Influence of pressure on the gasification kinetics of two high-temperature beech wood chars with CO2, H2O and its mixture

Christoph Schneider,Michael Zeller,Daniel Böhm,Thomas Kolb

doi:10.1016/j.fuel.2021.120523

Abstract

This paper presents experimental data and modeling approaches to describe the influence of CO2 and H2O partial pressure as well as absolute pressure on the gasification kinetics of two different beech wood chars. The chars were produced at 1400 °C (P1400) and 1600 °C (P1600) at high-heating rates and short residence times in a drop-tube reactor. The gasification experiments were conducted in a single-particle reactor with forced flow-through conditions reducing diffusional effects to a minimum. The interpretation of the experimentally determined reaction rates during gasification with CO2, H2O and its mixture is based on the char properties (graphitization, ash dispersion and morphology) presented in a previous publication.During gasification with CO2, P1600 shows higher reactivity as compared to P1400 for all CO2 partial pressures and temperatures applied. The higher reactivity of P1600 during CO2 gasification may be explained by a CaO film on the char surface catalyzing the char-CO2 gasification reaction. On the other hand, P1400 shows higher reactivity towards H2O which may be evoked by the lower graphitization degree and higher specific surface area. Reaction kinetic modeling for single atmosphere gasification was successfully carried out using a power law approach. The Langmuir-Hinshelwood model, however, only gave good results where a possible saturation of the char surface at high pressure was observed.Increasing the CO2 partial pressure during gasification in mixed CO2/H2O atmospheres leads to higher reactivity for both chars. The reaction rate rmix can be expressed by addition of the single atmosphere reaction rates in the low pressure area suggesting a separate active site mechanism. Catalytic activity of CaO increases the P1600 reactivity distinctively for lower H2O and CO2 partial pressures. For higher H2O and CO2 partial pressures, P1600 reactivity stagnates due to lower specific surface area and higher graphitization degree. Here, a common active sites mechanism can be assumed.

Highlights

The use of low-grade biogenic and fossil fuels in high-pressure entrained-flow gasification (EFG) allows for the production of highquality synthesis gas that can be converted into fuels and chemicals or used for power generation via integrated gasification combined cycle (IGCC) systems
This paper presents experimental data and modeling approaches to describe the influence of CO2 and H2O partial pressure as well as absolute pressure on the gasification kinetics of two different beech wood chars
The aim of this work is to determine the influence of pressure on the gasification kinetics for two beech wood chars that were produced under inert conditions at 1400 ◦C and 1600 ◦C at high-heating rates and short residence times in a drop-tube reactor imitating the conditions found during EFG

Summary

Introduction

The use of low-grade biogenic and fossil fuels in high-pressure entrained-flow gasification (EFG) allows for the production of highquality synthesis gas that can be converted into fuels and chemicals or used for power generation via integrated gasification combined cycle (IGCC) systems. In EFG, the fuel is converted via thermal and thermo-chemical processes i.e. drying, pyrolysis under high heating rates as well as the subsequent heteroge neous gasification reactions of the resulting char in a CO2– and H2O-rich atmosphere. Since the heterogeneous reactions are considered as the rate-limiting step for complete fuel conversion, the knowledge of the gasification kinetics is essential for the design of entrained-flow gasifiers [3]. The heterogeneous char gasification reactions with CO2 and H2O can be described by an oxygen exchange mechanism [9]. In the case of CO2 gasification, the following reaction mechanism presented in (R1)–(R3) is widely accepted [9,10]

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