3.1. Temperature Measurements
As explained before, in gasification experiments, the wall temperatures in the fluidised zone were regulated at about 850 °C with electric heating. The three in-bed temperature values were very close (less than 1 °C difference) in all gasification experiments except from the one with lignin residue. This is a sign that for all the other feedstock, the fluidisation was smooth with a uniform in-bed temperature. The in-bed temperature was a bit lower than the wall temperature, with a value between about 830 and 845 °C. For the experiment with lignin residue, a clear shift appeared between the three measurements after about 3 h feeding, although there was no change in wall temperature, pressure or gas flowrate. The gas temperature became then lower in the bottom part of the bed, with a difference of about 5 °C with the top of the bed. This can be a sign of degraded fluidisation, and could be linked to the beginning of agglomeration.
The temperatures measured in the fluidised bed zone are shown in
Figure 2 for the combustion of chars from bark, 50/50 wt% bark-wheat straw blend and lignin residue as examples. ‘Twall3’ and ‘Twall4’ are the temperatures measured on the external wall of the reactor at two axial locations (
Figure 1a). ‘Tbed3’, ‘Tbed2’ and ‘Tbed1’ are the in-bed gas temperature measurements, starting from the bottom of the bed (
Figure 1b). The inlet gas superficial velocity, as well as the produced CO and CO
2 flowrates, are plotted on the figures. For each combustion experiment, the values of Tbed1, Tbed2 and Tbed3 increase as soon as air is fed into the reactor, and reach higher values than Twall3 and Twall4, which is characteristic of exothermic oxidation reactions in the bed zone. After about 1–2 h, the in-bed temperatures decrease to values lower than Twall3 and Twall4, whereas the temperatures measured at the top of the reactor, in the metallic filter zone, slightly increase (not shown on the figures). This is due to combustion of the char entrained on the filters, after that the char in-bed has been completely oxidised.
In most cases, the CO flowrate is higher than the CO2 one in the whole first part of the oxidation, and the CO2 flowrate is higher only at the end. This is probably because stoichiometry of air relatively to char is too low for a complete oxidation.
The maximum temperature difference in the bed in the first part of the combustion test is very dependent on the combustion test: 10 °C for char from beech and bark, 25 °C for char from 85/15 wt% bark/wheat straw blend (not shown), 70 °C for char from 50/50 wt% bark/wheat straw blend, more than 100 °C for char from lignin residue.
This temperature difference in the bed can be a sign of heterogeneous fluidisation, and thus of possible bed partial agglomeration. However, after the combustion in bed is finished, the three in-bed temperature measurements converge toward the same value again for the char from beech, bark and blends. For the char from lignin residue, the temperature values remain far from one another (
Figure 2b). These results can be linked with the observations of the solid bed material after the tests, which are presented in
Section 3.4: Large agglomerates for lignin residue, and almost none for the blends.
3.2. Product Gas Yield and Composition in Gasification Experiments
First of all, elementary C, H and O mass balances were calculated for each feedstock gasification. These allow checking the overall accuracy of the results, and giving information about the distribution of the elements in the products (especially carbon which only comes from biomass).
At the inlet, we considered: the total mass of biomass fed into the reactor, together with elemental composition and moisture content (
Table 1), and the mass of steam fed into the reactor.
At the outlet, we considered:
The mass of dry gas obtained by integration over the whole gasification experiment (composition analysed by µGC, flowrate obtained with tracer gas method),
The mass of water trapped in condensers,
The mass and composition of condensable tar trapped in ‘tar protocol’,
The mass of fly residue on filters and cyclone recovered after gasification (this residue was supposed to be composed of carbon and ash only, and the ash content was measured on samples),
The mass of solid remaining in the bed determined with the combustion experiment, by quantifying the CO/CO2 released then (µGC analysis and tracer gas method).
For hydrogen and oxygen, the elemental balance was always incompletely closed with 7 to 20% lack. The difference to 100% was always the same for H and O, which is explained by the incomplete quantification of water at the outlet of the reactor. Indeed, the residual flow of water (steam) passing through the cold traps is not well quantified.
The carbon balance is presented in
Figure 3. The elemental balance is very close to 100% for every experiments, except from the first one with beech sawdust. This is attributed to the uncertainty of gas flowrate calculation using N
2 as tracer gas. This is one reason why helium was used for the subsequent experiments, giving better results.
