Catalytic pyrolysis of paper mill sludge over self-sourced CaO/biochar catalyst under different temperatures

Abstract

In order to realize the harmless treatment and resource utilization of paper mill sludge, the catalytic pyrolysis of paper mill sludge over a self-sourced CaO/biochar catalyst was studied in a two-stage fixed-bed pyrolysis reactor under different temperatures (500, 600, 700 °C) and catalyst/sludge mass ratios (0, 0.5, 1, 2). The experimental results showed that the catalyst addition and higher temperature could promote the decomposition of bio-oil vapors into small molecular gases such as H₂, CH₄, and CO, thus improving non-condensable gas yield and decreasing bio-oil yield. The optimal condition for gas production was when the proportion of catalyst to sludge was 1 at 700 °C. Under the condition, the gas yield was 176.80 ml/g sludge, and the H₂ concentration and yield were 31.97 % and 56.52 ml/g sludge, respectively. CaO/biochar catalyst could reduce the contents of oxygen-containing compounds in bio-oil including acids, esters, and ketones and improve the yield of hydrocarbons, indicating the improved quality of bio-oil. The N-heterocyclic compounds were the main nitrogen-containing species in bio-oil, mainly coming from the decomposition of protein at high temperature. CaO and biochar both played important roles during the catalytic pyrolysis process. In addition, the physical and chemical structures of the catalyst were greatly changed after the reaction. Carbonation of CaO with the CO₂ in the pyrolysis volatiles and the coke formation on the active sites of the catalyst would possibly happen during the catalytic pyrolysis process, resulting in reducing the specific surface area and pore volume of the catalyst. These research results prove that the self-sourced CaO/biochar catalyst plays an important role in the catalytic reaction and promotes the quality improvement of the pyrolysis bio-oil and gas.