hydropyrolysis and C0 2 gasification of the resulting chars) (d) the effect of the inherent inorganic matter on the hydropyrolysishydrogasification of lignites and on the C0 2 gasification of lignites and charsNon-isothermal investigations were mainly focused on the effect of operating conditions on C0 2 gasification weight loss, rates and yields. Parameters investigated included particle size (106 -1000 pm), heating rates (5 -100°C/min), C0 2 partial pressures (up to 1 atm) and the pre-charring conditions.The simultaneous determination of lignite weight loss and gaseous product distribution during non-isothermal TGA analysis employed here, provided a rational and detailed examination of the phenomena involved and permitted optimum selection of temperature regions for further investigations. Lignite behaviour during non-isothermal C0 2 gasification is characterised by the existence of two temperature regions that control conversions and yields, which however are not clearly distinguished. The first stage (pyrolysis, under 600 -650°C) is accompanied by volatiles evolution, fast weight decrease and formation of carbon oxides as well as light and heavy (mainly tars) hydrocarbons. Conversions in this region amount to -40% w/w of lignite on dry ash free basis (daf). During the second stage (C0 2 gasification) the heterogeneous Boudouard reaction (char -C0 2 ) dominates, accompanied by fast weight decrease and CO production. Traces of hydrocarbons are also produced in this stage coming from the remaining volatiles strongly bounded in the lignite structure. In this region lignite conversion reaches up to ~56% w/w daf. Lignite is found quite reactive with C0 2 and, thus, almost complete conversion is achieved by the end of the non-isothermal test. For the granulomeres studied (106 -1000 μιτι), the gasification rate and product yields are independent of particle size, thus lignite -C0 2 gasification does not suffer from mass transfer limitations. Although high heating rates result in increased weight loss and CO formation rates, total weight loss and CO yield decrease due to the reduction of solid residence time (i.e. reaction time). C0 2 partial pressure is a significant C0 2 gasification parameter over 600 -650°C (gasification region), where weight loss and CO formation rates increase significantly with P C o 2 -Thermal pre-treatment of lignite affects both C0 2 gasification regions. Increased pre-charring extend reduces weight loss and CO formation rates in the pyrolysis stage. In contrast, weight loss and CO formation rates are in...