The fraction of carbon in product gas is very similar for bark and for the 85/15 wt% bark/wheat straw blend (around 75%). It is a bit lower for the 50/50 wt% bark/wheat straw blend (69%), and for lignin residue (64%). Lignin residue is richer in lignin than the other feedstock studied here. The lower fraction of carbon in gas for this feedstock can be linked to the lower volatile content, and to the higher fixed carbon of lignin, compared to hemicellulose and cellulose [
21]. This could also explain why the fraction of carbon in char is between 20 and 25% for pelletised biomass, and seems to be a bit higher for lignin residue than for bark and bark-wheat straw blends. The fraction of carbon in fly residue (filter + cyclone) represents less than 7% of initial carbon. It seems to be higher for pellets than for sawdust, which is also observed for the fraction of C in char. This could be due to the initial form and size of the biomass, the pellets being globally heated at a slower rate than wood chips because of the internal thermal transfer limitation, which can lead to a lower gas yield [
22]. The fraction of carbon in tar is low (less than 1% for all feedstock except from lignin residue with about 3%).
The yield of each gas species analysed by µGC was determined all along each gasification experiment. So as to easily compare the results obtained with all feedstock, mean value of gas species yields and fractions were calculated for each experiment. The mean in-bed temperature was similar for all feedstock (834–840 °C). The mean dry gas composition (main gas species: H
2, CH
4, CO, CO
2, C
2H
4) is presented in
Figure 4. The results for beech sawdust, bark and the two bark/wheat straw blends are very close. The only differences concern CH
4 fraction, which seems to be slightly higher for beech, and slightly increasing with fraction of wheat straw blended to bark. Our results for bark/wheat straw blends are in good agreement with the dry gas composition obtained after the gasification of a 40% straw/60% wood blend in a dual fluidised bed gasifier [
11]: 38.5% H
2, 19.6% CO, 23.1% CO
2, 9.9% CH
4. The main differences come from lignin residue compared to the other feedstock: higher H
2 and CO fractions, and lower CH
4 and CO
2 fractions. Tian et al. [
21] investigated steam gasification of cellulose, hemicellulose, and lignin in a downdraft gasifier, and showed that the gas coming from lignin, at 900°C, was much richer in H
2 and poorer in CH
4, which is in agreement with our results. On the other hand, the authors [
21] found out that CO content in gas coming from lignin was lower than in gas coming from hemicellulose or cellulose, which is not in agreement with the present results.
The mean yields are presented in
Figure 5a for main gas species, and in
Figure 5b for minor ones. Beech sawdust results are not represented since the yields were probably underestimated, with a poor carbon balance as shown previously.
The differences observed between gas composition of lignin residue and of the bark containing feedstock are also visible for gas yields (higher H
2 and CO yields, and lower CH
4 and CO
2 yields for lignin residue). On the other hand, the H
2 yield seems to slightly decrease as the fraction of wheat straw blended to bark increases. These results concerning bark and bark/wheat straw blends seem to be in agreement with previous comparative results of wheat straw and wood steam gasification [
23], which tended to show that wheat straw gave a higher CH
4 yield and a lower H
2 one than woody biomass.
For the minor gas species (
Figure 5b), the main differences are also for lignin residue compared to the other feedstock, with higher yields in C
2H
2, H
2S, COS, C
6H
6 and C
7H
8. In particular, the benzene yield is nearly twice for lignin residue.
The cold gas efficiency (CGE), and LHV and yield of the dry product gas are presented in
Table 3 for each feedstock. The CGE is calculated as follows (Equation (2), in which
Qi is the mass flowrate of
i in kg·s
−1 and
LHVi is the lower heating value of
i in J·kg
−1):
Two values of CGE were calculated: the first one considering all gas species analysed by µGC, and the other without considering benzene and toluene which could be cleaned before the final synthesis, in a biomass-to-liquid process for instance.
For lignin residue, the cold gas efficiency is the lowest (66% without considering benzene and toluene against 73–78% for bark and blends), although the product gas LHV and yield are similar to those of the other feedstock. This is mathematically linked to the higher value of lignin residue LHV (
Table 1), which induces a lower cold gas efficiency (CGE) even if the energy content of the product gas is similar to the value for the other feedstock. The difference in CGE between lignin residue and the other feedstock is higher considering the product gas without benzene and toluene, as their yields were shown to be significantly higher for lignin residue (
Figure 5b). The lower CGE for lignin residue can be linked to a lower fraction of carbon in gas species than for the other feedstock (
Figure 3). Indeed, the carbon fraction to gas phase in gasification was shown to be much lower for lignin than for cellulose and hemicellulose [
24]. On the other hand, as the fraction of wheat straw in blend with bark is higher, the CGE tends to slightly decrease, which is linked to the decrease of product gas yield.
