Influence of Oxygen/Steam Addition on the Quality of Producer Gas during Direct (Air) Gasification of Residual Forest Biomass

Abstract

Biomass gasification is a relevant option to produce a gaseous fuel, it faces, however, several barriers regarding its quality for energetic applications. Therefore, in this study, air-steam and O₂-enriched air mixtures were used as gasification agents during the gasification of residual biomass from eucalyptus to improve the producer gas quality. The steam addition promoted an increase in CO₂ and H₂ concentrations, whilst decreasing the CO and CH₄ concentrations. The steam addition had no evident impact on the lower heating value of the dry producer gas and a positive effect on gas yield and the H₂:CO molar ratio, attaining the later values up to 1.6 mol_H2∙mol⁻¹_CO. The increase in O₂ concentration in the gasification agent (φ) promoted an increase in all combustible species and CO₂ concentrations. The lower heating value of the dry producer gas underwent an increase of 57%, reaching a value of 7.5 MJ∙Nm⁻³_dry gas, when the φ increased from 20 to 40 %vol._O2, dry GA. The gas yield had a significant decrease (33%) with φ increase. This work showed that the addition of steam or O₂ during air gasification of residual biomass improved producer gas quality, overcoming some of the barriers found in conventional air gasification technology.

1. Introduction

Our 21st-century society faces several challenges related to climate change and energy use. In 2015, all United Nations members approved and adopted the 2030 Agenda for Sustainable Development towards the improvement of life quality and promotion of the planet’s sustainability, defining various goals to be achieved by 2030 [1]. Later, and within the bioeconomy framework, the European Commission developed the European Green Deal, providing a set of objectives for 2050, targeting several subjects concerning energy use and climate change. Both frameworks state the importance of using cleaner and renewable energy and the urgency of reducing greenhouse gas emissions [2]. However, in 2021, energy-related CO₂ emissions reached 36.3 Gton, the highest annual value measured so far [3]. Moreover, although the amount of energy supplied by renewable resources increased by approximately 14%, between 2010 and 2019 [4], the share of renewables in the total energy consumption was only 17.7% in 2019 [5].

Therefore, different choices must be taken towards an environmentally sustainable planet. In this regard, the gasification of lignocellulosic biomass residues appears as a relevant option, providing a renewable green fuel gas with numerous applications and promoting energy independence whilst boosting the use of a non-intermittent endogenous renewable resource, and being highly available and widely distributed around the world [6,7,8]. Furthermore, this process favors the reduction in global CO₂ emissions and has a low environmental impact [9].

The gasification process is often described as a high-temperature (usually above 700 °C) conversion of a carbonaceous matrix into a fuel gas—producer gas—mainly composed of carbon monoxide, carbon dioxide, hydrogen, water, methane, and other light hydrocarbons, under a controlled amount of an oxidant or so-called gasification agent [10,11]. During the process, several reactions can occur (Table 1) and their direction and rate are highly dependent on the process conditions and biomass composition [12,13].

The method of introducing oxygen into the process has a major impact on the producer gas composition, heating value, and efficiency parameters [14]. The oxygen can be introduced using different gasification agents [16]. Owing to its availability and low price, air is the most common gasification agent, promoting the partial combustion of the fuel, thus providing the required energy to maintain the process in an autothermal regime [17]. However, although this technology is considered a promising solution for a green and clean energy transition, the process still faces several challenges, hindering its full implementation at the industrial level and commercial breakthrough [18]. Amongst different barriers, some technical issues can be identified regarding direct (air) biomass gasification, including [19]:

• Reduced lower heating value of dry producer gas (LHV_G), ~5 MJ∙Nm⁻³_{dry gas}, due to the dilution of N₂.

• Low H₂ concentration [~8 %vol._{dry gas}], the only carbon-free fuel gas in the producer gas.

• Reduced H₂:CO molar ratio [0.3 and 0.8 mol_H2∙mol⁻¹_CO] [19]—an important parameter for advanced applications, including Fischer-Tropsch synthesis for liquid fuels production ((0.6 or 2.0 mol_H2∙mol⁻¹_CO), depending on the catalyst applied), dimethyl ether (DME) synthesis (1 mol_H2∙mol⁻¹_CO), bioethanol synthesis (between 1 and 2 mol_H2∙mol⁻¹_CO) and methanol synthesis (2 mol mol_H2∙mol⁻¹_CO) and biomethane production (3 mol_H2∙mol⁻¹_CO) [7,20,21,22].

To overcome these barriers, other gasification agents can be applied, namely steam water, hereinafter referred to as “steam”, and pure oxygen (O₂). Steam leads to a H₂-rich producer gas, with H₂ concentrations between 40 and 60 %vol._{dry gas}, with a high H₂:CO molar ratio, up to 8 mol_H2∙mol⁻¹_CO, and an LHV_G ranging from 9 to 15 MJ∙Nm⁻³_{dry gas} [23,24,25,26], overcoming all the aforementioned constraints in air gasification. However, steam is less reactive than air or O₂, and most reactions involving steam are endothermic and slower when compared to oxidation reactions [14]. Therefore, to meet the energy needs of this process, an external source of heat is mandatory, increasing its overall costs [14].

