Study of Characteristics and Heavy Metals Migration During Co-Combustion of Coal and Sludge

ABSTRACT

Sludge is a complex mixture involving toxic substances, such as pathogenic microorganisms and heavy metals. Without effective treatment, sludge will pose a serious threat to the ecosystem and human beings. Feeding sludge into combustion boilers as auxiliary fuel and incinerating it as a low quality fuel together with coal high calorific value fuel can reduce the consumption of coal and realize the resourceful use of sludge. It can not only reduce the construction and operation costs of sludge disposal, but also realize the social and ecological benefits of coal-fired power plants. The combustion characteristics and the migration of heavy metals during the co-combustion of coal and industrial sludge were investigated in the paper. The experiments were performed at temperatures ranging from 20°C to 800°C at the constant heating rate of 15°C/min. It was found that the data of combustion index decrease with the increasing mixing ratio. The mixed proportion should not exceed 10%. In the ecological risk assessment of heavy metals, the risk index value of residues gradually increased with the increasing mixing ratio. To keep the residues at a low risk level, the mixing ratio should not exceed 5%. The results of FactSage simulation illustrated that the contents of PbCl₂ (g) and ZnCl₂ (g) increased with the increase of industrial sludge addition, showing that the combination effect of Cl with heavy metals was stronger than that of heavy metals captured by minerals.

KEYWORDS:

• Coal
• industrial sludge
• co-combustion
• heavy metals
• migration

Nomenclature

$T_i$	=	ignition temperature
$T_b$	=	burnout temperature
$T_{\mathit{max}}$	=	the temperature at the maximum weight loss rate
${\left(\mathit{dw}/dt\right)}_{\mathit{max}}$	=	the maximum weight loss rate
${\left(\mathit{dw}/dt\right)}_{\mathit{mean}}$	=	the average rate of weight loss
$\mathit{C}$	=	combustibility index
$\mathit{G}$	=	combustion stability index
$\mathit{S}$	=	combustion index
D	=	enrichment factor
$A_n$	=	the heavy metal content in sample after combustion
$m_a$	=	the mass of sample after combustion
$\mathit{A}$	=	the heavy metal content in sample before combustion
$\mathit{m}$	=	the mass of sample before combustion
RI	=	potential ecological risk index
$C^i$	=	the concentration of the element
$C_n^i$	=	a reference value of the element
$C_f^i$	=	the single element pollution factor
$T_r^i$	=	the toxic factor of an individual heavy metal
$E_r^i$	=	the potential ecological risk index of an individual heavy metal

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.

Consent to participate

All authors agreed with the content and that all gave explicit consent to submit and that they obtained consent from the responsible authorities at the institute/organization where the work has been carried out, before the work is submitted.

Consent to publish

All authors agreed on the content of the article and expressly consented to its publication.

Authors contributions

All authors contributed to the study conception and design. Data Curation, Investigation, Methodology, Writing – Original Draft were written by Chenyu Zhan. Formal Analysis, Funding Acquisition, Writing – Review & Editing were performed by Shuyuan Li. Formal Analysis, Funding Acquisition, Writing – Review & Editing were performed by Changtao Yue. Formal Analysis, Funding Acquisition, Writing – Review & Editing were written by Yue Ma. All authors read and approved the final manuscript.

Additional information

Funding

This research was funded by the Basic Research center for Energy Interdisciplinary, College of Science, China University of Petroleum, Beijing, 102249, China and the Science Foundation of China University of Petroleum, Beijing (Grant number 2462022YXZZ001), National key research and development program (2019YFC1906300).