3.3. Tar Content and Composition
The condensable tar production (without considering BTX: benzene, toluene, xylenes), as determined with tar protocol, is shown in
Figure 6 for each feedstock (in g/kg of daf biomass). The tar molecules were classified according to their number of aromatic rings. The ‘other’ class is composed of nitrogen and sulphur-containing tars (pyridine and thiophene respectively). Lignin residue clearly leads to a higher tar production (19 g/kg
daf), than all the other biomass, for which the tar production is 4 g/kg
daf at maximum. The highest contribution always comes from molecules with 2 aromatic rings, naphthalene being the major contributor, followed by acenaphtylene. The two main molecules with one aromatic ring are indene and styrene. The ‘3 aromatic rings’ class stands for phenanthrene, anthracene and fluoranthene, and the ‘4 aromatic rings’ for pyrene.
The condensable tar and BTX concentrations in dry gas are indicated in
Table 4. For beach, bark and bark-wheat straw blends, the BTX content is significantly higher than the condensable tar one. For lignin residue, the BTX content is still higher but closer to the condensable tar content. In agreement with the results in
Figure 6, the highest tar concentrations in dry gas are for lignin residue. This result is in agreement with CO
2/steam gasification results obtained in a dual fluidised bed with the same feedstock (oak bark and lignin residue) [
25]: even if the gasification was performed at higher temperature for lignin residue, the tar content was significantly higher than for oak bark.
On the other hand, the tar contents for bark and bark/wheat straw blends are not much different. The same tendency was observed in dual fluidised bed gasification of wood and wood/wheat straw blends, which showed that the biomass type had a low influence on tar content compared to process parameters, such as gasification temperature [
12]. The authors measured a gravimetric tar concentration of 2–3 g/Nm
3 for a gasification temperature of 845 °C, which is similar to our results. Moreover, the major tar molecule class was that comprised of naphthalene and 1 and 2-methylnaphthalene, followed by the class of HAP (without naphthalene). This is also in agreement with the present results.
3.4. Characterisation of the Solid Residue
The particle size distribution of the solid residue was investigated after gasification and after combustion with the objective to find out if some agglomeration happened during the tests.
For gasification, the mass of solid residue determined in each particle size range is reported in
Figure 7, after dividing it by the mass of input dry biomass. The ash content of the gasification residues according to their particle size range is shown in
Table 5. For combustion, the mass of ash residue is directly reported in
Figure 8. In both cases, the mass of residue recovered in the 200–500 µm range is not shown, as it is much higher than the others, being the range in which most olivine particles remain. Moreover, the ash content after gasification is always higher than 93% (
Table 5), assessing that it mainly contains olivine.
After gasification, a distinct pattern can be observed between beech wood on the one hand, and the other biomass on the other hand. Indeed, beech wood was fed as millimetre-size particles which produced a much lower fraction of larger residue particles (>0.5 mm) than the pelletised biomass. The mass of fine particles in the <0.2 mm range was unfortunately unavailable for beech wood. For the other feedstock, it represents a significant fraction of residue after gasification. This range can contain fines coming from biomass but also from olivine attrition. Indeed, P-XRD analyses showed the presence of Mg1.7Fe0.3SiO4 and MgSiO3, which are characteristic of olivine. For all feedstock but beech wood, the fraction of residue particles with size higher than 0.5 mm was majority. Only particles with a size higher than 4.5 mm can be considered as agglomerates, and these were only observed after lignin residue gasification. Slight agglomeration was indeed visually observed after lignin residue gasification (large particles composed of sticked olivine particles). These findings are in good agreement with the online measurements of in-bed temperature during gasification.