The use of pure O₂ is considered an interesting alternative to air as a gasification agent, increasing the LHV_G (ranging between 10 and 15 MJ∙Nm⁻³_{dry gas}), and the H₂ concentration, up to 40 %vol._{dry gas} [27], whilst allowing an autothermal regime. However, pure O₂ generation is an expensive process [27], and the use of this gasification agent requires a higher control of the process due to the increment of the solid fuel reactivity with increasing O₂ concentration [28]. Moreover, an increase in O₂ concentration will promote the increment of the oxidation reaction rate, according to the law of mass action [29]. This may lead to operating difficulties associated with an increase in the heat released and temperature.

To find a balance between the strengths and weaknesses of these gasification agents when applied alone, mixtures of air-steam or air-O₂, hereinafter referred to as “O₂-enriched air” can be used as a suitable alternative approach. Moreover, if these agents (pure O₂ and steam) are by-products from other processes (e.g., O₂ from water electrolysis for H₂ production) or industries (e.g., steam from the pulp and paper industry), the overall costs of the process can be reduced.

Gasification using a mixture of air and steam as a gasification agent has been investigated in several studies and its effect on the producer gas quality has been evaluated [9,30,31,32]. Tian et al. [30] carried out several experiments of air-steam biomass gasification in a bubbling fluidized bed reactor and observed an increase in both H₂ and CO₂ concentrations on the producer gas composition, accompanied by a decrease in the CO and CH₄ concentrations when the steam-to-biomass mass ratio (SBR) increased from 0.6 to 1.0 kg_steam∙kg⁻¹_biomass. However, Lv et al. [31] studied the impact of SBR on the producer gas quality during air-steam gasification of pine sawdust in a fluidized bed reactor and found that an increase from 0 to 1.35 kg_steam∙kg⁻¹_biomass in the SBR led to a decrease in the H₂ and CO concentrations and an increase in the CO₂ and CH₄ concentrations. The authors also observed that an increment in the SBR from 1.35 to 2.70 kg_steam∙kg⁻¹_biomass led to a gradual decrease in the CO and CH₄ concentrations, whereas CO₂ and H₂ concentrations showed a moderately increasing trend [31]. The LHV_G decreased when the SBR increased from 1.35 to 2.70 kg_steam∙kg⁻¹_biomass, having the gas yield and carbon conversion followed the same path [31]. The authors concluded that these results were caused by the decrease in the reactor temperature with the steam addition, lowering the producer gas quality [31]. Campoy et al. [9] performed experiments of air-steam gasification of wood pellets in a bubbling fluidized bed reactor and observed that both H₂ and CO₂ concentrations increased with the increment of SBR from 0 to 0.45 kg_steam∙kg⁻¹_biomass, whilst the CO and CH₄ concentrations decreased. Moreover, the authors also noticed an increase in the H₂:CO molar ratio in the increment of SBR, whereas the cold gas efficiency, LHV_G, and carbon conversion were unaffected [9]. Tian et al. [32] performed several experiments of air-steam biomass gasification using coal-bottom ash and observed that an increment in the SBR from 0.34 to 1.02 kg_steam∙kg⁻¹_biomass had a positive impact on the H₂:CO molar ratio, followed by a marginal effect on the LHV_G.

Some studies were found regarding the addition of O₂ during air biomass gasification [15,33,34,35]. Wang et al. [33] studied the O₂-enriched air gasification of biomass in a two-staged pilot-scale gasifier and observed an increment of all combustible species and CO₂ concentrations on the producer gas with the increment of O₂ in the gasification agent. Moreover, the authors also noticed that the gas yield decreased with the increase in the O₂ in the gasification agent, whereas the carbon conversion and LHV_G followed an opposite direction [33]. The authors concluded that these results were caused by the decrease in the nitrogen concentration in the gasification agent and the temperature increase in the reactor from the O₂ addition. Mastellone et al. [34] performed O₂-enriched air gasification of mixtures of coal, plastics, and wood in a fluidized bed gasifier and observed that, for the same temperature, the CO concentration (inert free) increased, whereas the CO₂ concentration (inert free) decreased with the increment of O₂ concentration in the gasification agent. The authors also observed an increase in the LHV_G and the cold gas efficiency, followed by a decrease in the gas yield [34]. Liu et al. [35] analyzed the effect of O₂ concentration in the gasification agent during the O₂-enriched air gasification of biomass in a fluidized bed reactor. The authors observed that the increment of O₂ concentration had a positive effect on the producer gas composition, increasing the volumetric fraction of all combustible species and CO₂ [35]. Moreover, this parameter also had a positive impact on the LHV_G, carbon conversion, and cold gas efficiency, with the gas yield being the only parameter negatively affected by the O₂ addition [35]. Liu et al. [15] studied the influence of the O₂ concentration in the gasification agent during biomass gasification. When the O₂ concentration increased from 21 %vol. (air) to close to 30 %vol., the authors observed an increment in H₂, CO, CO₂, and CH₄ concentrations, accompanied by an increase in the LHV_G, gas yield, carbon conversion, and cold gas efficiency [15]. However, even though the CO, CO_2, and CH₄ concentrations continued to increase with the O₂ concentration increase from 30 %vol. to, approximately, 45 %vol., the H₂ concentration firstly remained constant and then decreased [15]. For these conditions, the gas yield decreased and the carbon conversion remained constant, whilst the LHV_G and cold gas efficiency increased [15].

