Migration Behaviors of As, Se and Pb in Ultra-Low-Emission Coal-Fired Units and Effect of Co-Firing Sewage Sludge in CFB Boilers

Xuan Liu,Yang Teng,Kai Zhang

doi:10.3390/en15041544

Abstract

The migration behaviors of As, Se and Pb in ultra-low-emission coal-fired units were investigated and the effect of co-firing of sewage sludge and coal was explored in circulating fluidized-bed (CFB) boiler units. Samples of feed fuel including coal and sewage sludge, fly ash, bottom slag and desulfurization slurry were collected from five CFB units with a capacity between 150 MW and 350 MW and two pulverized coal boiler (PC) units with a capacity of 350 MW and 600 MW. The majority of As, Se and Pb in coal is released during coal combustion and enriched in fly ash. As, Se and Pb in fly ash and bottom slag are associated with inorganic matter, of which As, Se and Pb associated with silicates and aluminosilicates account for more than 60%. In the wet flue gas desulfurization (WFGD) unit, more than half of the As, Se and Pb migrates to the solid fraction and the concentrations of As, Se and Pb in fine solid particles are much higher than those in coarse solid particles and the liquid fraction. The proportions of exchangeable As, Se and Pb or associated with organic matter in gypsum are higher than those of fly ash and bottom slag. According to the two studied CFB units with blending ratios of sewage sludge below 10%, co-firing with sewage sludge has little effect on the migration and transformation characteristics of As, Se and Pb. Under a long-term acidic leaching environment, Pb in combustion by-products is in the range of low risk, while As and Se are in the range of medium risk.

Highlights

Arsenic (As), selenium (Se) and lead (Pb) are extremely toxic trace elements for human health due to their mobility, bioaccumulation and persistent toxicity [1,2]
Samples of feed fuel, fly ash, bottom slag and desulfurization slurry were collected from five circulating fluidized-bed (CFB) units with a capacity between 150 MW and 350 MW, two of which were co-firing with sewage sludge
Different from As and Se, the concentration of Pb in coal from CFB units and pulverized coal boiler (PC) units showed different regularity, i.e., the concentration of Pb in coal from CFB units ranged from 16.38 to 21.54 μg/g, which is higher than the average Pb concentration in Chinese coal (13.00 μg/g), while that in PC units was in the range of 8.01–8.22 μg/g, which is below the average Pb concentration in Chinese coal

Summary

Introduction

Arsenic (As), selenium (Se) and lead (Pb) are extremely toxic trace elements for human health due to their mobility, bioaccumulation and persistent toxicity [1,2]. The Environmental Protection Agency of the United States formulated the first national Clean Air Act standards to reduce toxic trace element emissions from coal-fired power units [4]. Co-firing coal with sewage sludge, biomass and other non-fossil energy in coal-fired power units is currently considered one of the most suitable processes for the resource utilization of waste, which could reduce CO2 net emission from coal combustion [9]. It is disappointing that only a few studies have focused on the effect of co-firing with sewage sludge on the migration behaviors of As, Se and Pb in coal-fired power units. It is of great significance to study the migration behaviors and environmental risks of As, Se and Pb in combustion by-products and the effect of co-firing sewage sludge

Objectives

Methods

Results

Conclusion