After combustion, all particles with a size higher than that of olivine (0.5 mm) could be considered as formed by agglomeration, and especially the largest ones (over 0.9 mm). For beech wood, bark and the 85/15 wt% bark-wheat straw blend, the mass of particles larger than 0.5 mm is quite low (less than 32 g—
Figure 8). It is a bit higher for the 50/50 wt% bark-wheat straw blend (about 80 g) and much higher for lignin residue (1230 g). The observation of the largest particles from the 50/50 wt% bark-wheat straw blend show the presence of some agglomerates of olivine particles, but also of some white particles, which seem to contain biomass ash only. Concerning lignin residue, the largest particles seem to be essentially agglomerates of olivine particles.
Defining the agglomeration rate as the ratio between the mass of residue with a size higher than 0.5 mm after combustion and the input mass of inorganic material (olivine + biomass ash), its values are: under 1% for beech wood, bark, and the 85/15 wt% bark-wheat straw blend, 1.7% for the 50/50 wt% bark-wheat straw blend, and 32% for lignin residue. Here again, these results are well correlated to the temperature heterogeneity in the bed during combustion. Especially it was noticed that it was maintained until the end of the combustion for lignin residue, which presents a much higher agglomeration rate, contrary to the other feedstock.
The observation and analysis of agglomerates coming from lignin residue, both after gasification and combustion, show that the olivine grains are surrounded by phases enriched in Mg (from olivine) and Na (from lignin) (
Figure 9). This is a sign that a chemical reaction happened between the ash from lignin residue and olivine.
3.5. Prediction of Defluidisation and Comparison with Experimental Results
Fryda et al. [
10] observed that the defluidisation temperature determined with their experiments was slightly higher than the one calculated with thermodynamic equilibrium simulations, and attributed this discrepancy to the need for a critical amount of slag before defluidisation. Balland et al. [
26,
27] introduced the criteria of ‘critical liquid ash volume fraction in bed’ from which bed defluidisation happens. Moreover, the authors derived from a whole set of results from biomass gasification experiments and experiments with simulants at ambient temperature, a proportional relation linking this critical liquid fraction to the ratio of the superficial gas velocity over the minimum fluidisation gas velocity:
τL,c: critical liquid volume fraction (vol% of bed material)
Ug: superficial gas velocity (m/s)
Umf: minimum fluidisation velocity (m/s).
Note that this relation was established for a fluidisation ratio (Ug/Umf) comprised between 2 and 6, similarly to the conditions of our gasification and combustion experiments.
From this proportional relation, the same authors [
26,
27] proposed a semi-empirical relation to predict the time for complete defluidisation:
tdef: time before complete defluidisation (h)
ms: mass of bed material (kg)
ρL(T): density of the molten ash at operating temperature T (kg/m3)
ρs: density of the bed material (kg/m3)
Qbio: dry biomass feeding rate (kg/h)
τash: biomass ash content (wt% on dry basis)
XL(T): molten ash fraction at operating temperature T (wt% of total ash)
This last parameter (molten ash fraction) can be estimated with thermodynamic simulations, as described in
Section 2.5. The mass fraction of liquid ash (in g/g of total biomass ash) in steam gasification conditions is represented as a function of temperature in
Figure 10. Only bark is not represented, as the mass fraction of liquid ash is 0 whatever the temperature.
Temperature has a major influence on the mass fraction of liquid ash for the 2 bark-wheat straw blends. This is especially visible for the 50/50 wt% bark/wheat straw blend in the 800–1000 °C range, as it then increases from 4% to more than 40%. On the contrary, for lignin residue, temperature has a slighter and even inverse influence, as the mass fraction of liquid ash decreases from 36 to 29% between 800 and 1000 °C. This trend can be explained with the composition of the liquid ash, which is a solution mainly containing sodium and potassium carbonates and sodium chloride under 850 °C. Above 850 °C, as temperature increases, the mass of this carbonate and salt solution decreases whereas the mass of an oxide solution (mainly containing Na, Si and O) increases. At the same time, the K, Cl and Na release to the gas phase increases. These changes globally lead to the slight decrease of the liquid ash mass fraction between 800 and 1000 °C.
These liquid ash contents are qualitatively well correlated with the signs of agglomeration after gasification. In particular, the liquid ash content is the highest for lignin residue for which agglomerates were observed. However, for the 50/50 wt% bark-wheat straw blend, no agglomerate could be observed after gasification, even if the simulation predicts a small fraction of liquid ash.