Despite the growing interest in air-steam and O₂-enriched air biomass gasification process, information regarding the effect of steam or O₂ air on the producer gas quality during air gasification of lignocellulosic residues is still scarce. Moreover, there are still some gaps in the literature that prevent a thorough knowledge of the influence of the addition of steam/O₂, without the side effects of other parameters (e.g., temperature, equivalence ratio). Temperature plays a major role during biomass gasification and the addition of steam or O₂ will affect this parameter, changing the reactions kinetics of the process, gas composition, and the overall producer gas quality.

Therefore, this work intends to deliver new information on the effect of steam or O₂ addition on the producer gas characteristics, during air gasification of residual forest biomass from eucalyptus, whilst maintaining other process parameters constant (e.g., temperature and equivalence ratio), in order to evaluate their true influence on: (a) average producer gas composition and gas compounds yield; (b) LHV_G; (c) performance parameters, namely specific dry gas production, cold gas efficiency, and carbon conversion efficiency; and (d) H₂:CO and CO:CO₂ molar ratios.

The results attained may add knowledge in the field of biomass gasification, and help this technology reach technical feasibility and full implementation.

2. Materials and Methods

2.1. Feedstock Characterization

Residual forest biomass (RFB) from Eucalyptus globulus (a lignocellulosic residue) is a waste stream from the pulp and paper industry, with more than 75,000 tons generated in 2020, in Portugal [36]. Bearing that in mind, a mixture of RFB from eucalyptus (E. globulus) was used in the gasification experiments carried out in this work. The feedstock included biomass from two different activities: forestry operations, including tree logging (branches, bark, and foliage); and industrial operations, namely woodchip production for pulp production (sawdust). Once prepared, the mixture was air-dried to ensure the thermochemical needs of the process and pelletized to increase the physical characteristics in terms of feedstock uniformity. Finally, the pellets were crushed and sieved between 2 and 4 mm to fulfill the requirements of the feeding system.

Table 2 shows the residual biomass characterization. The proximate analysis was performed following the CEN/TS standards for the determination of moisture content, ash content, and volatile matter content, whilst the Fixed Carbon content was calculated by difference. The ultimate analysis was performed by an external laboratory using a CHNS Elemental analyzer (Fisons Instruments, model EA1108). The LHV_B calculation was made considering the higher heating value of dry biomass (HHV_B) estimation, following the empirical correlation developed by Channiwala and Parikh [37], based on the ultimate analysis of the fuel, as shown by Equations (1) and (2).

(1)${\mathrm{HHV}}_{\mathrm{B}}=0{.3491\mathrm{w}}_{\mathrm{CB} }+1{.1783\mathrm{w}}_{\mathrm{HB}}+0{.1005\mathrm{w}}_{\mathrm{SB}}- 0{.1034\mathrm{w}}_{\mathrm{OB}}- 0{.0151\mathrm{w}}_{\mathrm{NB}}- 0{.0211\mathrm{w}}_{\mathrm{ashB}}$

(2)${\mathrm{LHV}}_{\mathrm{B}}{=\mathrm{HHV}}_{\mathrm{B}}-{ \mathrm{h}}_{{\mathrm{H}}_2\mathrm{O}, 298\mathrm{K}} \times \left({\mathrm{w}}_{{\mathrm{H}}_2\mathrm{OB}}{+\mathrm{w}}_{\mathrm{HB}} \times \frac{{\mathrm{M}}_{{\mathrm{H}}_2\mathrm{O}}}{{\mathrm{M}}_{{\mathrm{H}}_2}}\right)$

2.2. Experimental Infrastructure

The gasification experiments were carried out in a bench-scale bubbling fluidized bed (BFB) gasifier (3 kW_th) (Figure 1), previously described in other works [19,22,38,39]. The experimental facility is composed of a refractory steel (Sandvik 253MA) reactor with an internal diameter of 49 mm and a 340 mm high reaction chamber. The heat was supplied to the system by means of a 4.2 kW_e electric furnace, ensuring sufficient thermal energy to maintain the set bed temperature, measured by a k-type thermocouple placed in the bed and adjusted by a proportional–integral–derivative (PID) controller.

The bed material was quartz sand (150 g, particle size between 180–250 μm). The flow rate of dry synthetic air—20 %vol O₂ in N₂, hereinafter referred to as “air”—and O₂ were adjusted with two mass-flow controllers—Bronkhorst, Prestige model—measuring in a range of 0 to 10 NL∙min⁻¹ and 0 to 1 NL∙min⁻¹, respectively. The steam flow rate was produced and controlled by pumping liquid water, with a high-performance liquid chromatography HPLC pump (Jasco, PV-980 model, 0–10 mL∙min⁻¹) to an electrically heated chamber—plenum, operating at 320 °C and atmospheric pressure. The gasification/fluidization agent was injected into the reactor through the plenum, before passing through the distribution plate.

Apart from the reactive system, the experimental infrastructure includes a stainless-steel biomass feeding unit (AISI 316) composed of a storage silo, a screw feeder, and a vertical discharge tube. The closed and purged (Ar) silo has an agitation device to promote the downward movement of the fuel particles towards the screw feeder. The vertical discharge tube was responsible for transporting the fuel from the screw feeder to the bed surface.

The producer gas leaves the reactor through an electrically heated stainless-steel pipeline (>400 °C) to the exhaustion to prevent tar and water vapor condensation.

