Runs#1,3 were performed under allothermal conditions (solar-only heating), Run#2 was similar to Run#1 but with oxygen injection for partial oxy-combustion (hybrid operation), whereas Run#4 was first carried out in allothermal followed by hybrid operation mode (with the injection of 0.20 NL/min of O
2). For the hybrid solar/autothermal experiments (Runs#2,4), the particles were partially burnt under oxygen lean conditions. The Equivalence Ratio ER = (B/O)/(B/O)
st Biomass to Oxygen mass ratio (equivalent to the fuel to oxidizer ratio [
43]) was 4.95 in Run#2 and 2.74 in Run#4 based on Equations (3) and (4).
The duration of gasification experiments corresponds to the time required to inject the whole amount of feedstock loaded in the feeding system. This duration could be increased by decreasing the feeding rate or by increasing the amount of feedstock. The experimental duration is not the main parameter influencing the gasification performance, given that the injection is continuous. The total experimental duration also includes the heating and stabilization periods as well as transition periods (feedstock injection stopped) when switching from allothermal to hybrid operation. The feeding rate is actually the key parameter affecting the process, as previously evidenced in the case of wood biomass gasification [
44]. In this study, it was chosen so as to offer stable gasification performance at the reactor operating conditions. Indeed, a low feeding rate means the reactor is not used at its nominal capacity, whereas a too high feeding rate results in continuous operation failure because the input energy is not high enough to gasify the fed particles. Thus, the optimal feeding rate existing for given operating conditions (reactor temperature and solar energy input) was selected.
The average syngas composition and gas yields (in mmol/g
dry,feedstock) were thereafter evaluated thanks to the time integration of the gas production rates. The performance of the reactor was evaluated using three relevant performance metrics: the carbon conversion efficiency (CCE), Equation (6); the cold gas efficiency (CGE), Equation (7); and the solar-to-fuel efficiency (SFE). Equation (8). The CCE quantifies the extent of feedstock conversion inside the reactor (ratio of carbon contained in the syngas to carbon in the initial feedstock). The CGE (also called energy upgrade factor) is the ratio of the calorific value of produced syngas to that of the initial feedstock, each multiplied by its mass (where LHV is the low heating value (J/kg) and m
syngas, m
feedstock are the mass of produced syngas and initial feedstock (kg)). The SFE represents the ratio between the calorific value of produced syngas over the total thermal energy that enters the reactor, including both solar and injected feedstock energy.
3.1. Beechwood Gasification
The measurements of outlet gas production rates by online syngas analysis (continuous lines) and GC analysis (dots) are shown in
Figure 3. Syngas flow rate evolutions fluctuate around a mean value throughout the experiment. These fluctuations were due to the screw feeder outlet that could not be inserted in the reaction zone due to high temperatures, hence, the woody particles in the injection tube were pushed by preceding ones and fell by gravity into the cavity in the form of small parcels. In Run#1, 1.2 g.min
−1 of biomass was gasified by 0.2 g.min
−1 of steam (
Figure 3a). In Run#2, 0.25 NL.min
−1 of O
2 was added to the system with 0.2 g.min
−1 of extra-wood injection (
Figure 3b); the oxygen flow corresponds to the quantity required to completely burn the extra-wood injection.
The solar power in Run#1 was about 1.2 kW
th, it decreased significantly in Run#2 to 0.8 kW
th (
Figure 4a) as part of the energy was provided by internal exothermic gas/solid oxy-combustion reactions. The experiments were performed at 1300 °C and above (according to T3 inserted inside the cavity;
Figure 4b). Thanks to the achieved high temperatures, the gas in Run#1 (
Figure 3a) was predominantly composed of H
2 and CO. CO
2 and light hydrocarbons (CH
4 and C
nH
m, mainly C
2H
2 and to a lesser extent C
2H
4) were produced in lesser amount in accordance with previously reported works [
45,
46]. Although the biomass feeding rate was increased in Run#2 from 1.2 g.min
−1 to 1.4 g.min
−1, H
2 and CO flow rates declined consistently (oxidation of these gases by the added O
2 thus likely occurred), and CO
2 sharply increased (H
2O should also be produced from H
2 oxidation, but it was not quantified because it was trapped in the outlet bubbler). Accordingly, the syngas composition was substantially affected by the presence of O
2, as expected. The time-averaged volume fractions for Runs#1,2 were respectively: H
2 (52.2%, 37.1%), CO (41.6%, 39.4%), CO
2 (3.4%, 16.8%), CH
4 (2.5%, 4.7%), and C
nH
m (1.3%, 2.0%).
