Pyrolysis of Waste Plastics into Fuels (2010)

Waste plastic disposal and excessive use of fossil fuels have caused environment concerns in the world. Both plastics and petroleum derived fuels are hydrocarbons that contain the elements of carbon and hydrogen. The difference between them is that plastic molecules have longer carbon chains than those in LPG, petrol, and diesel fuels. Therefore, it is possible to convert waste plastic into fuels. The main objectives of this study were to understand and optimize the processes of plastic pyrolysis for maximizing the diesel range products, and to design a continuous pyrolysis apparatus as a semi-scale commercial plant. Pyrolysis of polyethylene (PE), polypropylene (PP), and polystyrene (PS) has been investigated both theoretically and experimentally in a lab-scale pyrolysis reactor. The key factors have been investigated and identified. The cracking temperature for PE and PP in the pyrolysis is at 450 ºC, but that of PS is lower, at 320 ºC. High reaction temperature and heating rate can significantly promote the production of light hydrocarbons. Long residence time also favours the yield of the light hydrocarbon products. The effects of other factors like type of reactor, catalyst, pressure and reflux rate have also been investigated in the literature review. From the literature review, the pyrolysis reaction consists of three progressive steps: initiation, propagation, and termination. Initiation reaction cracks the large polymer molecules into free radicals. The free radicals and the molecular species can be further cracked into smaller radicals and molecules during the propagation reactions. β-scission is the dominant reaction in the PE propagation reactions. At last, the radicals will combine together into stable molecules, which are termination reactions. There are three types of cracking of the polymers: random cracking, chain strip cracking, and end chain cracking. The major cracking on the polymer molecular backbone is random cracking. Some cracking occurs at the ends of the molecules or the free radicals, which is end chain cracking. Some polymers have reactive functional side group on their molecular backbones. The functional groups will break off the backbone, which is chain strip cracking. Chain strip cracking is the dominant cracking reaction during polystyrene pyrolysis. The reaction kinetics was investigated in this study. The activation energy and the energy requirement for the pyrolysis are dependent on the reaction process and the distribution of the final products. Following the equations from other literatures, the theoretical energy requirement for pyrolyze 1kg PE is 1.047 MJ. The estimated calorific value of the products is about 43.3 MJ/kg. Therefore, the energy profit is very high for this process. The PE pyrolysis products are mainly 1-alkenes, n-alkanes, and α, ω-dialkenes ranging from C1 to C45+. The 1-alkenes and the n-alkanes were identified with a special method developed in this research. It was found that secondary cracking process has a significant influence on the distribution of the product. This process converts heavy hydrocarbons into gas or light liquid product and significantly reduces 1-alkenes and α, ω-dialkenes. This secondary process can be controlled by adjusting the reflux rate of the primary product. The product of PE pyrolysis with maximized diesel range output consist of 18.3% non-condensable gases, 81.7% w/w liquid product, and less than 1% pure carbon under high reflux rate process. Some zeolite catalysts were tested to reduce the heavy molecular weight wax. It was found that NKC-5 (ZSM-5) was the most effective catalyst among zeolites tested. The proportion of the non-condensable gases was promoted from 17% w/w to 58% w/w by adding 10% w/w NKC-5 into the PE feedstock. The products of PP pyrolysis are mainly methyl- oligomers. The reflux effect on the product from pyrolysis of PP is not as great as that on PE. The PP pyrolysis product with high reflux rate consists of 15.7% non-condensable gases, 84.2% condensed liquid product, and less than 0.25% char. Cyclohexane is the dominant component, 21%w/w in the liquid product. 44%v/v of the non-condensable gases is propene. In the pyrolysis product of PS, there are 4% non-condensable gases, 93% liquid, and 3% char. Styrene accounts for 68.59%w/w in the PS liquid pyrolysis products due to the chain strip reactions. There was 19% v/v hydrogen in the gas product, which did not exist in the PE pyrolysis gas product. The composition of the char is almost pure carbon, which is similar to that from PE pyrolysis. The mixture of virgin and post-consumer PE, PP and PS have also been investigated to identify the feedstock interaction and the effect of the contamination on the product. The interaction promotes the production of non-condensable gases. However, the effect of the interaction on the distribution of total product is not significant. Contamination of paper labels on the post-consumer plastics may result in higher solid residue in the product but no significant effect on the product was found in this study. Based on the achievements, a continuous semi-scale reactor has been designed and constructed at maximum capacity of 27.11kg/hr in this research. From the experiments of pyrolysis of both virgin PE and post-consumer PE on this semi-scale pyrolysis reactor, it was found that the major components are 1-alkenes, n-alkanes, and α, ω- dialkenes. The distribution of the condensed products of PE pyrolysis from the semiscale reactor is the same as that of the products from low reflux rate process with the lab-scale reactor. However, the proportion of non-condensable gases is much higher than that from pyrolysis in the lab-scale tests with low reflux rate because the semiscale plant has higher reaction temperature and heating rate. Lower proportion of unsaturated hydrocarbons was found in the condensed product from the post-consumer PE pyrolysis than in the virgin PE product because of the contamination on the postconsumer PE. The actual energy consumption for cracking and vaporizing PE into fuels is 1.328 MJ/kg which is less than 3% of the calorific value of the pyrolysis products. Therefore, the pyrolysis technology has very high energy profit, 42.3 MJ/kg PE, and is environmental-friendly. The oil produced has very high quality and close to the commercial petroleum derived liquid fuels. The experience of design and operation of the semi-scale plant will be helpful for building a commercial scale plant in the future.