2.3. Gas Sampling and Analysis

A sample of raw producer gas from the heated exhaustion was cooled to a set of impingers immersed in liquid water in thermal equilibrium with ice—0 °C, 101,325 Pa—to remove condensable compounds (water and tar) at ambient conditions. Afterwards, the gas passed through a quartz filter (Outer diameter—47 mm) to remove particulate matter and aerosol. As a precautionary measure, the gas passed through a U-tube immersed in liquid water in thermal equilibrium with ice (0 °C, 101,325 Pa), placed after the quartz filter, to ensure the removal of all tar compounds and water.

The dry and clean gas was collected into a sampling bag (FlexFoil bag) and analyzed in a gas chromatography analyzer with a thermal conductivity detector (μGC-TCD), for the determination of CO, H₂ CO₂, CH₄, C₂H₄, C₂H₆, C₃H₈, and N₂ concentration. The μGC-TCD operated with a double column (module A and module B). Using Argon as a carrier gas, module A operated at 70 °C and 241,317 Pa, giving information regarding the concentration of H₂, O₂, N₂, CH₄, and CO. Module B used Helium as a carrier gas to determine the concentration of CO₂ and light hydrocarbons (C₂H₄, C₂H₆, and C₃H₈) at 50 °C and 206,843 Pa.

2.4. Experimental Procedure and Data Analysis

The main operating conditions of the gasification experiments carried out in this work are described in Table 3. The equivalence ratio (ER) is the ratio between the amount of O₂ supplied to the system and the stoichiometric O₂ required for the complete combustion of the biomass. The stoichiometric needs were calculated based on the ultimate analysis of the fuel and moisture content (Table 2). The ER was 0.25, falling within the optimal operating range often referred to in the literature [40,41], and the reactor was operated at 800 °C (atmospheric pressure), both attaining an appropriate balance between producer gas quality, process stability, and efficiency parameters, as suggested in previous works for bubbling fluidized bed systems [19,38,41].

The steam-to-biomass mass ratio (SBR) is given by dividing the steam mass flow added to the gasifier by the air-dried biomass mass flow fed to the gasifier. The SBR varied between 0.0 and 0.6 kg_steam∙kg⁻¹_biomass, ranging within the values referred to in the literature [9,30,31,32]. The φ is defined as the O₂ concentration in the dry gasification agent (GA). The φ varied between 20 %vol._{O2,dry GA} (air), and 40 %vol._{O2,dry GA}, falling within the range found in the literature [15,33,34,35].

Experiment #1 is the reference condition (air gasification, without any added steam or oxygen, see Table 3). On the one hand, experiments #2 to #4 refer to the steam addition, having the G_{v,liquid H2O} increased to meet the SBR increment. On the other hand, experiments #5 to #7 refer to the O₂ addition. In those experiments, the air flow rate suffered a decrease, whereas the O₂ flow rate increased to comply with the increment of φ (see Table 3). In addition, the biomass flow rate in those experiments also increased to ensure the same ER in all experiments (ER = 0.25).

Prior to each experiment (see Table 3), the system was completely cleaned, and a new batch of sand was loaded into the bed. The impact of SBR and φ was analyzed after the system reached steady-state conditions, in terms of temperature and gas composition, with the latter monitored in real time using an online analyzer—SICK, Figure 1. Once the system reached steady-state conditions, three sampling bags were used to collect the dry and clean producer gas for 45 min (15 min per bag), which were thereafter analyzed in the μGC-TCD (see Section 2.3).

The influence of steam or O₂ addition during air gasification of RFB from eucalyptus was assessed based on the producer gas composition, LHV_G, and three efficiency parameters determined based on the experimental data: specific dry gas production (Y_gas), cold gas efficiency (CGE) and carbon conversion efficiency (CCE). The LHV_G was determined considering the concentration of the combustible species measured the producer gas and their respective LHV (at standard conditions, 273.15 K and 101,325 Pa) [42]. Y_gas was calculated using Equation (3), giving the ratio between the volumetric flow rate of dry and clean producer gas and the mass flow rate of dry biomass [43]. The dry gas volumetric flow rate was determined based on a nitrogen mass balance. CGE was the ratio between the chemical energy in the producer gas and the chemical energy in the biomass fed to the process (Equation (4)) [43]. The CCE is the ratio between the carbon present in the gaseous compounds measured the producer gas and the carbon present in the feedstock fed (Equation (5)) [43]. Moreover, the H₂:CO and CO:CO₂ molar ratios were also determined and analyzed. The H₂:CO molar ratio is a relevant parameter concerning the producer gas application for biofuels and chemicals synthesis [43] and CO:CO₂ molar ratio is an indicator of the process efficiency, analyzing the rate of the gasification/combustion reactions [44].

(3)${\mathrm{Y}}_{\mathrm{gas}}=\frac{{\mathrm{G}}_{\mathrm{v},\mathrm{gas}}}{{\mathrm{G}}_{\mathrm{m},\mathrm{dry} \mathrm{biomass}}}$

(4)$\mathrm{CGE}={\mathrm{Y}}_{\mathrm{gas}}\times \frac{{\mathrm{LHV}}_{\mathrm{G}}}{{\mathrm{LHV}}_{\mathrm{B}}}$

(5)$\mathrm{CCE}=\frac{{\mathrm{Y}}_{\mathrm{gas}}\times \frac{{\mathrm{P}}_{\mathrm{N}}}{\mathrm{R}\times {\mathrm{T}}_{\mathrm{N}}}\times {\mathrm{M}}_{\mathrm{C}}\times {\sum }^{}{\mathsf{\epsilon }}_{\mathrm{C},\mathrm{i}}\times {\mathrm{y}}_{\mathrm{i}}}{{\mathrm{w}}_{\mathrm{CB}}}$

3. Results and Discussion

Table 4 shows the main results of this work, concerning the producer gas composition, yield of the gaseous compounds in the producer gas (excluding N₂), LHV_G, Y_gas, CGE, CCE, and H₂:CO, H₂:CO_2, and CO:CO₂ molar ratios. The following sections present the analysis of the results obtained regarding the influence of steam and O₂ addition during air gasification of residual forest biomass from eucalyptus on the parameters previously described in Section 2.4.