The C
nH
m molar composition was very similar in the two runs: C
2H
2 (87.0–85.9%), C
2H
4 (13.0–14.0%) and C
2H
6 (<0.1%). Their time-dependent production rates, measured by the GC, are plotted in
Figure 5.
Mass balance, syngas yields, and reactor performance indicators are shown in
Table 3 and
Table 4. The results reveal that H
2 and CO yields both declined respectively by 43% and 26% while CH
4 and C
nH
m increased by 48% and 21%, probably due to lower gas residence time (0.51 s in Run#1 against 0.48 s in Run#2). CO
2 yield was highly impacted and increased by almost 3 times due to oxy-combustion reactions. The greater decrease in hydrogen would be related to its higher concentration in the cavity [
42] and its greater reactivity with oxygen [
47]. This suggests that the gaseous products from gasification are oxidized/combusted by oxygen rather than the raw biomass. CO also decreased but, to a lesser extent, probably because it is also produced by partial oxidation of gases (methane, light hydrocarbons and tars) and char.
Although gas residence time decreased in Run#2, oxygen injection globally favored biomass conversion. In fact, the CCE was slightly improved from 83.3% in Run#1 to 84.6% in Run#2 due to the very fast O2-char reaction kinetics. Moreover, given the decrease in H2 and CO yields, the CGE declined strongly from 112.9% in Run#1 to 84.7% in Run#2. In return, the SFE was almost the same at around 17.6% in both heating configurations given that the total solar energy consumed in Run#2 was significantly lower by about 41% than in Run#1.
Globally, beechwood particles were successfully converted in both solar-only and hybrid solar/autothermal operating modes. The hybrid process provided additional combustion heat to the reactor and reduced the solar energy consumption at the expense of lower H2 and CO yields.
In the following, the ability of the reactor to operate with SRF particles in both solar-only (allothermal) and hybrid solar/autothermal heating modes is investigated.
3.2. Solid Recovered Fuels Gasification
Figure 6 shows the measurements of the syngas flow rates with the SRF particles (Runs#3,4). Noticeable fluctuations in the gas flow rates reflected by variable peaks and valleys all along the experiments were observed. They were due to the unstable injection of the particles caused by the heterogeneity of the sample and the possible melting of plastics and inorganics at the screw feeder tip and at the injection tube that affect the particles’ flowability. In spite of this issue, complete feedstock load injection was achieved. Run#3 was performed at 1300 °C under solar-only heating conditions while Run#4 was also partly operated in solar hybrid mode.
Just as in Run#1 (using beech wood particles), H
2 and CO flow rates were the highest, followed by CO
2, CH
4, and C
nH
m. GC measurements of the C
nH
m are shown in
Figure 7. They were primarily composed of C
2H
2, whereas C
2H
4 and C
2H
6 were found in negligible amounts.
Thermodynamic equilibrium calculations (
Figure 8) have been performed thanks to the open source software CANTERA [
48] to compare with the experimental data (the initial system compositions used in thermodynamic calculations take into account the feedstock composition and the inlet gas atmosphere for Run#3 and #4). The reaction equilibrium thermodynamics agree with the experimental values observed in terms of trends of the syngas species (especially H
2 and CO predicted at temperatures above ~900–1000 °C). More specifically, the results evidence the increasing amount of CO
2 and H
2O, and the decreasing amount of H
2 and to a lesser extent of CO when O
2 is introduced at high temperatures (above 800 °C). However, kinetic limitations explain the presence of additional compounds such CH
4 or C2 light hydrocarbons in the experiments. The main reactions are pyrolysis followed by steam gasification of carbon (char) that yields H
2 and CO. The additional steam reforming and water gas shift reactions proceed in the gaseous phase and are thus influenced by the gas residence time. Thermodynamics consider that gas residence times are sufficiently long to reach reaction equilibrium conditions, which is not the case experimentally.
