Evaluation of high-temperature pyrolysis and CO₂ gasification performance of bituminous coal in an entrained flow gasifier

Highlights

• This is the first-ever gasification study for Bangladeshi Barapukurian coal using an entrained flow gasifier.

• A complete release of volatile matter and full carbon conversion occur at a temperature of 1200°C and 1400°C respectively.

• Based on the CO/H₂ ratio, syngas conditioning is required while using a temperature of 1200°C or above.

• The key mineral detected from the XRD study of gasified char/ash was β-quartz.

Abstract

This study used an entrained flow gasifier to assess the pyrolysis and gasification performance of Bangladeshi Barapukurian bituminous coal. The pyrolysis and CO₂ gasification were conducted at temperatures of 1000°C–1400 °C under atmospheric pressure. The carbon conversion, syngas yield and pollutant emissions of coal using two different particle size of 90–106 and 250–300 μm have been analysed under CO₂ concentration of 10–80 vol%. Solid residue (char/ash) was analysed by using scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD) methods. Results show that the release of volatile matter increases up to the temperature of 1200 °C. However, the temperature needs to increase at 1400 °C for the complete carbon conversion. Based on the CO/H₂ ratio, syngas conditioning is required while using a temperature of 1200 °C or above, especially over the stoichiometric CO₂ concentrations. Considering the heating value of syngas for power generation, the use of CO₂ should be limited to less than 20% at a temperature of 1200–1400 °C. Particle size is important for carbon conversion but there is no significant impact on the heating value of syngas. Installation of the syngas cleaning system is required to drop the pollutants emission from the ppmv range to the ppbv range.

Introduction

Entrained flow gasification of coal is a promising choice for the power generation and the synthesis of valuable fuels and chemicals. Coal is the cheapest and most reliable fuel source dominating the world energy market by supplying 30% of global energy [1]. However, the critical challenges of coal to energy production are lower thermal efficiency but higher emissions (NOx, SOx, HCN, etc.) using conventional techniques. About 75% of current coal-fired power plants are operated under subcritical technology, with an average efficiency of 33% [2]. Besides, 41% of the global CO₂ emission is caused by coal [3]. Therefore, advanced technologies are required to maintain emission regulations besides increasing the efficiency of coal to product process as this source will continue to supply energy for many more decades especially in the developing nations. One such initiative is cleaner coal or zero-emission technology via gasification technique. An increase in the thermal efficiency of up to 12%, while a decrease in the CO₂ emission of up to 37% can be achieved using gasification technique [4]. Gasification is also an important process for chemical production from the synthesis gas that is produced.

Three main types of gasifiers have been used for the gasification of coal, which includes the fixed bed, fluidised bed and entrained flow gasifier [5]. However, the entrained flow gasifier is the most advanced one used by 70% of the commercial gasification plants [6]. The advantages of entrained flow gasifier include higher efficiency, lower residence time, uniform temperature profile and fuel flexibility [7]. Entrained flow gasifiers are operated at high temperatures between 1200 and 1600 °C, thus are capable of gasifying coal with low reactivity. Xu and Bhattacharya [8] studied the effect of residence time at temperatures between 700 °C and 1000 °C using Victorian Brown coal (VBC). It was reported that increasing residence time increased the carbon conversion and syngas yield. A residence time between 17 and 21s was required to achieve full carbon conversion, which was more than a double (6–10s) compared to that of commercial entrained flow gasifier. The gasification behaviour of VBC coal char was compared with German lignite at high temperature (>1000 °C) using CO₂ as a reactant [9]. The results showed that almost 100% conversion was achieved at a temperature of 1200 °C under 80% CO₂. A study conducted by Tremel et al. [10] showed that the carbon conversion and syngas yield are predominantly influenced by the physicochemical properties of coal and operating conditions. The study considered three different types of coal, such as lignite, bituminous and anthracite. Under identical operating conditions, lignite and bituminous released about five to six times higher syngas than that of anthracite coal. The yield of pyrolysis gases was consistent with their corresponding inherent volatile matter. The gasification study was carried out only for lignite coal and found that increasing temperature and residence time increase the carbon conversion almost linearly. Lee et al. [11] studied four different bituminous coals under low temperatures between 600 °C and 800 °C and pressure between 60 and 80 atm. It was observed that increasing temperature and operating pressure increased carbon conversion and syngas yield. However, a significant difference in carbon conversion and syngas yield among the coals were reported. A maximum carbon conversion of 54% was observed under most favourable operating conditions. Adeyemi et al. [12] studied the syngas yield at different axial locations of the gasifier using bituminous coal under entrained flow condition. Results showed that increasing temperature, pressure and equivalence ratio led to increasing the yield of CO and H₂.

As is known, low-rank coals (i.e., lignite, sub-bituminous) have higher reactivities compared to bituminous coal regardless of the gasifying agents [13]. The higher reactivity of lower-rank coals is because of the physicochemical properties, microporous structure and catalytic effect from alkali and alkaline earth materials (AAEM)[13]. However, the issues of gasification with low-rank coal include high moisture content in fuel, and low energy content [14]. These issues cause a significant energy penalty for the drying of as well as resulting in a low calorific value syngas. Furthermore, the emissions from bituminous coal and their difference between net and gross calorific value are lower than those of low-rank lignite. Therefore, the use of bituminous coal in gasification is the choice for many current gasification plants.

According to the literature review, the gasification characteristics of different coals with the same rank shows a significant variation; thus, fuel-specific data are crucial for the development of gasification-based plants. Also, the effect particle size on CO₂ gasification characteristics of bituminous coal has rarely been reported in the literature. Hence, as a first-ever study, this investigation examined the CO₂ gasification characteristics of Bangladeshi Barapukurian bituminous coal by analysing the effect of industrially relevant operating parameters on carbon conversion, combustible syngas and pollutant emission. The operating parameters include different particle size, temperatures and reactant concentrations. As a complementary study, the solid phase char and ash were also analysed to understand the morphology and mineral matter characteristics. The results of this study will enable the technical assessment for this coal for the generation of fuels and chemicals. Furthermore, the baseline data generated from this study will be useful for the modelling purpose, which is essential for scale-up. The reason for conducting CO₂ gasification is its industrial relevance. In a practical gasifier air/O₂ blown gasifier, the dominant gas introduces from the combustion zone of the gasifier to the reduction zone is CO₂ with minor steam, which comes out from the moisture of the coal [15]. Thus, to understand the gasification phenomena in the reduction zone, the study of CO₂ gasification is crucial.