3.1. Effect of Steam Addition during Air Gasification of Biomass

This section presents the analysis of the results obtained regarding the influence of steam injection during the air gasification of RFB from eucalyptus on producer gas quality.

3.1.1. Producer Gas Composition

Figure 2 shows the influence of SBR on the producer gas composition (CO, CO₂, H₂, and CH₄ concentration), during the air gasification of RFB from eucalyptus. The concentration of CO decreased from 12.9 %vol._{dry gas}, when no steam was injected (SBR = 0.0, reference condition), to 9.0 %vol._{dry gas}, when the SBR was 0.6 kg_steam∙kg⁻¹_biomass. The CO₂ concentration followed an opposite trend to the one observed for CO concentration, increasing from 16.2 %vol._{dry gas}, when no steam was injected, to 19.0 %vol._{dry gas}, with an addition of 0.6 kg_steam∙kg⁻¹_biomass. Analogous to CO₂ concentration, H₂ concentration ranged between 8.7 and 14.6 %vol._{dry gas}, with the highest value found for an SBR of 0.6 kg_steam∙kg⁻¹_biomass. The CH₄ concentration decreased with the increment of SBR, with the lowest concentration value (2.72 %vol._{dry gas}) observed for an SBR of 0.6 kg_steam∙kg⁻¹_biomass. Light hydrocarbons (C₂H₄, C₂H₆, and C₃H₈) concentration did not follow a clear trend with the increase in steam in the process (Table 4); the concentration of these compounds was not presented in Figure 2 due to their vestigial presence.

The presence of steam had a major impact on producer gas composition, with H₂ and CO₂ concentrations increasing with the increment of SBR, whilst the CO and CH₄ concentrations decreased. The injection of 0.6 kg_steam∙kg⁻¹_biomass led to an increase in the H₂ and CO₂ concentrations of, approximately, 68% and 17%, respectively, when compared to the reference condition. In contrast, CO and CH₄ concentrations decreased, by approximately, 30 and 12%, for the same conditions. For the maximum SBR tested, the H₂ concentration in the producer gas was 14.6 %vol._{dry gas}, decidedly higher than the one found in the literature for air biomass gasification (~8 %vol._{dry gas}—see Section 1). Campoy et al. [9] and Tian et al. [30], in their studies of biomass gasification using mixtures of air and steam, also observed an increase in H₂ and CO₂ concentrations and a decrease in the CO and CH₄ concentrations, with the increment of SBR. The results obtained in this work are indicative of a possible increased relevance of the water gas-shift reaction (R9), favoring the formation of H₂ and CO₂, whilst consuming CO. The decrement in the CH₄ concentration with the SBR increase suggests a higher occurrence of methane-steam reforming reactions (R10) and (R11). Moreover, the H₂:CO₂ molar ratio increased from 0.54 to 0.77 mol_H2∙mol⁻¹_CO2 when the SBR increased from 0.0 to 0.6 kg_steam∙kg⁻¹_biomass (Table 4). This result indicates a possible higher occurrence of dry (CO₂) reactions (R13) and (R14) with the increment of steam addition, favoring the consumption of CO₂, whilst producing H₂, having the latter a higher relative increase in comparison with CO₂ concentration increase.

Figure 3 presents the effect of SBR addition during the air gasification of RFB from eucalyptus on the CO, CO₂, H_2, and CH₄ yield. The CO yield varied between 215 and 289 g_CO∙kg⁻¹_{dry biomass}, with the lowest value found for an SBR of 0.6 kg_steam∙kg⁻¹_biomass. The CO₂ yield increased with the increment of steam addition in the system, ranging from 570 to 714 g_CO2∙kg⁻¹_{dry biomass}, with the highest value observed for an SBR of 0.6 kg_steam∙kg⁻¹_biomass. Steam addition had a positive effect on the H₂ yield, increasing from 14 to 25 g_H2∙kg⁻¹_{dry biomass}, when the SBR increased from 0.0 to 0.6 kg_steam∙kg⁻¹_biomass. The CH₄ yield suffered a minor decrease with the increment of steam addition, from 40 to 37 g_CH4∙kg⁻¹_{dry biomass}.

The addition of 0.6 kg_steam∙kg⁻¹_biomass led to an increase of, approximately, 80% and 25%, in the H₂ and CO₂ yield, respectively, whereas the CO and CH₄ yield decreased, by approximately, 26% and 6%, respectively, when compared to the reference condition (Figure 3). These results are in agreement with those obtained in Campoy et al. [9], where the H₂ and CO₂ yield increased with the increment in SBR, whilst the CO and CH₄ yield decreased for the same conditions. As stated about the gas species concentration behavior (Figure 2), these gas species yields are indicative of a higher occurrence of the water–gas shift reaction (R9) and reforming reactions (R10)–(R14), privileging H₂ formation.

