Simultaneous removal of sulfur dioxide and nitrogen oxide from flue gas by phosphorus sludge: The performance and absorption mechanism

Abstract

Developing low-cost and green simultaneous desulfurization and denitrification technologies is of great significance for sulfur dioxide (SO2) and nitrogen oxide (NOx) emission control at low temperatures, especially for small and medium-sized coal-fired boilers and furnaces. Herein, phosphorus sludge, an industrial waste from the production process of yellow phosphorus, has been developed to simultaneously eliminate SO2 and NOx from coal-fired flue gas. The key factors affecting the experimental results indicate that desulfurization and denitrification efficiency of over 95% can be achieved at a low temperature of 55 °C. Further, the absorption mechanism was investigated by characterizing the solid and liquid phases of the phosphorus sludge during the absorption process. The efficient removal of SO2 is attributed to the abundance of iron (Fe3+) and manganese (Mn2+) in the absorbent. SO2 can be rapidly catalyzed and converted to SO4^2- by them. The key to NOx removal is the oxidation of NO toward water-soluble high-valent nitrogen oxides by oxidizing reactive substances induced via yellow phosphorus, which are then absorbed by water and converted to NO3-. Meanwhile, yellow phosphorus is oxidized to phosphoric acid (H3PO4). The spent absorption slurry can be reused through wet process phosphoric acid production, as it contains sulfuric acid (H2SO4), nitric acid (HNO3), and H3PO4. Accordingly, this is a technology with broad application prospects.

1. Introduction

In recent decades, fossil energy such as coal, has injected inexhaustible impetus into China's industrial development, but sulfur dioxide (SO2) and nitrogen oxides (NOx) have inevitably discharged into the atmosphere, which poses a great threat to the ecosystem and human health [1,2]. Since the implementation of ultra-low emissions of flue gas in China, SO2 and NOx emissions have decreased significantly [3,4]. Nevertheless, the efficient and low-cost removal of these pollutants from the flue gas of small- and medium-sized coal-fired boilers is still a difficulty.

At present, there are hundreds of control technologies for SO2 and NOx removal. For SO2 control, wet flue gas desulfurization techniques still dominate the market, among which calcium-based processes are the most widely used. However, the calcium method will solve the problem of waste gypsum and acid waste water. It would not only take up a lot of ground but also pollute the environment and groundwater resources [5e7]. In terms of denitrification, catalytic reduction of NOx was the most widely used denitrification method. The selective catalytic reduction (SCR) purification method uses ammonia (NH3) as a reducing agent to reduce NOx to nitrogen (N2), which is very popular in thermal power plants [8,9]. Unfortunately, this technology is difficult to adapt to the flue gas of small and medium-sized coal-fired boilers because the low-temperature SCR catalysts are expensive [10e12]. Additionally, the traditional flue gas desulphurization and denitrification technology is difficult to widely introduce in medium- and small-sized enterprises due to the high investment costs and operating expenses. Based on the above, the simultaneous desulphurization and denitrification technology has received a lot of attention in recent years, owing to its characteristics of high efficiency and low cost.