Products formation and analysis in fixed bed pyrolysis of plastics and biomass

Wenhao Hu

Research output: Book/ReportPh.D. thesis


The rapid growth of the economy and urbanization has led to a significant global municipal solid waste (MSW) generation. An adequate treatment of solid waste provides a pathway to convert waste into valuable products, which may reduce potential environmental pollution and alleviate the depletion of fuel energy sources simultaneously. Thus, it has attracted an abroad attention of researchers worldwide. The main components of MSW include plastic and biomass-based waste. In the past decades, research has shown that pyrolysis is an effective and reliable way to dispose of mixed solid waste without complicated segregation requirements. Fixed bed pyrolysis is a low-cost and effective pyrolysis method that makes it possible to treat large amounts of mixed solid waste. This thesis is dedicated to the exploration of pyrolysis of plastic and biomass, possible synergy effects between plastic and biomass during co-pyrolysis.

The experiments presented in this thesis were mainly conducted in a two-stage fixed bed lab-scale reactor. The normal temperature used in the first reactor was 550 ℃, where the sample was loaded, and the second reactor temperatures were in the range of 300-750 ℃ depending on the catalytic material investigated. Different catalytic materials were tested in the second reactor such as cement raw meal and cement calcined raw meal.

Pure LDPE (Low-Density Polyethylene) pyrolysis was investigated using the lab-scale two-stage fixed-bed pyrolyzer with different catalytic materials in the second reactor for upgrading the vapors. The yields of gas and char were determined, and the condensed product was thoroughly examined by gas chromatography-mass spectrometry (GC-MS) and thermogravimetric analysis (TGA). The use of different catalytic materials such as HZSM-5, cement raw meal, calcined cement raw meal, and silicon carbide was investigated. A wax-rich condensed product (intermediate molecular weight hydrocarbons) was observed in the following type of experiments: no catalyst, SiC, and cement raw meal in the second reactor. The wax was mainly composed of alkanes and alkenes. The use of an HZSM-5 catalyst and applying temperatures from 350 ℃ to 500 ℃ in the second reactor highly decreased the chain length of hydrocarbons in the condensed product and converted the wax into a liquid. When the second reactor temperature was higher than 400 ℃, the main product components were aromatics and naphthalene. Using calcined raw meal in the second reactor at a temperature of 600 ℃ also led to the conversion of the wax to a liquid product with a large fraction of cyclic aliphatics.

Co-pyrolysis of LDPE (Low-Density Polyethylene) and spent coffee grounds (SCG) in a two-stage fixed-bed reactor using cement raw meal and calcined cement raw meal as catalytic materials in the second reactor was performed to investigate the upgrading of the vapors and to study synergetic effects during pyrolysis of a mixture of plastic and biomass. The yields of gas, water, wax/oil, coke, and char were examined, and the composition of gas and wax/oil was investigated using gas-chromatography-mass spectrometry (GC-MS). It was found that the wax product was converted to liquid oil in the two-stage co-pyrolysis experiments with a calcined raw meal as the catalyst using first and second reactor temperatures of 550 ℃ and 600 ℃, respectively. Moreover, wax-rich products could be observed in most co-pyrolysis and LDPE pyrolysis cases. The main components in the oil from the pyrolysis of SCG are hydrocarbons and oxygenated hydrocarbons, while in the case of co-pyrolysis and LDPE pyrolysis, hydrocarbons were the main product. The chain length of components in the condensed phase from LDPE was decreased when calcined raw meal was applied in the second reactor. Both raw meal and calcined raw meal lead to a decreased condensed phase yield and an increased gas yield from SCG, while the oxygen content of the SCG condensed phase product was not reduced. Due to the longer chain length of the LDPE pyrolysis product, no significant co-pyrolysis synergy was observed. In the co-pyrolysis of LDPE and SCG using thermogravimetric analysis, a minor synergy effect was observed, leading to a reduction in the maximum hydrocarbon chain length.

A possible synergy of co-pyrolysis of different plastics (LDPE, PP, and PS) and biomass (wood, straw, and coffee ground) was discussed, and the influence of inorganic elements (K and Ca) that possibly exist in the biomass on the plastic pyrolysis was investigated in TGA. Co-pyrolysis of LDPE (Low-Density Polyethylene) and coffee ground in a one-stage fixed-bed reactor (slow heating rate, 10 ℃/min) and fast pyrolysis reactor (Pyrola, fast heating rate, 300 ℃/min) was conducted to investigate the synergy effect on the condensed phase composition using gas-chromatography-mass spectrometry (GC-MS). Moreover, in most cases of pyrolysis, the influence of inorganic elements (K and Ca) on plastic pyrolysis was limited. The liquid yield of fixed bed reactor co-pyrolysis experiments was equivalent to the predicted values, which were calculated based on the individual LDPE and SCG pyrolysis indicating no significant synergy can be observed. In the analysis of condensed liquid from co-pyrolysis of LDPE and coffee ground, the peaks of waxes from LDPE pyrolysis are dominant. The yield of alcohol in the case of fast co-pyrolysis experiments was higher than the theoretical value and the slow pyrolysis results, indicating that fast co-pyrolysis contributes to the conversion of acid to alcohol.

To conclude, the understanding of LDPE plastic conversion to specific chemicals and valuable products in a fixed bed was improved through detailed analysis. The synergy between LDPE and spent coffee grounds was also discussed from the perspectives of product yield distribution and product composition to mimic municipal waste pyrolysis in real cases. A comparison of slow and fast heating rates during the co-pyrolysis of LDPE and spent coffee grounds was also given to investigate the influence of heating rate.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages159
Publication statusPublished - 2023


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