3.1.2. LHV_G and Efficiency Parameters

Figure 4 shows the influence of SBR increment on the LHV_G and efficiency parameters during air gasification of RFB from eucalyptus. LHV_G fell between 4.7 and 4.9 MJ∙Nm⁻³_{dry gas}, whilst the CGE and CCE ranged from 44% and 67% to 47% and 70%, respectively. The Y_gas underwent an increase with the increment in SBR, from 1.79 to 1.92 Nm³_{dry gas}∙kg⁻¹_{dry biomass}.

The SBR increase had no evident impact on the LHV_G, following the trend presented in Campoy et al. [9]. This can be justified by a trade-off between the increment in the H₂ concentration and the decrease in the CO concentration, both combustible species with a major impact on the LHV_G, followed by the increment of CO₂ concentration, resulting in almost no change in the LHV_G.

Similarly to that observed in Campoy et al. [9], even though steam addition had a clear impact on the yield of the different gas species (mainly H₂, CO, and CO₂—Figure 3), the CCE had only a minor increase, meaning that the increment of SBR did not clearly promote heterogenous reactions nor enhanced the char conversion to gaseous species. This may be explained by the fact that steam is less reactive with char than O₂ (in the air) [14]. Therefore, since the ER was kept constant and the biomass mass flow rate did not change, the amount of O₂ (in the air) was the same throughout the experiments, possibly overcoming the steam potential for heterogenous reactions, and leaving, perhaps, the steam to be mainly involved in gas-phase reactions.

Moreover, although the increment of SBR had a minor impact on the CGE and CCE, the presence of steam slightly improved the CGE and the CCE, when compared to the reference condition (air gasification). The CGE and CCE are highly dependent on the Y_gas (Equations (4) and (5)), and this parameter increased with the presence of steam, probably caused by a higher occurrence of reforming reactions (R10)–(R14), thus increasing the overall gas yield. As a result, an increase in the Y_gas might have a positive effect on the CGE and CCE.

3.1.3. H₂:CO and CO:CO₂ Molar Ratios

Figure 5 presents the impact of SBR increase, during air gasification of RFB from eucalyptus, on H₂:CO and CO:CO₂ molar ratios. The H₂:CO molar ratio varied between 0.67 and 1.63 mol_H2∙mol⁻¹_CO, with the highest value found for SBR equal to 0.6 kg_steam∙kg⁻¹_biomass. Contrariwise, the CO:CO₂ decreased with the increment of SBR, ranging from 0.47 to 0.80 mol_CO∙mol⁻¹_CO2.

The addition of 0.6 kg_steam∙kg⁻¹_biomass led to an increase of, approximately, 142% in the H₂:CO molar ratio, and a decrease of, approximately, 41% in the CO:CO₂ molar ratio, when compared to the reference condition, following the trend observed in Campoy et al. [9]. The increment in the H₂:CO molar ratio with the SBR increase was probably caused by the increasing relevance of reforming reactions ((R10)–(R14)), favoring the production of H₂, and the enhancement of water gas-shift reaction (R9), consuming CO whilst producing H₂. The decrease in CO:CO₂ molar ratio can be justified by the increased relevance of the water gas-shift reaction (R9).

With 0.6 kg_steam∙kg⁻¹_biomass, a producer gas with a H₂:CO molar ratio of 1.63 mol_H2∙mol⁻¹_CO was generated, remarkedly higher than the H₂:CO molar ratio range for air biomass gasification (between 0.6 and 0.8 mol_H2∙mol⁻¹_CO—Section 1), fulfilling the established limits of H₂:CO molar ratio for several advanced applications, including [7,20,21]:

• Production of liquid fuels through Fischer-Tropsch synthesis (0.6 or 2.0 mol_H2∙mol⁻¹_CO, depending on the applied catalyst);

• DME synthesis (1 mol_H2∙mol⁻¹_CO);

• Bioethanol synthesis (between 1 and 2 mol_H2∙mol⁻¹_CO).

3.2. Effect of O₂ Addition during Air Gasification of Biomass

This section describes the results obtained regarding the influence of O₂ addition during air gasification of RFB from eucalyptus on producer gas quality.

3.2.1. Producer Gas Composition

Figure 6 shows the impact of O₂ concentration on the gasification agent on the producer gas composition (CO, CO₂, H_2, and CH₄ concentration), during air gasification of RFB from eucalyptus. Apart from N₂ concentration, all gaseous compounds concentration increased with the increment of O₂ concentration (φ ranging from 20 to 40 %vol._{O2,dry GA}) in the gasification agent. The CO concentration increased from 12.9 to 22.2 %vol._{dry gas}, and the CO₂ concentration increased between 16.2 and 24.4 %vol._{dry gas}. The H₂ concentration increased from 8.7 to 12.9 %vol._{dry gas}, and the concentration of CH₄ increased from 3.1 to 4.6 %vol._{dry gas}. Light hydrocarbons (C₂H₄, C₂H₆, and C₃H₈) concentration also increased with the increment of φ (Table 4); these compounds’ concentrations are not shown in Figure 6 due to low concentration values.

