Inﬂuences of the Pretreatments of Residual Biomass on Gasiﬁcation Processes: Experimental Devolatilizations Study in a Fluidized Bed

: The European research project CLARA (chemical looping gasiﬁcation for sustainable production of biofuels, G.A. 817841) investigated chemical looping gasiﬁcation of wheat straw pellets. This work focuses on pretreatments for this residual biomass, i.e., torrefaction and torrefaction-washing. Devolatilizations of individual pellets were performed in a laboratory-scale ﬂuidized bed made of sand, at 700, 800, and 900 ◦ C, to quantify and analyze the syngas released from differently pretreated biomasses; experimental data were assessed by integral-average parameters: gas yield, H 2 /CO molar ratio, and carbon conversion. A new analysis of devolatilization data was performed, based on information from instantaneous peaks of released syngas, by simple regressions with straight lines. For all biomasses, the increase of devolatilization temperature between 700 and 900 ◦ C enhanced the thermochemical conversion in terms of gas yield, carbon conversion, and H 2 /CO ratio in the syngas. Regarding pretreatments, the main evidence is the general improvement of syngas quality (i.e., composition) and quantity, compared to those of untreated pellets; only slighter differentiations were observed concerning different pretreatments, mainly thanks to peak quantities, which highlighted an improvement of the H 2 /CO molar ratio in correlation with increased torrefaction temperature from 250 to 270 ◦ C. The proposed methods emerged as suitable straightforward tools to investigate the behavior of biomasses and the effects of process parameters and biomass nature.


Introduction
The EU's Renewable Energy Directive (RED II) has set the goal of achieving a 14% renewable energy share in the transport sector by 2030 [1], and residual biomasses and agro-industrial waste can be exploited as sources to produce sustainable second generation biofuels [2], which are expected to significantly reduce greenhouse gas emissions [2,3].
The gasification of residual biomass, to produce advanced biofuels, is a promising technology to achieve the goals of RED II. Gasification is a mature thermochemical conversion process suitable for biomasses, with syngas (mixture of H 2 , CO, CO 2 , CH 4 , possibly diluted by steam and/or N 2 [4]) as the main product; syngas is primarily used to generate heat and electricity, and is potentially exploitable to synthesize advanced biofuels (the latter option has not yet been fully implemented at the industrial scale) [5]. Gasification consists of partial oxidation of the carbon contained in the biomass (or in other carbonaceous fuels) at high temperature (750-1150 • C [6]), using a controlled amount of an oxidant agent (air, pure oxygen, steam, or mixtures of them) [6]. Pure oxygen ensures the production of a high heating value and nitrogen-free syngas, the latter feature being advantageous for the synthesis of biofuels; however, the provision of pure oxygen requires an air separation This work thoroughly investigates the influence of some pretreatments on the thermochemical decomposition of wheat straw and the produced syngas. In this regard, experiments were carried out involving devolatilizations in a fluidized bed made up of sand. Devolatilization is a key step of a generic gasification process, and strongly influences the amount and composition of the produced gas [22], so devolatilization results may detect possible primary effects of pretreatments on the thermochemical behavior of wheat straw. The fluidized bed made up of inert sand was chosen in so that: (i) biomasses could be studied in a reactor configuration similar to that of CLG developed in the CLARA project; (ii) possible redox effects from solids (such as OC) could be excluded and only those influences strictly due to biomass pretreatments could emerge. Devolatilization tests were performed on pellets of differently pretreated biomasses, firstly elaborating data by methods described elsewhere [10,23]. In addition, a further analysis of the same data was introduced, based on information taken from the instantaneous peaks of gas release during devolatilizations, treated by simple regressions with straight lines. This represents a point of novelty, since the introduced method is quite straightforward as far as both experimental and mathematical approaches are concerned.

