Optimal conditions to maximize the fixed carbon content and the gas higher heating value of chrome-tanned leather shavings (CTLS) processed through fast pyrolysis

Abstract

Chromium remains a major obstacle to environmentally friendly management of the massive amounts of leather waste produced by tanneries worldwide. Pyrolysis has been considered an interesting cost-benefit solution because the remaining char fixes carbon and the gases contain energy that can be reused in the process. Nevertheless, the optimal pyrolysis conditions must be determined to maximize carbon sequestration, minimize the volume of Cr-containing char and produce gases with the highest heating value. This study investigated the fast pyrolysis of chrome-tanned leather shavings (CTLS) containing 2.84 wt% Cr (dry basis) to determine the optimal conditions for industrial scaleup. The partitioning of CTLS into char, liquid, and gas was investigated at heating rates of 41–76 °C/min, temperatures of 400–800 °C, and residence times of 15–60 min. The best pyrolysis condition was 15 min at 400 °C, which produced 659 g_char/kg_CTLS, with 38 wt% fixed carbon. This condition also resulted in 106 g_gases/kg_CTLS, with 17.2 wt% light hydrocarbons (C1-C5), and an estimated higher heating value of 7.2 MJ/kg. The resulting chars had Cr content in the trivalent state (Cr³⁺) ranging from 4.3 to 5.2 wt%, and despite the high calorific value, they should not be used as fuel without a robust flue gas post-treatment system.