The increase in φ had a major impact on the producer gas composition, following the trend presented in Wang et al. [33] and Liu et al. [35]. Compared to the reference condition, the increment of φ to 40 %vol._{O2, dry GA} led to an increase in the H₂ and CO concentrations of, approximately, 48% and 72%, respectively. Moreover, the concentration of CO₂ and CH₄ increased by approximately 50% and 48%, respectively, for the same conditions. The increase in φ had a corresponding decrease in the N₂ concentration in the gasification agent, thus leading to a reduction in this species in the producer gas composition, resulting in an inevitable increase in the concentration of all combustible species and CO₂. Moreover, the results also showed a higher impact of φ increase on the CO concentration, in comparison to the other gaseous compounds. This may be justified by a higher occurrence of the Boudouard reaction (R5), where the CO₂ reacts with the solid carbon in the char, producing CO [33]. The promotion of the Boudouard reaction (R5) was probably caused by a higher inventory of char on the bed. In fact, to meet the O₂ concentration increment in the gasification agent, whilst keeping the ER constant, the biomass feeding rate increased, thus increasing the amount of char available in the bed for heterogeneous reactions.

Figure 7 presents the behavior of CO, CO₂, H₂, and CH₄ yield with the increment of O₂ concentration in the gasification agent, during air gasification of RFB from eucalyptus. The CO yield varied between 287 and 323 g_CO∙kg⁻¹_{dry biomass}, whilst the CO₂ yield ranged from 567 to 578 g_CO2∙kg⁻¹_{dry biomass}. The H₂ and CH₄ yields did not suffer major changes with the increase of O₂ concentration in the gasification agent, varying between 13 g_H2∙kg⁻¹_{dry biomass} and 15 g_H2∙kg⁻¹_{dry biomass}, and 38 g_CH4∙kg⁻¹_{dry biomass} and 40 g_CH4∙kg⁻¹_{dry biomass}, respectively.

The yield of the different gas species provides information regarding the effect of O₂ concentration in the gasification agent on the conversion of biomass into those gas species. One can observe that an increment in φ led to a slight increase in the CO yield and to a minor decrease in the CH₄ yield. However, H₂ and CO₂ concentrations did not show a clear trend, undergoing an increase when the φ increased from 20 to 30 %vol._{O2, dry GA}, followed by a decrease for the highest φ tested, although no major changes were observed. These results are not in agreement with the results presented by Wang et al. [33]. One might say that an increase in the yield of all combustible species and CO₂ should be expected when the O₂ concentration in the gasification agent increased. That increase could be expected because the local temperature at the surface of the reacting particles and its boundary layer would potentially increase with the increment of O₂ concentration in the gasification agent, due to the heat released from enhanced exothermic reactions. Thus, the increase in the temperature would favor endothermic reactions, including water–gas (R2), Boudouard (R5), reforming ((R10)–(R14)) and tar cracking reactions, promoting tar destruction, and char conversion, thus increasing H₂, CO, CH₄ and light hydrocarbons yield [35]. Nevertheless, the bulk bed temperature was controlled at 800 °C for all experiments (Table 3) to obtain deeper knowledge about the effect of O₂ concentration in the gasification agent on the producer gas characteristics, without being misunderstood by side effects from other parameters, i.e., ER, T_bed, with a strong impact on reactions rate and producer gas composition. Therefore, the increase in CO yield may be related to a higher relevance of the Boudouard reaction (R5) with the increment of φ.

Moreover, Mastellone et al. [34] during their study observed that, whilst maintaining the temperature constant, the concentration of CO and CO₂ (inert free) followed opposite trends, where the former increased and the latter decreased with the increment of O₂ concentration in the gasification agent. Those results are also not in alignment with the results obtained in this work. One possible explanation may lie in the fuel composition. The fuel used in their study was a mixture of coal, plastics, and biomass, resulting in a material with a higher content of carbon (above 59 %m/m_db) [34] when compared to the residual forest biomass used in this work (see Table 2). Hence, the reaction mechanisms may have taken different paths, showing, perhaps, a more pronounced relevance of the Boudouard reaction (R5) when compared to this work. In addition, although the temperature of the process remained constant, local temperature increases (so-called hot spots) that cannot be monitored by the thermocouple placed in the bed may have occurred due to the reduced thermal action of nitrogen [34], prompting oxidation reactions, which, together with the higher relevance of the Boudouard reaction (R5), may explain the marginal change in the CO₂ yield.

In short, the results obtained may be explained as resulting from a trade-off between oxidation and reducing reactions, prevailing Boudouard reaction (R5), thus increasing the CO yield.

3.2.2. LHV_G and Efficiency Parameters

Figure 8 shows the influence of the φ increase in gasification agent on the LHV_G and efficiency parameters during air gasification of RFB from eucalyptus. LHV_G fell between 4.7 and 7.5 MJ∙Nm⁻³_{dry gas}, with the highest value found when the φ was 40 %vol. _{O2, dry GA}. The CGE and CCE ranged from 44 and 67% to 46 and 70%, respectively. The Y_gas underwent a significant decrease from 1.8 to 1.2 Nm³_{dry gas}∙kg⁻¹_{dry biomass}, when the φ increased from 20 to 40 %vol._{O2, dry GA}.