Investigated Biomasses
The biomass investigated in this work is wheat straw, one of the biogenic residues selected within the CLARA project [10,15,16,24]. Wheat straw was in the form of pellets, useful to facilitate their transport, storage, and handling, and closer to its possible commercial utilization. Those wheat straw pellets underwent some pretreatments, i.e., torrefaction and torrefaction followed by washing, as described extensively elsewhere [10,[15][16][17]25]. The torrefied and torrefied-washed pellets were also compared to the untreated wheat straw pellets (studied elsewhere [10]), which were considered as a reference material to infer the effects from pretreatments on syngas, if any. From here on, biomasses are named as indicated in Table 1. These biomasses were characterized by proximate and ultimate analyses, which allowed determining the moisture and ash contents, and the elemental composition; some of these data are available in [17,24] and were used in Equation (3) of this work. Chemical analysis and ash melting tests were also performed on investigated biomasses, as reported by Di Giuliano et al. [15], to quantify respectively the content of inorganics and the melting temperature of biomass ashes.

Bed Material and Conditions of Devolatilization Tests
Devolatilizations were carried out in a laboratory scale fluidized bed reactor. The granular bed was made up of sand, an inert material used to perform devolatilizations in the absence of particles with proven oxidizing properties (such as OC exploited in CLG). The physical properties of sand are summarized in Table 2. Nitrogen (N 2 ) was used as the fluidizing gas, to avoid the provision of external oxygen, at 1.5 times the minimum fluidization velocity of sand, so to ensure similar fluid-dynamic conditions for all tests. Under the selected conditions (700, 800, and 900 • C in N 2 ), sand particles (Table 2) belong to the Group B of generalized Geldart classification [26]. The minimum fluidization velocities (u mf ), calculated according to the method adopted by Di Giuliano et al. [15,27,28], are summarized in Table 2. Table 2. Physical and fluid-dynamic properties of sand, adapted from [10]: particles diameter (d p ) and particle density (ρ p ); minimum fluidization velocity (u mf ) in N 2 as a function of temperature (T), with the indication of the related generalized Geldart Group [26].

Material
Sand

Experimental Apparatus and Procedure for Devolatilization Tests
Devolatilization tests, as anticipated in Section 2.2, were carried out for all biomasses listed in Table 1 at three temperature levels (700, 800, and 900 • C), with N 2 as the fluidizing agent, in a fluidized bed made up of sand. The related experimental apparatus at laboratoryscale was depicted and fully described in detail elsewhere [10,23]. For the sake of clarity, it is also briefly described in the following.
A mass flow controller allowed N 2 to be fed into the windbox of a cylindrical quartz reactor (5 cm internal diameter), in which the devolatilizations took place. Sand was loaded inside, in such a quantity to form a 7.5 cm high bed (1.5 times the internal diameter of the reactor). The reactor was heated by a cylindrical electric furnace, with temperature controlled by a thermocouple directly submerged in the bed. The syngas produced by devolatilizations left the reactor freeboard together with N 2 , and both passed through an ice trap, which operated a first separation of condensable species and entrained fine solids. Downstream, a double-pipe condenser (ethylene glycol on the shell-side at −4 • C, gas flow on the tube-side) allowed the forced separation of water and other condensable substances. The dry and cold syngas passed through filters for a further cleaning, then reached gas detectors: (i) a micro-gas chromatograph (µGC) (Agilent 490, Agilent Technologies Italia S.p.A., Cernusco sul Naviglio (MI), Italy), to identify the hydrocarbons in the syngas (qualitative identification from a not exhaustive list of detected species, as discussed in [10,23]); (ii) an online ABB station, with analyzers measuring the volumetric concentrations of H 2 , CO, CO 2 , CH 4 , and hydrocarbons expressed in ppm of "equivalent C 3 H 8 ". From here on, equivalent C 3 H 8 is named "C 3 H 8,eq. " and such quantity excludes CH 4 , separately measured and accounted.

