Thermodynamic analysis of waste heat recovery of aluminum dross in electrolytic aluminum industry

ABSTRACT

In order to recover the waste heat of aluminum dross and carbon resource in carbon residue, the waste heat recovery process of aluminum dross by coupling of physical and chemical method was proposed. In this process, waste heat in the high-temperature range was recovered by carbon residue gasification reaction and waste heat in the low-temperature range was recovered by physical method (PM). Enthalpy and exergy analysis of PM and physical–chemical coupling method (PCCM) processing the aluminum dross was performed by thermodynamic compass method. Sensible heat of aluminum dross can be recovered by PM, but exergy efficiency is 20.4%. And the exergy loss (EXL) is 429 MJ. A lot of hot water is produced by this method, but the use of hot water is limited. The exergy efficiency for PCCM is 38.4% in System 2(a) and 51.2% in System 2(b), respectively. Compared with the PM, the EXL of PCCM is lower, and the exergy efficiency is higher. Each ton of aluminum dross can produce 47.11 m³ CO. in System 2(a) and Each ton of aluminum dross can produce 56.88 m³ CO and H₂ in System 2(b). The carbon resources in carbon residue are effectively utilized. Comparing with the energy level of chemical reaction, the influence of specific temperature for exergy efficiency is more significant during the physical–chemical coupling of waste heat recovery process.

KEYWORDS:

• Waste heat recovery
• energy conversion
• aluminum dross
• carbon residue
• thermodynamic analysis
• exergy analysis

Nomenclature

ΔHj	=	The enthalpy changes of in-flow and out-flow, MJ
Δεj	=	The exergy changes of in-flow and out-flow, MJ
T	=	Temperature of aluminum dross, K
T0	=	Environmental temperature, K
ΔG	=	Gibbs free energy change, kJ
ηh	=	Heat efficiency, %
ηe	=	Exergy efficiency, %
Δε	=	Exergy entered in system or equipment, MJ
Δε’	=	Exergy recovered in system or equipment, MJ
ΔH’	=	The enthalpy of energy recovered in system or equipment, MJ
ΔH	=	The enthalpy of energy entered in system or equipment, MJ.

Abbreviations

PAC	=	Polyaluminum chloride
ST	=	Specific temperature
PM	=	Physical method
PCCM	=	Physical-chemical coupling method
EXL	=	Exergy loss

Additional information

Funding

This work was supported by the The Major State Research Development Program of China [2017YFB0603603];the National Natural Science Foundation of China [51604077,51704071];the Fundamental Research Funds for the Central Universities [N170204016,N172504019].