3.1 Impact of feedstocks component on H2 yield
Figure 2 showed the product distribution, syngas yields and characteristics of different feedstocks. The yield of solid phase obtained from paper was the highest (17.83 wt.%) among all the tested components, due to its high ash content. Gas percentage and syngas yield obtained from textile and wood tests were relatively high, corresponding to the highest H2 yields among the feedstocks tested in plasma gasification. The H2 yields are shown in the Figure 2(b) and were 36.92 mol/kg and 36.98 mol/kg, respectively. Waste textile and biomass were widely used to produce syngas 18-20. The percentage of liquid products from plastic (0.91 wt.%) was the highest compared to other single components, due to the high bond-energy of C-Cl in PVC polymer16. The gas amount obtained by the mixed components was 85.30 wt.%, slightly higher than that of each individual component. Previous studies indicated that the mixed materials of textile/plastic with biomass enhance hydrogen production because alkali and alkaline earth metals in biomass could enhance the char reactivity, promoting the free radical reaction and hydrogen output21, 22.
As shown in Fig 2b, H2 accounts for the largest part of the syngas ranging from 56.34 wt.% to 64.29 wt.% in the syngas produced by all five types of materials, followed by CO (18.17 – 26.23 wt.%). H2 amount from textile (61.21 wt.%) and plastic (64.29 wt.%) were higher than that of other mono-component feedstocks despite the highest H content in wood and paper. This result may be attributed to the high volatile content of textile and plastic and the higher reactivity of these material. Coincide with the highest H content and syngas yield, the H2 yield (36.98 mol/kg) produced from plasma gasification of wood was higher than other single component feedstocks. The tests performed with the mixed components as feedstock produced the syngas having the largest LHV value of 11.88 MJ/Nm3. Compared with the weighted average H2 yield of each component (35.38 mol/kg), the actual mixed component produced higher H2 production of 35.44 mol/kg (Figure 2b). The H2 proportion from mixed materials is around 60 wt.% in plasma gasification, while that of conventional gasification is 20-50%23-27. These results indicated that the extreme temperature reached in this kind of technology and the plasma effect on particle activation is beneficial for H2 production and energy output.
3.2 Impact of operating parameters on H2 yield
3.2.1 Impact of S/C ratio
The water vapor flow rates were set to 0.6 kg/h, 0.7 kg/h, and 0.8 kg/h, respectively, corresponding to S/C ratio of 0.72, 0.84 and 0.96. As described in Figure 3(a), with the increase of S/C ratio from 0.72 to 0.84, the proportion of gas products increased from 81.97 wt.% to 85.89 wt.% and the syngas yield reached the peak value of 1.43 Nm3/kg. Thus, when the S/C ratio reached 0.96, the gas proportion descended to 75.60 wt.% and the syngas yield decreased to 1.26 Nm3/kg. In addition, as shown in Fig. 3b, H2 yield and LHV reached the peak value of 36.42 mol/kg and 11.98 MJ/Nm3 at the S/C ratio of 0.84, respectively. However, as the S/C ratio climbed to 0.96, the H2 yield declined to 33.55 mol/kg. The continuous increase of H2 content (59.87% – 61.04%) with the increase of vapor flow rates from 0.6 kg/h to 0.8 kg/h could be explained by the promotions of vapor on the water-gas reaction (Eq. (1)) and Boudourad reaction ( Eq.(3))28. However, higher water vapor content would lower the temperature in the furnace and had an adverse effect on the gas phase proportion, syngas yield and H2 content11, 17. To maximize the H2 proportion and reduce the energy input, the appropriate experimental vapor flow rate should be selected as 0.7kg/h.
3.2.2 Effect of running power
As can be seen from Figure 4(a) and (b), as the running power is raised from 16 kW to 28 kW, the gas content gradually increased and reached the peak value of 90.28 wt.% and the syngas yield got the maximum value of 1.52 Nm3/kg while the liquid content was not detected. According to Figure 4(b), in the same range of power the H2 yield climbed from 35.44 mol/kg to 39.77 mol/kg and reached the peak value of 39.77 mol/kg.
Actually, the increase in running power could improve the heating rate between particles, which resulted in effective destruction of particles and complete gasification reactions. The consequent rise in temperature also had a positive influence on gasification and tar, char reforming reaction (Eq. (1-3)). Thus, the syngas yields show an uptrend with the increase of running power. In addition, the increase of power could also improve the concentration of hydrogen radicals and hydroxyl radicals, accelerating the global reaction rate and H2 information. As shown in Fig. 2(b), the H2 proportions in syngas increased slightly from 59.78% to 60.34% and the H2 yields rose significantly from 35.44 mol/kg to 39.77 mol/kg.
3.3 Effect of continuous feed experiment
Figure 5 comprehensively compared the syngas characteristics of three groups of experiments with feed rates of 45 g/min, 55 g/min, and 65 g/min. As the feed rate increased from 45 g/min to 65 g/min, the H2 contents (57.30- 59.68%) showed a first increase then a decrease trend and reached the maximum value when the feed rate was 55 g/min. However, due to the increase of syngas yield, the H2 yield increased steadily to 35.37 mol/kg. Comparing with batch feeding experiment results at the same feeding rate (65g/min), the syngas from continuous feeding experiment had a similar H2 yield (around 35.4 mol/kg) while showed a higher LHV (12.18 MJ/Nm3) and CO content. This result may be explained considering that in the continuous feeding experiment a more stable running operating and higher running temperature were reached, which is beneficial for the decomposition of volatiles and the formation of CO, C2-C3 component.
3.4 Comparison with conventional gasification technologies
Table 3. Comparison between this study and other MSW gasification studies in literature.
Compared with other MSW gasification technologies (Table 3), the plasma gasification had a larger processing capacity and higher dry gas yield. This phenomenon could be explained considering that thermal plasma resulted in a dynamic increase of temperature and formation of radicals, which accelerated the fracture of chemical bonds as well as the reforming reaction of tar and char. The H2 proportions and H2 yield of plasma gasification and dolomite catalytic gasification are higher than other technologies, confirming that plasma gasification is an attractive MSW disposal option to overcome the adverse environmental influence of MSW and obtain high value products. Conversely, the LHV of syngas from plasma and dolomite catalytic gasification are lower than other technologies, partly attributing to the lower concentrations of hydrocarbon (CnHm) and CO. Moreover, the low mass share of tar and char from plasma gasification indicated the additional environmental benefits.