Processing of Devolatilization Data
For each pair "biomass kind-temperature", three repetitions of devolatilization were performed (i.e., three pellets of the same kind were devolatilized at each temperature).
Each pellet was devolatilized individually and completely before feeding the following one. Because of this procedure, as already evidenced in [10,23], the experimental process is intrinsically at unsteady-state.
Thanks to the hypothesis of N 2 as the internal standard, it was possible to determine the instantaneous molar flow rates of the gases (F i,out , with i as the generic gaseous species produced by the devolatilization tests, quantified by the ABB system: H 2 , CO, CO 2 , CH 4 , C 3 H 8,eq. ). Figure 1 (Section 3.1) shows examples of these instantaneous flow rates from individual devolatilizations as functions of time (t), characteristically shaped as asymmetric peaks [29,30]. C3H8,eq.). Figure 1 (Section 3.1) shows examples of these instantaneous flow rates from individual devolatilizations as functions of time (t), characteristically shaped as asymmetric peaks [29,30]. The evaluation of devolatilization performances by integral-average values, already proposed by the same research team in [10,23], was adopted in this work to calculate: gas yield (η av , Equation (1)); H2/CO molar ratio (λ av , Equation (2)); carbon conversion (χc av , Equation (3)); the superscript "av" means "integral-average".
As to these parameters (Equations (1)- (3), the arithmetic average out of the three repetitions and the related standard deviation were calculated for each set "biomass kind-temperature", and the resulting values were represented by bar-charts in Figure 2 (Section 3.1).

