Investigation of oxy-fuel combustion for methane and acid gas in a diffusion flame

Abstract

Co-combustion of methane (CH4) and acid gas (AG) is required to sustain the temperature in Claus reaction furnace. In this study, oxy-fuel combustion of methane and acid gas has been experimentally studied in a diffusion flame. Three equivalence ratios (ER = 1.0, 1.5, 2.0) and CH4-addition ratios (CH4/AG = 0.3, 0.5, 0.7) were examined and the flame was interpreted by analyzing the distributions of the temperature and species concentration along central axial. CH4-AG diffusion flame could be classified into three sections namely initial reaction, oxidation and complex reaction sections. Competitive oxidation of CH4 and H2S was noted in the first section wherein H2S was preferred and both were mainly proceeding decomposition and partial oxidation. SO2 was formed at oxidation section together with obvious presence of H2 and CO. However, H2 and CO were inclined to be sustained under fuel rich condition in the complex reaction section. Reducing ER and increasing CH4/AG contributed to higher temperature, H2S and CH4 oxidation and CO2 reactivity. Hence a growing trend for CH4 and AG to convert into H2, CO and SO2 could be witnessed. And this factor enhanced the generation of CS2 and COS in the flame inner core by interactions of CH4 and CO2 with sulfur species. COS was formed through the interactions of CO and CO2 with sulfur species. The CS2 production directly relied on reaction of CH4 with sulfur species. The concentration of COS was greater than CS2 since CS2 was probably inhibited due to the presence of H2. COS and CS2 could be consumed by further oxidation or other complex reactions.

1. Introduction

Hydrogen sulfide, a flammable and toxic gas, is widely spotted in crude natural gas, petroleum well and coal seams and is a common by-product during a number of industries such as coal or biomass gasification, oil refining, gas sweetening, biogas utilization, waste gas purification, waste water treatment and other desulfurization processes [1–4]. Generally, acid gas (AG) mainly consists of H2S and CO2 and other impurities with low concentration such as hydrocarbons, NH3 and CS2 and COS [3,4]. Meanwhile, it is well known that venting acid gas or its combustion products (mainly SO2) directly into atmosphere without any post processing is forbidden since it can cause various environmental pollution and health hazards [5]. Therefore, acid gas must be processed properly to meet the increasingly strict regulations and consideration of environment and safety.

Currently, the modified Claus process is widely used for the treatment of acid gas and the recovery of economically valuable sulfur products [1,6]. This process consists of three consecutive sections. In the first step, the oxidation of H2S to SO2 (Eq. (1)) and the Claus reaction between H2S and SO2 (Eq. (2)) to form sulfur (mainly S2) occur simultaneously at over 1273 K in the reaction furnace [7]. The second part is a further Claus reaction (Eq. (3)) occurring over catalyst bed at about 573 K in the catalytic converters, rendering S8 as the primary product [8,9]. The final section is the tail gas treatment or incinerating to meet emission standards. The reactions of the Claus process can be described as follows: H2S + 1.5O2 → SO2 + H2O (ΔHr = -520.72 kJ·mol⁻¹) (1); 2H2S + SO2 → 1.5S2 + 2H2O (ΔHr = 42.45 kJ·mol⁻¹) (2); 2H2S + SO2 → 3/8S8 + 2H2O (ΔHr = -104 kJ·mol⁻¹) (3).

The partial oxidation of H2S in the reaction furnace is deemed as the most critical and complex section for the Claus process [10–13]. It includes the generation of up to approximately 70% sulfur and adjustment of the suitable H2S/SO2 ratio for subsequent catalytic reaction [14,15]. Meanwhile, the impurities in acid gas need to be destroyed to mitigate the negative impact on the catalytic section. However, this process is hard to achieve since the CH4 co-combustion is required to maintain the furnace temperature especially when the H2S content is below 50% [16,17]. The presence of additional CH4 and pre-existing CO2 can significantly complicate the reactions in the reaction furnace and further adversely affect the Claus reaction. Not only is there competitive oxidation of H2S and CH4, but unwanted COS and CS2 from various side reactions and soot also proceed in the combustion progress, which can degrade catalyst performance and reduce the sulfur recovery efficiency once existing in the catalytic section due to the sulfate corrosion and coverage on the catalytic pores [18,19]. To understand the effects of CH4 and CO2 on partial oxidation of H2S in the reaction furnace, researchers have been conducting intensively studies in recent decades [20–24]. Li et al. [18] has studied the oxidation process of acid gas in a non-premixed flame. As the reactivity in fuel-rich condition was mainly expressed as ...