Flue gas-to-ash desulfurization of combustion of textile dyeing sludge: Its dependency on temperature, lignocellulosic residue, and CaO
Graphical abstract
Introduction
Textile dyeing sludge is the inevitable by-product of printing and dyeing wastewater treatment plants and contains higher quantities of heavy metals, hazardous organic components, and recalcitrant organics than does sewage sludge [1]. About 20 million tons of high moisture TDS are discharged annually from the treatment plants of printing and dyeing wastewater in textile industries in China [2]. Given the growing worldwide concern about environmental pollution, the thermochemical conversions such as incineration, pyrolysis, and gasification are dominating landfills as the sustainable disposal method of TDS with the goals of reducing waste stream volume, environmental impacts, and economic cost [3], [4]. In addition to the degradation of harmful organics, the fixation of heavy metals, and the avoidance of water and soil contamination, the thermochemical conversion enables cleaner energy and by-product recoveries [5]. (Co-)combustions have been widely used to treat various kinds of sludge such as sewage sludge [6], pickling sludge [7], and textile dyeing sludge [8], with the multiple purposes of waste stream reduction, detoxification, and energy recovery [9].
One of the major environmental issues caused by combustions is the emission of air pollutants, in particular, SO2, which leads to acid rains and the corrosion of the combustion equipment. The S content of TDS was reported as 7.9 and 10.4% by Liu et al. [8] and Peng et al. [10], higher than what the S-rich coal contains. In other words, the control on SO2 emission is essential to the design and operation of boilers and desulfurization devices of fuel gas and is affected by the composition of fuels, the combustion temperature, and the residence time [7]. Better knowledge of the combustion-induced transformation and migration of S and interactions between S and the major elements such as chlorine (Cl), phosphorous (P), heavy metals, and alkali metals bear important implications for controls on SO2 emission. For example, hydrogen chloride (HCl) is more easily combined with potassium (K) than SO2 to form KCl, thus inhibiting the formation of potassium sulfate (K2SO4) [11]. Guo et al. [12] reported that the alkali and alkali-earth minerals in wood pellets captured S to form sulfate and decreased SO2 emission from the co-combustion of anthracite coal and wood pellets. The presence of silicon (Si) and P was reported to react with CaO preferentially, to reduce the desulfurization ability of CaO, and to promote the volatilization of SO2 [8]. Large knowledge gaps still exist about S migration and transformation patterns in the combustion by-products (flue gas, bottom ash, and fly ash), in particular, when TDS is co-combusted with lignocellulosic biomass or desulfurization agent to control SO2 emission.
The recent focus to reduce SO2 emission has been on co-combustion with lignocellulosic residues or a desulfurization agent (CaO). Co-combustion with biomass may offset the drawbacks of TDS such as its high ash and moisture contents, low heating value, and pollutant emissions [13]. For example, Wang et al. [14] found that the addition of wheat straw improved the combustion characteristic index of TDS and its residue due to the catalytic effect of alkali metal oxides of wheat straw ash. Spent mushroom substrate (SMS) is an abundant source of lignocellulosic residues in China generated from mushroom cultivation industries and was reported to promote the decomposition of TDS [15]. Co-combustion performances of TDS and SMS and their kinetic behaviors and qualitative analysis of emissions such as NOx and SO2 were previously posed using thermogravimetry and mass spectrometry (TG-MS) [15]. SO2 emissions from other (co-)combustions were also characterized using different methods such as TG-MS and flue gas analyzer. When CaO was used to improve the sludge dewatering performance, Ca was found to be retained in sludge [16]. However, the impacts of CaO and SMS on S distribution in flue gas and bottom and fly ashes still remain poorly understood.
Therefore, the objectives of this study were three-fold: to (1) quantify the S transformation characteristics of the (co-)combustions of TDS with SMS versus CaO; (2) explore the effects of temperature and blend ratio on desulfurization, typical crystalline phase, and surface morphology with or without the addition of SMS and CaO; and (3) simulate S distributions in gas and solid phases via FactSage 7.1.
Section snippets
Sample preparation
TDS samples were collected from a textile dyeing wastewater plant that used ferric chloride (FeCl3) as a sludge conditioner. The conditioned TDS samples were filtered using a plate and frame press. After being naturally dried, they were crushed using a laboratory grinder and passed through a 200-mesh sieve to obtain powder samples of less than 74 μm. The obtained powder samples were dried in an oven at 105 °C for 24 h. The dried samples were put in the sample bag and stored in the desiccator
Temperature dependency of S distribution
The S content of TDS (8.26%) was higher than that of sewage sludge, lignite, and biomass materials [5]. High SO2 emission occurred from the TDS mono-combustion depending on the temperature dependency of (in)organic S species in TDS. Temperature was a major driver of the distribution of S to the combustion by-products of flue gas, bottom ash, and fly ash, as can be seen in Fig. 2.
With the rising temperature from 600 to 1000 °C, the conversion of S to the flue gas increased from 63.1 to 92.9% but
Conclusions
The S distribution patterns of the flue gas and the bottom ash during the (co-)combustions of TDS and its blend with SMS or CaO were quantified both experimentally and with simulations. The temperature was one of the most important factors on SO2 emission. The decomposition of inorganic sulfates such as CaSO4 at the high temperature accelerated the release of S into the flue gas. The increased temperature and blend ratio of SMS reduced SO2 emission, while the S retention in the ash changed
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was financially supported by the National Natural Science Foundation of China (No. 51978175), the Science and Technology Planning Project of Yunfu, Guangdong Province, China (No. 2020040401), the Research Fund Program of Guangdong Key Laboratory of Radioactive and Rare Resource Utilization (No. 2019-LRRRU04; 2018B030322009) and the Science and Technology Planning Project of Guangdong Province, China (No. 2019B020208017).
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