Figure 1.
Example of H 2 , CO, CO 2 , CH 4 , and C 3 H 8,eq . outlet molar flow rates (F i,out ) as functions of time, produced by devolatilizations in the sand fluidized bed of (a) WSP at 800 • C; (b) WSP-T1 at 900 • C. WSP data adapted from [10].
with j = CO, CO 2 , CH 4 and C 3 H 8,eq. ; m p = mass o f pellet (g) n j = number o f carbons atoms in j %moisture ar = moisture content as wt% in as recieived (ar) biomass%ash db = ash content as wt% in biomass on dry basis (db) %C da f = elemental carbon as wt% in biomass on dry ash f ree basis (da f ) As to these parameters (Equations (1)-(3), the arithmetic average out of the three repetitions and the related standard deviation were calculated for each set "biomass kind-temperature", and the resulting values were represented by bar-charts in Figure (1)); (b) integral average H2/CO molar ratio (λ av , Equation (2)); (c) integral average carbon conversion (χc av , Equation (3)); WSP data adapted from [10].
This work introduces a further method to analyze devolatilization performances, which focuses on the quantitatively most representative moment of unsteady-state devolatilizations of individual pellets, i.e., the top of Fi,out devolatilization peaks as functions of time (see Figure 1), when the highest gas release occurred.
The procedure to elaborate this data follows: 1. for each set "biomass kind-temperature" and for each of the three repetitions, the highest released flow rate (i.e., peak top of Fi,out in Figure 1) was identified for each quantified species (i = H2, CO, CO2, CH4, and C3H8,eq.); 2. a neighborhood of 7 Fi,out experimental points was selected, centered on the considered peak top; 3. the arithmetic average ( , , , Equation (4), where "p" superscript means "peak") was calculated out of these 7 points.
This work introduces a further method to analyze devolatilization performances, which focuses on the quantitatively most representative moment of unsteady-state devolatilizations of individual pellets, i.e., the top of F i,out devolatilization peaks as functions of time (see Figure 1), when the highest gas release occurred.
The procedure to elaborate this data follows: 1. for each set "biomass kind-temperature" and for each of the three repetitions, the highest released flow rate (i.e., peak top of F i,out in Figure 1) was identified for each quantified species (i = H 2 , CO, CO 2 , CH 4 , and C 3 H 8,eq. ); 2.
a neighborhood of 7 F i,out experimental points was selected, centered on the considered peak top; 3.
the arithmetic average (F ,p i,out , Equation (4), where "p" superscript means "peak") was calculated out of these 7 points.
In addition, the distribution among peaks of released gases-namely H 2 , CO, CO 2 , CH 4 , and C 3 H 8,eq. -was calculated, in terms of molar fractions on a nitrogen-free basis  (5)). For each temperature value and each gaseous species, three points were obtained (one per test), corresponding to the three repetitions for each kind of biomass; therefore, for the generic gaseous species i, 9 values of Y p i,out were obtained, evenly distributed on the three temperature levels 700, 800, 900 • C.
Moreover, a parameter called "specific maximum gas production" (SMGP) was introduced and calculated by Equation (6). This parameter is a local value expressed as a specific gas yield per unit of biomass and unit of time, which focuses on the devolatilization phenomenon around the peak top of released gas flow rate.
m p with i = H 2 , CO, CO 2 , CH 4 and C 3 H 8,eq. ; m p = mass o f pellet (g) (6) Analogously to Y p i,out , 9 SMGP values resulted for each kind of biomass, evenly distributed on the three temperature levels 700, 800, 900 • C.
For each 9-points set of Y p i,out or SMGP as functions of devolatilization temperature, a regression was performed by means of dedicated Microsoft Excel tool, under the assumption of straight line (Equation (7)) as the modeling equation for Y p i,out or SMGP dependency on devolatilization temperature (T). This assumption was supported by observing the approximately linear trends of devolatilization performances experimentally determined by Zeng et al. [30], with tests at different temperature levels, progressively increased by 50 • C in the range 600-900 • C.
3. Results Figure 1 shows two examples of results (out of 63), obtained from devolatilizations of individual pellets, expressed in terms of F i,out . As already reported by [10,23], F i,out curves as functions of time have an asymmetrical shape, due to the unsteady-state of each devolatilization. Figure 2 shows the overall results of the devolatilization tests carried out using sand as the bed material, in anoxic conditions due to N 2 supply, at three temperature levels (700, 800, and 900 • C). The data of the three repetitions of the untreated WSP pellets were adapted from [10]. The bar-charts in Figure 2 summarize the devolatilization results in terms of integral-average parameters: (i) gas yield (η av , Equation (1)), (ii) H 2 / CO molar ratio (λ av , Equation (2)), and (iii) carbon conversion (χ av C , Equation (3)), calculated by the procedure described in Section 2.4. For each set "biomass kind-temperature" in Figure 2, the bar heights represent the average values of the considered quantity out of the three repetitions, the associated error bars represent the related standard deviations.

Results from Devolatilization Peaks: Regression Analyses
As described in Section 2.4, the molar fractions on N 2 -free basis (Y p i,out , Equation (5)) of the gases and the SMGP (Equation (6)) were calculated, focusing on the peaks top of gas release during devolatilizations of individual pellets. Figure 3 shows the results of this calculations from devolatilizations of WSP pellets at each temperature level, provided with regression straight lines (Equation (7)). For the sake Appl. Sci. 2021, 11, 5722 8 of 18 of clarity, the slopes (m, Equation (7)) and y-axis intercepts (q, Equation (7)) of regression straight lines were collected in Table 3 for devolatilizations of WSP pellets.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 19 As described in Section 2.4, the molar fractions on N2-free basis ( , , Equation (5)) of the gases and the SMGP (Equation (6)) were calculated, focusing on the peaks top of gas release during devolatilizations of individual pellets. Figure 3 shows the results of this calculations from devolatilizations of WSP pellets at each temperature level, provided with regression straight lines (Equation (7)). For the sake of clarity, the slopes (m, Equation (7)) and y-axis intercepts (q, Equation (7)) of regression straight lines were collected in Table 3 for devolatilizations of WSP pellets.

