APERTURA DE PROPOSICIONES Y PROPUESTA DE ADJUDICACIÓN

Model 1 developed in this study can be used to predict the effect of a change in mass of each of the components investigated as part of a mixed MSW fuel. The total volume, HHV and composition of the produced gas can be estimated for pyrolysis at 550°C for a range of residence times. The model has shown good comparisons with the composition of gases produced in commercial rig 2 although significant differences in the volumes of gas produced in commercial rig 1.

Several limitations of Model 1 have been identified. A difference in the pyrolysis conditions from that studied in laboratory investigations can cause significant challenges in comparison of model predictions with data from other pyrolysis rigs. It is suggested that model predictions are therefore most accurate for pyrolysis processes with a high heating rate up to 550°C without the addition of excess O2.

The unidentified gases detected during the pyrolysis of several MSW components, especially PET and PVC, although assumed to be hydrocarbons, have not been identified. Without identification, the effect of these gases on the HHV of the produced gas is unknown and therefore so too is the effect on HHV predictions using Model 1.

A low gas yield was found in this study from the pyrolysis of HDPE, PVC and textiles. This was lower than that reported in other studies [66, 69]. This could partly be attributed to the challenges presented in comparison between data from TGA test and that from

142 laboratory scale pyrolysers as discussed in section 5.1.1. This could also be partly attributed to the thick tar produced during laboratory investigations that may have inhibited the progression of the produced gas through the tar trap to the gas analysis instrumentation. If the low gas yields are due to inaccuracies in laboratory data, the predictions of Model 1 for waste mixtures including these components would also contain inaccuracies.

A 5 g mass of waste is unlikely to pyrolysis in exactly the same way as a much larger mass of waste. A change in the temperature profile throughout a larger mass of waste could have a significant effect on the gas produced as shown in comparisons with commercial rig 1. Model 1 is based on laboratory data from the pyrolysis of 5 g samples and any predictions using this model are made using the assumption that pyrolysis behaviour of the mass of waste is the same as that of a 5 g sample.

5.4 SUMMARY

Empirical models have been developed based on laboratory data from the laboratory scale pyrolysis reaction rig found in this study. The first models have been developed with the aim of predicting the mass reduction behaviours of MSW components during pyrolysis for temperatures ranging from 300 °C to 900 °C for a residence time of 0-50 minutes. The reduction of mass was found to change exponentially with a change in residence time and sigmoidally with a change in pyrolysis temperature.

Data was extrapolated for pyrolysis temperatures below 300 °C and show good comparisons with previous research using TGA methods for the temperatures at which thermal degradation began for each component. However, it has been established that the larger mass of waste used in laboratory studies did not follow the same trend in terms of mass reduction during pyrolysis as that shown in TGA tests. This was attributed to the variation in temperature profile throughout the larger mass which reduced the rate of pyrolysis reactions and inhibited gas production causing a slower reduction in mass. This was also found by Yang et al [97]. This has shown that mass reduction predictions based on TGA tests are unrealistic when predicting the behaviour of a larger mass of waste such as that in commercial scale rigs. Although the 5 g mass of waste used in this study is also significantly smaller than that in commercial rigs, it is significantly larger than that used in TGA tests. Results presented in this study can therefore provide an estimation of the effect of a larger mass. It is suggested that a mass of waste larger than 5 g would follow the same trend and

143 have a slower mass reduction rate due to the increased variation in temperature throughout the increase mass.

An empirical model was also developed based on laboratory data with the aim of predicting the composition, HHV and volume of the gas produced from pyrolysis at 550 °C for any composition of MSW that is based on the components investigated in this study. This model was used to predict the composition and quantities of the gas produced from the pyrolysis of a range of waste mixes as well as to establish the effect of each individual MSW component on the gas produced. It was found that the addition of newspaper to a waste mix led to the highest HHV an increased volume of gas produced. This model could be especially useful for predicting the composition of MSW needed to for a variety of optimum conditions i.e. to maximise HHV or the production of a specific gas.

There were significant challenges in the comparison of model predictions with data from both commercial rig 1 and commercial rig 2. This is due to fundamental differences in pyrolysis process for both commercial rigs and laboratory investigations. However, it was found that the composition of gas as predicted using Model 1 was similar to the composition of gas analysed from both commercial rigs. Comparisons with predictions from Model 1 and data from commercial rig 2 suggested that commercial rig 2 was operating at a temperature higher than 550 °C.

Comparisons between model predictions and data from commercial rig 1 were difficult due to the lower pyrolysis temperature, slow heating rate and the introduction of a high flow rate of air. Extending the parameters of Model 1 to include the effect of a range of pyrolysis temperatures on MSW components as well as the effect of post-pyrolysis gasification would allow for much closer comparison between model predictions and data recorded from commercial rig 1. Conclusions of this study along with other recommendations for future work can be found in Chapter 6.

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