Gasification characteristics of coal in an entrained flow gasifier for syngas production

Abstract

Each country is trying to develop clean coal technology (CCT) since it became more significant to convert coal into clean energy and higher added value product with low emission of environmental pollu-tants. Coal gasification technology is one of the most important platform for CCT utilization. It provides the opportunity of adding value to coal by converting it into higher value-added materials (e.g., electricity, chem-icals, and liquid fuel). Entrained-flow gasification, which is one of the representative gasification technologies, has rapid heating, high-temperature, and high-pressure operating characteristics. Experimental data similar to actual operating conditions are needed to design, construct, and operate entrained flow gasifiers. The present study examines the characteristics of the coal water mixture (CWM) used as an entrained-flow gasifier fuel, the devolatilization characteristics of coal under a rapid heating condition, the char gasification characteris-tics under high-temperature and high-pressure conditions, and the operation characteristics of synthetic oil production using syngas produced from 10.0 ton/day pilot entrained flow gasifier. First, the CWM used as the fuel in a wet-type entrained-flow gasifier behaves more like liquid com-pared to coal. There are no problems with spontaneous ignition and dust being generated, while there are ad-vantages of easy transportation and being able to be injected at high pressure. A good CWM should have a high coal content and excellent fluidity and stability for coal gasification fuel. These CWM characteristics have a direct effect on the improvement of the coal gasifier performance. In the present study, The CWM thickeners such as xanthan gum, modified acrylic polymer, and carbomer are compared in terms of stability and fluidity. The xanthan gum and the modified acrylic polymer added to improve the stability of the CWM in existing processes have a problem of reduced fluidity, which is caused by the increased viscosity of the CWM. However, carbomers, which are pH responsive thickener, show the highest viscosity at pH 7. Moreo-ver, they have the advantage of allowing viscosity control back to the previous level by changing the pH. Compared to the existing xanthan gum (27.3 wt.%) and the modified acrylic polymer (22.72 wt.%), car-bomer, which is a pH responsive thickener, has an excellent characteristic of allowing viscosity control by using a small amount(22-27 wt.%). In the storage stage of the CWM, the storage stability can be maximized by pH change (pH 7？9) through the addition of cabomer. In the usage stage, the state of the carbomer can be changed to return the viscosity back to its original state by adding a basic material (e.g., Potassium hy-droxide) that would change pH value more than 13. Therefore, pH change would allow satisfactory stability and fluidity of CWM. This technology makes it possible to use it to supply fuel to CWM boiler equipment or small-scale gasification plants, where the establishment of the CWM production equipment may be difficult. Coal gasification reaction comprises two major step

devolatilization and char-gasification. During devolatilization step, where coal undergoes a very rapid pyrolysis under high temperature, and gaseous sub-stances with $CO_2$, CO, $H_2O$, $H_2$, and $CH_4$ as the main components, a liquid substance called tar, and a residue char can be produced depending on the heating condition. The devolatilization reaction itself does not have a major effect on the gas atmosphere, but devolatilization products are transformed into more stable products through continued devolatilization and reaction with various gases. The devolatilization in an entrained-flow gasifier is a fast devolatilization. Hence, significant differences can be found in the product distribution or degree of volatilization compared to slow devolatilization. The devolatilization characteristics of the high-volatility coal under a fast pyrolysis condition were identified using a wire mesh reactor heating rate of $1,000 \circ C/s$ for the experimentation on the gas component distribution and char, tar, and gas production characteristics based on the effects of the peak temperature. The experimental results showed that devolati-lization was terminated at approximately $1,000 \circ C$ or higher. Furthermore, the amount of tar produced was the highest at approximately $800 \circ C$, but tended to slightly decrease as the temperature increased. $CH_4$ was observed at approximately $600\circ C$ or higher, whereas $H_2$ was produced at $1,000 \circ C$ or higher. With respect to gas production, the amount of $H_2$, CO, $CH_4$, and $C_nH_m$ increased with the increase in temperature. The devolatilization was completed within 1s at $1,000 \circ C$ in a WMR. Therefore devolatilization was expected to be completed within 1 s inside the reactor because the operating temperature of the entrained-flow reactor was approximately $1,500 \circ C$ or higher. The second step in the coal gasification reaction involves the reactions between completed devolatilized char and gasifying agents such as oxygen, steam, and carbon dioxide. A drop tube reactor was used at $1,200 \circ c$ and under nitrogen atmosphere to produce uniformly sized char via fast devolatilization. A pressurized drop tube reactor (PDTR) designed to allow the operation and continuous sampling at a peak temperature of $1,400\circ C$ and a peak pressure of 15 bar was used in the char gasification experiment to obtain the syngas composition and the carbon conversion rate according to the partial pressure of the oxidants, reactor temperature, and reactor pressure. Based on these results, the random pore model, which is one of the gas-solid reaction models, was used on the char gasification reaction of coal to obtain the reaction order (n), activation energy (E(kJ/mol)), and frequency factor ($A_0$) needed for the gasification reaction modeling. Through the experiment on char gasification using the PDTR, the present study obtained data by various temperatures, partial pressures, and types of oxidants that can be used to analyze the gasification reaction that simultaneously occurred within a very short period inside the gasifier. These calculated values can be used to predict the experimental results and the gasification reactivity based on the coal used. Moreover, the findings can be used as basic data for designing gasifiers by combining with numerical modeling. Finally, based on the devolatilization and gasification characteristics of coal in an entrained-flow gasifier, 10.0 ton/day pilot entrained flow gasifier was designed and operated to produce syngas. Syngas can be made into electricity, synthetic petroleum, synthetic natural gas, and chemical fuel (methanol, ammonia) through a conversion process. The increasing global demand for fossil fuels necessitates the search of cheaper alternatives. To address this issue, we describe the design and operation of an indirect coal liquefaction plant with integrated coal-water slurry manufacturing, entrained flow gasifier, synthesis gas purification ($Rectisol^{circledR}$), and Fisher-Tropsch processes to produce liquid fuels for transportation vehicles. The aforementioned plant contained an entrained flow gasifier (10 ton/day of coal test rig) operated using oxygen as a gasifying agent (21 bar, $1,100\circ c$) and could stably produce synthesis gas (37.8 vol.% $H_2$, 36.4 vol.% CO, 23 vol.% $CO_2$, < 1.0 vol.% of $CH_4$) of ca. 600 $Nm^3/h$. Due to the importance of impurities in synthetic liquid fuel production, more than 99 % of acidic impurities ($H_2S$) contained in synthesis gas were removed by the $Rectisol^{\circledR}$ process employing chilled methanol. An iron-based catalyst allowed liquid fuels containing wax, light/heavy oil, and alcohol fractions to be obtained by the Fisher-Tropsch process at a rate of 6 barrel per day (BPD), with detailed analysis confirming their compliance with various quality standards and thus their suitability for use as automobile diesel after distillation.