Discussion
Before a detailed discussion of devolatilization results, it is worth to stress that this study aimed to strictly examine the influences from wheat straw pretreatments and devolatilization temperatures on pellets thermochemical behavior. For this reason, tests were carried out in an inert atmosphere and with a unique bed material, devoid of those oxidative properties typical of the OC investigated within CLARA project [5,10,15,31]. In such a way, results did not depend on the type of bed material or any external oxygen supply. Figure 2 highlights that the devolatilization temperature is a parameter with a significant effect on devolatilization performances, whatever the considered biomass kind; for all biomasses, the gas yield (η av , Equation (1), Figure 2a) and the H 2 /CO molar ratio (λ av , Equation (2), Figure 2b) grew as the temperature was increased. With regard to the carbon conversion (χ av C , Equation (3), Figure 2c), the difference between values at 800 and 900 • C was not always evident (net of standard deviations), so that trends with respect to temperature were not as much clear as in the case of η av and λ av . Anyway, one can state that the temperature increasing from 700 to 800 • C always improved χ av C . As a matter of fact, Wang et al. [32] reported how the gas yield, the carbon conversion and the H 2 and CO content in the syngas increased as the devolatilization temperature was increased, with experiments on sawdust pellets in a fluidized bed reactor, within the range 750-950 • C. Consequently, considerations about Figure 2 may suggest that one should obtain the best performance of thermochemical conversion of wheat straw biomasses by operating at the highest tested temperature (900 • C).

Integral-Average Quantities
In general, the increase in gas yield (η av , Equation (1), Figure 2a) is not necessarily accompanied by an improvement in syngas quality (e.g., in terms of H 2 /CO ratio for Fischer-Tropsch synthesis). Remarkably, in the case of study of this work, the H 2 /CO molar ratio (λ av , Equation (2), Figure 2b) grew together with gas yield (η av , Equation (1), Figure 2a) as the temperature was increased; in other words, there is a general improvement in the quality and quantity of the syngas due to the increase of devolatilization temperature, which in turn appeared to enhance the extent of reforming and cracking reactions.
In addition to the devolatilization temperature influence, minor effects due to pretreatments were observed on devolatilization performances.
A comparison between the results of torrefied pellets (WSP-T1, WSP-T2, WSP-T3) and those of WSP, suggested that: • the η av of torrefied pellets was close to that of WSP (Figure 2a), with differences even less evident if standard deviations are taken into account; • the λ av of torrefied pellets is slightly higher than that of WSP ( Figure 2b); • with torrefied pellets, a substantial decrease of the χ av C emerged in comparison to the same quantity of WSP ( Figure 2c); this is in agreement with the expected effects of the torrefaction pretreatment (defined elsewhere [17]). As highlighted by Fan et al. [33], torrefaction can lead to a reduction of carbon conversion in the thermochemical conversion of the biomass, because of devolatilization, polycondensation, and carbonization which occur during the pretreatment; as a matter of fact, Niu et al. [34] referred that torrefaction increased the elemental carbon content per unit of mass, because of the release of volatiles (such as water and CO 2 ), which in turn made the biomass properties shift towards those of coal [35].
With regard to torrefied samples (WSP-T1, WSP-T2, WSP-T3), a further focus was performed on effects of torrefaction temperature: • no evident influences emerged on gas yield (η av , Equation (1), Figure 2); The highest H 2 /CO molar ratio (λ av , Equation (2), Figure 2b) resulted for WSP-T3; Zhang et al. [36] found that 270 • C was the best torrefaction temperature for pelletized pine and spruce sawdust, among investigated values of 240, 270, 300, and 330 • C: they carried out devolatilizations by thermogravimetric measurements and determined, by kinetic analyses, that the activation energy of H 2 release was minimum when the torrefaction temperature was equal to T3 (270 • C).
Concerning the torrefied-washed samples (WSP-T1W, WSP-T2W, WSP-T3W), a comparison with the corresponding torrefied pellets (WSP-T1, WSP-T2, WSP-T3) evidenced that η av (Figure 2a), λ av (Figure 2b), and χ av C (Figure 2c) did not substantially vary, net of standard deviations. The washing pretreatment after torrefaction did not produce significant improvements, at least in terms of gas yield, H 2 /CO molar ratio, and carbon conversion. These results can be justified by considering that the washing pretreatment  Figure 5 and Table 5 summarize the results from the regression data analysis regarding the SMGP parameter (Equation (6)). Figures 3b and 5 suggest that the predominant effect on SMGP derives from the increase of the devolatilization temperature: for all the biomasses, SMGP increased as the devolatilization temperature was increased, in agreement with the integral average gas yield (η av , Equation (1), Figure 2a). This corroborates the reliability of both analysis methods.
Overall, by comparing SMGP lines in Figure 6f, substantial differences did not emerge in relation to pretreatments.