The chemical composition of the syngas during this Run#3 was thus: H2 (56.8%), CO (32.1%), CO2 (7.3%), CH4 (2.3%), CnHm (1.5%). To study the flexibility of the solar process and its ability to deal with solar energy variations, a small amount of O2 (0.25 NL/min), calculated assuming the complete combustion of about 33% of the feedstock, was added to the system in Run#4 at time = 13 min. According to the previous results of Run#2, O2 injection is expected to elevate and stabilize the reactor temperature at a higher setpoint value.
The reactor temperature measurements during Runs#3,4 are plotted in
Figure 9. While the temperature was quite stable in Run#3, the hybrid operation (Run#4) showed a fluctuating temperature pattern. The fluctuations were increased significantly during the hybrid phase at around 1350 °C and were due to the rapid oxy-combustion of the particles. In fact, the consumption of the particles was faster than the injection process leading to sharp temperature variations by about 80 °C. During the hybrid phase of Run#4, T3 temperature increased unsteadily. As a consequence, the desired effect of increasing and stabilizing the reactor temperature at a higher setpoint value through partial feedstock oxy-combustion was not totally effective due to the injection issues. Therefore, the impact on the gas composition was considerable and affected essentially the H
2 and CO
2 content.
Figure 10a shows the gas volume fractions before and after hybridization (Run#4). The H
2 volume fraction decreased by 24% (58.9–40.0%), and CO
2 increased by more than 5 times (3.5–24.0%), while CO concentration was only slightly affected with a decrease of 6% (33.8–31.8%).
To assess the consistency and repeatability of the measurements, the syngas compositions of the Run#3 and Run#4-allothermal were compared in
Figure 10b (even if the water flow-rate varied slightly between the two runs). It can be observed that globally the composition was not significantly modified, except for the CO
2 that was somewhat higher and CH
4 that was slightly lower during Run#3 (CO
2, 7–4%; CH
4, 2–3%). This confirms that syngas composition outputs are similar for comparable operating conditions, which demonstrates results repeatability. Indeed, in spite of the experimental instabilities linked to the SRF injection, a good agreement was achieved.
The mass balance, syngas yields (integrated over the total experiment), and reactor performance metrics are shown in
Table 5 and
Table 6. The injection of oxygen significantly affected the gas yields. Although the first part of the hybrid run (Run#4) was carried out in allothermal mode, the drop in the H
2 and CO yields, calculated over the whole run#4, was noticeable (by respectively 31% and 20%). Light hydrocarbons slightly decreased and the CO
2 yield in the hybrid run increased by about 37%.
The achieved CCE in Run#3 was 88.1%, which is in the range of the reported CCE values (generally between 80–92%) for waste gasification in autothermal fluidized beds operating up to 800 °C [
49]. The CGE was 104.5%, which was much greater than the reported CGE values for SRF gasification in autothermal fluidized bed gasifiers that vary between 70% and 85% at best [
6,
50,
51]. The CGE higher than 1 confirms that solar energy was effectively stored in the gas products, and this demonstrates the interest and benefits of solar heating for gasification when compared with autothermal operation (involving partial feedstock combustion). The SFE was 15.8%. In Run#4-hybrid phase, the O
2 injection impaired to a certain extent the production of H
2 and CO in comparison with Run#3. It also affected SRF conversion and led to a relative CCE drop by 11%. Likewise, the CGE and the SFE were also decreased to reach barely 78.0% and 11.9%.
These experiments further allowed identifying technical issues to be addressed regarding SRF solar gasification. In addition to the difficulties encountered for the transport during the injection of the SRF particles, the process suffered from ash melting and agglomeration issues.
Figure 11 depicts the cavity and ash deposition at the bottom at the end of Run#4. A thick block of solidified ash stuck to the cavity wall. It surrounded the fixed Al
2O
3 particles bottom layer that was used to protect the oxidant injection tube. Future work should be dedicated to manage ash accumulation in the reactor.