The liquid ash fraction calculated for each feedstock at 850 °C, in the steam gasification conditions, was used for the prediction of the defluidisation time according to the relation presented above. These values of X
L (850 °C) are respectively 0 for bark and the 85/15 wt% bark-wheat straw blend, 6% for the 50/50 wt% bark-wheat straw blend, and 32% for lignin residue (
Figure 10). The mass of bed material is 5 kg for each experiment, and the bed material density is 3040 kg/m
3. The fluidisation ratio and the biomass feeding rate are given in
Table 2, and the biomass ash content is presented in
Table 1. At last, the density of molten ash was 2000 and 1810 kg/m3 for the bark-wheat straw blend and for lignin respectively. Indeed, according to the simulations, the liquid ash from the 50/50 wt% bark-wheat straw blend is an oxide solution. On the other hand, for lignin residue, the liquid ash at 850 °C is a solution mainly containing sodium and potassium carbonates. Thus the density of the molten ash was adapted accordingly [
27].
For all feedstock but lignin residue and the 50/50 wt% bark-wheat straw blend, no defluidisation is predicted since no liquid ash is predicted to be formed in the conditions of the experiment. For the 50/50 wt% bark-wheat straw blend and lignin residue, the time until complete defluidisation according to the previous relation is 5.2 h and 2.5 h respectively. The blend gasification duration was 1.5 h (
Table 2) and no sign of agglomeration or defluidisation was observed. As for lignin residue, the in-bed temperature started to become heterogeneous after 3 h (
Section 3.1) which was probably a sign of defluidisation, even if not complete. These experimental results are thus in good relative agreement with the calculated defluidisation times.
Thermodynamic simulations were also performed in air oxidation conditions as detailed in
Section 2.5. The mass fraction of liquid ash is represented as a function of temperature for each feedstock in
Figure 10. For each feedstock, the values are quite close to what was obtained for steam gasification. Note that this is probably linked to the sub-stoichiometric air oxidation conditions considered for the calculations to fit the measured CO and CO
2 release (
Section 2.5). A further simulation with the 50/50 wt% bark-wheat straw blend at 950 °C to investigate the influence of the equivalence ratio shows that above a value of 1, the mass fraction of liquid is 36%, slightly higher than 32% in sub-stoichiometric conditions.
The liquid mass fraction for the 85/15 wt% bark-wheat straw blend remains low even at 1000 °C. For the 50/50 wt% bark-wheat straw blend and lignin residue, the liquid mass fraction is quite similar to that of lignin residue at 900–950 °C, but differs substantially from it at lower temperatures. Moreover, further thermodynamic calculations considering olivine and lignin showed that the fraction of liquid ash was then even higher in the presence of olivine.
The oxidation experiments being performed as batch experiments, the relation predicting the time before defluidisation cannot be tested in these conditions. However, a defluidisation criteria was derived by extension of the findings of [
26], by stating that defluidisation occurs as soon as the liquid ash volume fraction in bed is higher than
τL,c. This criteria can be re-written, using the above-stated relations, as:
The parameters used for the calculations of this
C criteria are given in
Section 2.4, otherwise similar to what was used for the calculation of the defluidisation time.
The
C values are represented for each feedstock as a function of temperature in
Figure 11. For each feedstock, the
C value dependence on temperature is similar to the one of the mass fraction of liquid ash in
Figure 10. Indeed, in Equation 5, a constant value was considered for
ρL(
T) for the blends, while it only varied between 1810 and 2000 kg/m
3 for lignin residue, depending on the relative proportions of carbonate and oxide solutions in liquid ash. So the major temperature dependence comes from
XL(
T), which is shown in
Figure 10. For lignin residue, the
C value is slightly higher than the defluidisation limit up to 950 °C, and equal to 1 at 1000 °C. For the 50/50 wt% bark-wheat straw blend,
C increases a lot with temperature similarly to the mass fraction of liquid ash (
Figure 10). It is under the defluidisation limit for temperature under 900 °C approximately, and above the limit for higher temperatures. As for the 85/15 wt% bark-wheat straw blend, the
C value remains well under the limit whatever the temperature. These calculation results could explain why the in-bed temperature heterogeneity observed for lignin residue char combustion (
Figure 2b) and attributed to defluidisation, remained until the end of the test, whereas the three in-bed temperature measurements converged toward the same value again (around 900 °C) for the char from 50/50 wt% bark-wheat straw blend. For lignin residue, according to
Figure 11, the conditions were still propitious to defluidisation, whereas for the blend, with in-bed temperature decreasing under 900 °C, re-fluidisation could have happened.