Conclusions
In this work, devolatilization tests of untreated and pretreated wheat straw pellets were carried out at three temperature levels (700, 800, and 900 • C), in a fluidized bed made up of sand.
Integral-average gas yield, H 2 /CO molar ratio, and carbon conversion were determined from gas release data obtained by devolatilizations of individual pellets. Whatever the considered biomass, all these parameters increased as the temperature was increased, with a general improvement in syngas quality and productivity. Concerning the specific pretreatments:

•
No evident influences on the integral-average gas yield emerged; • All pretreated wheat straw pellets showed integral-average H 2 /CO molar ratios higher than those of untreated wheat straw: the highest value was recorded for wheat straw pellet torrefied at 270 • C (the highest explored devolatilization temperature); • Integral-average carbon conversion of untreated wheat straw pellets was significantly higher that of pretreated pellets; • The washing pretreatment after torrefaction did not produce significant improvements in term of integral-average gas yield, H 2 /CO molar ratio, and carbon conversion, when compared to only-torrefied ones.
Because of the intrinsically unsteady-state of devolatilizations (performed for individual pellets), a new analysis method of devolatilization data was proposed, focused on the peak in the experimental curves of released flow rates of syngas components (CO, CO 2 , H 2 , CH 4 , and hydrocarbons as C 3 H 8,eq. ). Trends of syngas compositions as functions of devolatilization temperature were obtained by regressions with straight lines. Similarly, trends regarding the parameter "specific maximum gas production" were also obtained. As to fractions of gas species, the regressed trends offered some further information, which were not inferred from the previous integral-average analysis:

•
The higher the devolatilization temperature, the greater the H 2 fraction in the syngas, at the expenses of CO 2 , CH 4 , and hydrocarbons; • All pretreatments improved the H 2 /CO molar ratio related to peaks, in comparison to the same ratio obtained from untreated wheat straw; • A direct influence from torrefaction temperature emerged on H 2 /CO molar ratio related to peaks, corroborating the less clear indication obtained by the integral average analyses.
Observations from the two kinds of analysis were in fair agreement with literature. The integral average estimations and the regression peak analysis both appeared as general and straightforward methods to investigate the thermochemical behavior of biomasses, as well as the influences from operating conditions and biomass nature. Together with the experimental procedure of devolatilization of a few pellets, they constitute a faster and simpler procedure to select the more promising biomasses and operating conditions during a preliminary screening phase, in comparison to a more complex and time-demanding experimental campaign based on a continuous gasification process.
An additional outcome of this work is the provision of elaborated experimental data for further studies with modeling purposes, which also allow careful extrapolation (out of the experimentally explored temperature range) by means of linear regressed trends.
As a general remark, the torrefaction pretreatment brings in several advantages (e.g., grindability, pelletability, storability, increased heating value, higher H 2 /CO molar ratio in devolatilized syngas), while the related operational costs may be limited-thanks to the low required temperatures-and easily compensated via heat recoveries in the intensified industrial configuration of a CLG plant.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.