Pyrolysis of mixed plastics and paper to produce fuels and other chemicals

Sammanfattning: As the world population and economy grow, higher consumption results in higher waste packaging, plastics and paper residues. Pyrolysis offers a way to recover fuels and other chemicals from this waste fraction. By applying heat to these materials in the absence of oxygen, pyrolysis process can convert these feedstocks into more valuable products in the forms of gas, liquid and char.One important issue in the pyrolysis process which requires an investigation is the interactions between the feedstocks which consist of cellulose as the main component of paper and different types of plastics. Regarding this topic, 3 subtopics were investigated which are: the effect of mixing methods on the co-pyrolysis products, the interactions between the plastics and cellulose, and the formation of H, OH and water during cellulose pyrolysis. All these experimental investigations were based on microscale pyrolysis experiments using Py-GC/MS technique.In the first work, polyethylene and cellulose were mixed by melting and by putting side-by-side. It was found that some interactions occurred during co-pyrolysis of these materials which slightly altered the yields of some anhydrosugars, aldehydes and ketones when the two feedstocks were mixed together by melting. Nevertheless, the main pyrolysis products from each feedstock were not affected. In the second study, the investigation continues on the interactions between different types of plastics (PE, PP, PS, PET) and cellulose. By using Py-GC×GC/MS, a good separation of the mixed pyrolysis products could be achieved, thus assisting the analysis. It was found that although the main pyrolysis products from each feedstock were not affected by the co-pyrolysis, small interactions occurred such that the interactions between different plastics were more pronounced than the interactions between plastics and cellulose. Nevertheless, some hydrogen transfer reactions occurred when PS was co-pyrolyzed with cellulose. However, the source of hydrogen was not clear.Therefore, the investigation on the formation of H and OH radicals during cellulose pyrolysis was performed. This work combined first-principle calculations with experimental investigations. The author of the thesis was responsible for the experimental part. It was found from the first-principle calculations that it is energetically more favorable for the generation of a pair of H and OH radicals with subsequence formation of water than to generate a single radical because the formation of a double bond on the resulting cellulose helps stabilize the structure. With Gibbs free energy calculations, it was predicted that the water would be released at 280 °C. This agree well with the experimental findings from multistep pyrolysis of cellulose in Py-GC/MS which showed that water was generated at two different temperature ranges with the first peak around 280 °C.As the interactions between the feedstocks during co-pyrolysis do not much improve the liquid products’ properties, and the nature of the products produced from plastics and paper pyrolysis are significantly different; it might be more beneficial to separate the pyrolysis products from the two feedstocks. Moreover, the hydrocarbons produced from plastics pyrolysis and the oxygenated products from paper pyrolysis require different upgradation methods. Stepwise pyrolysis was then proposed to produce and collect these two products separately. With simulated feedstock mixtures (PE, PS, cellulose) and real waste fractions which are paper rejects, it was successfully demonstrated that the stepwise pyrolysis with a temperature of the first step of 300-350 °C and a temperature of the second step of 500 °C could be used to produce two products streams as previously described. However, an optimization of the process and further investigations on product properties and upgradation are still required.As a continuation on the investigation of the stepwise pyrolysis, an upgradation of the products from the first pyrolysis step was studied. When PVC plastic is present in the feedstock, dehydrochlorination of PVC occurs in the temperature range of the first pyrolysis step together with the pyrolysis of cellulose. Calcium oxide (CaO) was then tested for the simultaneous adsorption of HCl and reforming of cellulose pyrolysis products. The experiments were performed in a two-stage reactor system which was a pyrolysis reactor connected in series to a catalytic reactor containing CaO. It was found that the catalytic temperature should be between 300-350 °C because the desorption of HCl occurred when the temperature was higher than 400 °C. This was partly due to a reaction between water and CaCl2 which caused the desorption of HCl.From all the studies, stepwise pyrolysis has a great potential to produce fuels and other chemicals from mixed plastics and paper. Further investigations are needed to develop, evaluate and realize this promising process.