Solid-gas products and reaction mechanism of pyrolysis of the sheath material of a typical flame-retardant low-voltage cable in substations during a fire

Substations are key power system components, so substation fires can have serious consequences on system operations. In particular, more research is needed on the pyrolysis mechanism of cables in substations. There is little research on the chemical reactions and the gas products produced during pyrolysis of substation cables. The current study analyzed the pyrolysis kinetics, solid-gas product characteristics and reaction mechanism of the sheath material of a typical flame-retardant low-voltage power cable (type ZRB-VV22-0.6/1.0 kV) in a substation in the absence of oxygen. Thermogravimetric (TG) experiments were used to investigate the pyrolysis of the sheath material in a nitrogen atmosphere. The experiments showed that the pyrolysis can be divided into two major stages with activation energies of 145.17 and 241.71 kJ/mol for the two stages based on the Friedman, FWO and KAS methods. A field scanning electron microscope and an X-ray energy spectrometer (EDS) were used to analyze the elemental mass fractions of the raw material before pyrolysis and the residue after pyrolysis. The carbon mass fraction decreased due to volatilization of the carbon in the sheath. Ca, Mg and Si were not volatilized but remained in the residue with significantly greater mass fractions. Cl and O were partially volatilized with somewhat higher mass fractions in the residue. The volatilization of Cl infers the existence of HCl in the gas products. The use of Py-GC/MS to quantify the volatile products showed that the products of the first pyrolysis stage (< 623 K) are mainly HCl and benzene. When the pyrolysis temperature rises up to 773 K, the products contain polycyclic aromatic hydrocarbons and benzene series. The main pyrolysis process is attributed to the pyrolysis of the PVC in the sheath material. Therefore, the main products are similar to PVC pyrolysis. The HCl yield was 0.077 g/g and the benzene yield was 0.266 g/g while the benzene series yields were relatively low.