Treatment of pyrolytic wastewater and adsorption of heavy bio-oil vapor with biochar: Experimental investigation and molecular simulation

Alireza Zehi Mofrad

Australia contributes to a noticeable portion of global greenhouse gas emissions due to its heavy reliance on fossil fuels for energy generation. Nearly 74% of Australia’s net electricity was produced via coal and natural gas in 2020. Only 1% of electricity generation is associated with bioenergy. The share of bioenergy can be increased by up to 20% of total Australia’s electricity in the long run. Wheat production in Australia is around 19 − 25 million tons of wheat per year, producing 22 − 28 million tons of wheat straw. Farmers typically open-burn 85% of this amount, equivalent to 2.6 − 3.5 × 108 GJ of energy. Such energy is 5 − 7 times larger than the energy production in Bayswater coal-fired power station in NSW with a maximum capacity of 2640 MW. Furthermore, burning the straw emits nearly 28 − 36 million tons of CO2 into the atmosphere and 1.4 − 1.9 million tons of other pollutants such as CO and CH4. Figuring out a way to utilize such renewable energy would: (1) reduce the carbon emission from the grain industry, (2) avoid the waste of energy and inject renewable electricity into the national electricity grid, and (3) improve air quality by avoiding open burning of straw. Transportation, storage, and usage of wheat straw and other biomass sources are problematic due to their low bulk and energy density. Hence, pre-treatment of biomass is required. Pyrolysis is an effective way to pre-treat biomass and produce biofuels of improved properties. Typical products of biomass pyrolysis are biochar, pyrolysis oil, pyrolytic wastewater (PWW), and combustible gases such as CH4, H2, and CO. The energy density of wheat straw improves from 2 GJ/m3 to 9 GJ/m3 in biochar and 30 GJ/m3 in pyrolysis oil. Several hurdles are present in the usage of biochar and pyrolysis oil. Biochar’s low bulk density (0.13 − 0.46 kg/m3) and dusty properties hinder its transportation, storage, and utilization. Bio-oil’s high oxygen content (up to 40 wt%), low thermal stability during storage and handling, presence of water, and possessing a wide range of molecular weights and functionalities prevent its application. PWW, a stream containing acids, aldehydes, ketones, aromatics, sugars, and alcohols, also needs to be treated in a cost-effective way. Here, three innovative research tasks were performed to address these challenges. Firstly, a new process is proposed to treat PWW from biomass pyrolysis with char-assisted drying, which can be operated in two modes: (I) one-stage drying for PWW treatment; and (II) two-stage drying and condensation for both treating PWW and recovering valuable chemicals (e.g., acetic acid). The process is proved via drying the mixture of a char and a synthetic PWW solution containing acetic acid, acetol, furfural, and phenol at different temperatures (60, 80, and 105 °C) and char-to-PWW mass ratios (1, 1.5, and 2). The results, supported by density functional theory (DFT) calculations, demonstrate that under Mode I, the char captures over 96.4% of the organic compounds in the PWW at 60 °C and a char-to-PWW ratio of 2 while evaporating almost all the water. The evaporated stream can be discharged into the atmosphere without condensation or condensed into wastewater with a significantly reduced total organic carbon content. Under Mode II, the char-PWW mixture is sequentially dried at 60 and 105 °C, with the exhaust gases being separately condensed. This yields a stream of purified acetic acid solution (concentration: ~76 wt%) concentrated by a factor of ~6.2 (compared with the PWW), with a recovery rate of ~29%. Secondly, an enhanced char-based process to treat PWW from biomass pyrolysis was developed. The process was investigated by drying the char and synthetic PWW solution (char-to-PWW ratios of 0.5, 1.0, and 2.0) at 60 °C and further discharging the organics at higher temperatures (105, 125, 150, 200, 250, and 300 °C). In the char-to-PWW ratio of 2.0 and 60 °C, ~ 95.3-99.6 % of the organic compounds were retained in the char, while ~94.6 % of water was removed. Single-step discharging recovery and concentration data depicted that at 125 °C and a char-to-PWW ratio of 2.0, the highest acetic acid concentration achieved is 94.0 wt%. Hence at a char-to-PWW ratio of 2.0, an integrated three-stage method was developed, resulting in two main streams: (I) a water vapor stream with reduced VOC content that can safely be discharged to the atmosphere generated at 60 °C, and (II) a high concentration steam of acetic acid with the concentration of ~97 wt% acetic acid and recovery rate of ~ 75% attained at 125 °C. Char can be recovered and utilized in the process for the second time to provide a self-sustained process. The recovery rate of acetol/furfural/phenol at 300 °C was 26.5/68.3/63.7 % at a char-to-PWW ratio of 2.0 and 24.2/83.7/74.8 at a char-to-PWW ratio of 0.5, respectively. Neither of the temperature gaps could successfully separate the remaining organics, and only a lumped concentrated stream can be achieved at 300 °C, which requires further separations. Finally, char application as a novel approach to separate heavy molecules of pyrolytic liquid was studied. Guaiacol was used as a typical model compound for lignin-derived bio-oil. Hence adsorption of guaiacol in the gas phase on char was studied. Two different bed temperatures (250 °C and 350 °C) were chosen for the experiments. Since some sources of char might be rich in iron, the effect of iron on guaiacol adsorption on char was also investigated (5.28% Fe w/w). Increasing the bed temperature led to a 43% and 21% decrease in the adsorption capacity of raw char and Fe-loaded char samples, respectively. Although the addition of iron had a negligible effect on guaiacol adsorption capacities at 250°C (only a 10% increase), it caused a 52% increase in adsorption capacities at 350 °C. Thus, the maximum adsorption capacity was 55.2 mg/g char for Fe-loaded-char samples at 250 °C. The effect of iron on guaiacol adsorption was also explained using density functional theory (DFT) simulations. Pseudo-first-order kinetic is very well suited to the experimental data of guaiacol adsorption on char samples. Keywords: Biomass, Pyrolysis, Pyrolytic wastewater, Acetic acid, Char

Treatment of pyrolytic wastewater and adsorption of heavy bio-oil vapor with biochar: Experimental investigation and molecular simulation

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