(536h) Removal of By-Product VOCs from Biomass Gasification Catalyzed By Ni/HZSM-5 : Optimization of Process Conditions

Background, Aims and Scope. Biomass cracking and gasification technology, as a kind of thermochemical conversion technology, is widely used due to its advantages of strong raw material adaptability and high conversion efficiency. However, the organic pollutants produced in the biomass gasification process not only seriously affect the conversion efficiency, but also cause a certain degree of pollution to the atmospheric environment and endanger human health. These organic pollutants are condensable macromolecular gaseous organics produced during a series of complex gasification processes of biomass. Therefore, it can be considered that the organic pollutants produced in the biomass gasification process belong to the category of VOCs (Volatile Organic Compounds). At present, there are lots of VOCs treatment technologies, including adsorption, absorption, membrane separation, incineration, biodegradation, and catalysis. Many of these methods are prone to secondary pollution, poor stability and low removal efficiency during the treatment process, which are not conducive to the efficient elimination of VOCs. Catalytic method has high removal efficiency, does not cause secondary pollution, and is considered to be the most efficient removal method. For the VOCs produced by biomass gasification, catalytic cracking can be selected to remove them. In the method, a catalyst is added in the biomass gasification process, and the carbon-carbon bonds in the generated VOCs molecules are broken under the action of the catalyst, which reduces the activation energy of the cracking. This enables VOCs to be reformed and converted into small molecular gases at lower temperatures. This not only improves the removal efficiency of VOCs, but also further increases the yield of cracking gas. Undoubtedly, the core of catalytic cracking technology is the development of catalysts. Among various catalysts, nickel-based catalysts are the most widely used due to their low price and high activity. Active metal nickel has a certain activation effect on C-C bonds and C-H bonds in VOCs molecules, which can effectively promote the decomposition of organic macromolecules and effectively control the gas composition. The catalyst prepared with molecular sieve as carrier has large specific surface area, excellent thermal stability and special pore structure, so it has better catalytic activity for the catalytic cracking of VOCs. Previous studies have shown that HZSM-5 zeolite was considered as an ideal catalyst support due to its good surface acidity, hydrothermal stability and selectivity. Therefore, HZSM-5 molecular sieve is supported by active metal Ni, hoping to prepare a catalyst for the efficient removal of VOCs.

Methods. Molecular sieves were pretreated before catalyst preparation. After grinding the solid HZSM-5 molecular sieve into 80 mesh powder, it was calcined at 600 °C for 3 h, in order to improve the mechanical strength of the molecular sieve and remove impurities. The preparation of the nickel-based catalyst was based on HZSM-5 molecular sieve as the carrier, supported metal nickel as the active component, and traditional impregnation, ultrasonic assistance, ion exchange and rotary evaporation were used as auxiliary methods. The solids obtained by different auxiliary methods were dried in an oven at 105 °C for 12 h, ground into powder and calcined in a muffle furnace at 550 °C for 3 h to obtain the prepared Ni/HZSM-5 catalyst.

The performance test of the catalyst was carried out in a two-stage fixed bed reactor. The model compound was injected into the first stage reactor through a peristaltic pump, the catalyst was placed in the second stage, and the model compound was catalytically pyrolyzed under the action of the catalyst. In the whole process, high-purity N2 was used as the carrier gas, the gas product was collected by the air bag, and the liquid product was recovered after passing through the cooling system for subsequent qualitative and quantitative analysis.

Results and Discussion. In this study, the prepared nickel-based supported molecular sieve catalyst was used to study the catalytic cracking of VOCs model compounds. The influence of different catalyst preparation processes and cracking conditions was investigated, and the catalytic cracking conversion paths of various VOCs model compounds were explored. The conclusions are as follows: 1) The optimal preparation method of self-made nickel-based supported catalyst was ultrasonic-assisted excess impregnation method, and the optimal loading amount of Ni was 8 wt.%. The preparation method was convenient and easy to operate. Ultrasonic assistance not only promoted the refinement of the particle size of the active components and improved the dispersion of the active components, but also effectively controlled the loading of active metals through impregnation. 2) Through the study of cracking temperature, it was found that with the increase of temperature, the conversion rate of model substances and the yield of cracking gas increased significantly, and 800 °C was selected as the cracking temperature within the temperature selection range. 3) Catalytic cracking of various types of VOCs model compounds was carried out at 800 °C using Ni/HZMS-5 catalyst, and it was found that the model compounds could be converted from macromolecular organic compounds to small molecular gases under catalysis, and the conversion rate could mostly reach more than 90%. Model compounds such as toluene, phenol, and acetic acid can generate more hydrogen by cracking. The selectivity of H₂ and CH₄ from toluene cracking reached 93%, and cyclohexane reached more than 98%.

Conclusion. This study selected toluene and phenol as the representative of VOCs monocyclic aromatic hydrocarbon compounds, furan as the representative of heterocyclic compounds, and cyclohexane and acetic acid as the model compounds of aliphatic hydrocarbons. The catalytic cracking experiments were carried out using a self-made nickel-based supported molecular sieve catalyst to obtain the optimal preparation conditions of the catalyst. The transformation path of catalytic cracking of different VOCs model substances was explored, aiming to provide a new idea for the removal of VOCs and the efficient utilization of biomass resources.