The Y_gas suffered a decrease of, approximately, 33% with the increment of φ from 20 to 40 %vol._{O2, dry GA}. This was probably caused by the reduction in N₂ concentration in the gasification agent, and the respective decrease in the gas flow rate, thus reducing the N₂ concentration in the producer gas and decreasing the overall gas production. However, even though the amount of producer gas decreased with the increase in O₂ concentration, its quality in terms of LHV_G increased. Each increment in the O₂ concentration in the gasification agent led to an increase in the LHV_G. In short, less producer gas production, but a higher quality producer gas. Comparing the reference condition to the maximum φ tested, the LHV_G underwent an increase of, approximately, 57%, reaching a value of 7.5 MJ∙Nm⁻³_{dry gas}, considerably higher than the average LHV_G for air biomass gasification (see Section 1), meaning that the increment of φ enhanced the producer gas quality for energetic applications. The improvement in the LHV_G was mainly associated with the reduction in N₂ concentration on the producer gas (caused by the decrease of N₂ concentration in the gasification agent), leading to an increase in all combustible species concentration. Moreover, the possible promotion of the Boubouard reaction (R5) with increased production of CO concentration had a positive impact on the LHV_G.

The increment in O₂ concentration in the gasification agent had a marginal effect on the CGE and CCE. However, it could be said that a higher impact of φ increase was expected in these parameters due to the increment of LHV_G and combustible species and CO₂ concentration. Since the CGE and CCE are highly influenced by the Y_gas, and this parameter significantly decreased with φ, the resulting values of these efficiency parameters can be regarded as a trade-off between the improvement of producer gas quality and gas yield. Moreover, as the temperature was kept constant, the reduction in N₂ concentration in the gasification agent was, possibly, the main reason for the improvement of LHV_G and combustible species and CO₂ concentration, masking the different gas species’ yield. As a result, no evident impact was found on the CGE and CCE.

3.2.3. H₂:CO and CO:CO₂ Molar Ratios

Figure 9 presents the influence of O₂ concentration in the gasification agent, during air gasification of RFB from eucalyptus, on H₂:CO and CO:CO₂ molar ratios. The H₂:CO molar ratio varied between 0.58 and 0.67 mol_H2∙mol⁻¹_CO, with the lowest value found when the φ was 40 %vol._{O2, dry GA}. Contrariwise, the CO:CO₂ increased with the increment of φ, ranging from 0.80 to 0.91 mol_CO∙mol⁻¹_CO2.

The injection of 40 %vol._{O2, dry GA} led to a decrease of, approximately, 15% in the H₂:CO molar ratio, and an increase of 15% in the CO:CO₂ molar ratio. These results are indicative of the promotion of kinetic mechanisms favoring CO formation, following the analysis performed in Section 3.2.1. This outcome can be explained by the possible promotion of the Boudouard reaction (R5), consuming CO₂ and producing CO, thus improving the CO:CO₂ molar ratio and decreasing the H₂:CO molar ratio. Nevertheless, other reactive mechanisms involving other species as tars cannot be disregarded. This should be the subject of further studies to understand the role of O₂-enriched air as a gasification agent during residual biomass gasification on that matter.

4. Conclusions

In this work, the gasification of RFB from eucalyptus, using air-steam or O₂-enriched air as gasification agents in an externally heated 3 kW_th bench-scale BFB, was successfully performed and analyzed. The main objective was to evaluate the effect of steam/O₂ addition on the producer gas quality, whilst maintaining the ER and T_bed constant.

Regarding the effect of steam addition, this parameter had a major impact on the producer gas composition, increasing the concentration of CO₂ and H₂, and decreasing the CO and CH₄ concentrations, probably caused by the promotion of the water gas-shift reaction. Moreover, the increment in steam had a positive effect on the gas yield and the H₂:CO molar ratio, with the latter attaining values up to 1.6 mol_H2∙mol⁻¹_CO, thus fulfilling the established limits of this parameter for several advanced applications. SBR had a slightly positive impact on the CGE and CCE, and no evident impact on the LHV_G.

The effect of the O₂ concentration increase in the gasification agent was very notorious on the producer gas composition, leading to an increase in all combustible species and CO₂ concentrations. The results suggest that this increment was mainly caused by the reduction in N₂ concentration in the gasification agent, decreasing the N₂ concentration on the producer gas. Moreover, the yield of the main compounds in the producer gas (excluding N₂) was fairly constant throughout the experiments, apart from the CO yield, which increased with φ, suggesting the possible promotion of the Boudouard reaction (R5). Comparing the reference condition to the maximum φ tested (40%), LHV_G underwent an increase of, approximately, 57%, reaching a value of 7.5 MJ·Nm⁻³_{dry gas}, meaning that the increment in φ improved the producer gas quality for energetic applications. In addition, the Y_gas suffered a significant decrease with the φ increase, caused by the reduction in dilution by N₂, meaning that less gas was produced, but with higher quality. The φ had a minor positive impact on the CGE and CCE.

As such, this work successfully showed that the addition of steam or O₂ during the air gasification of RFB from eucalyptus improved the producer gas quality in terms of H₂ concentration, H₂:CO molar ratio, and LHV_G, showing the potential of these primary measures (modification of gasification agent composition) on improving producer gas quality, and thus overcoming, to some extent, the barriers found for the conventional air gasification technology. The results shown in this work clarified some effects (at the macroscopic level) of adding steam or increasing O₂ concentration in the gasification agent, on the producer gas quality, thus being relevant to support the scale-up of the process. Moreover, the results also highlighted the need to further develop detailed experiments to understand the reactive mechanisms underlying those effects.

Author Contributions

Conceptualization, H.G.M.F.G. and L.A.C.T.; methodology, H.G.M.F.G., M.A.A.M. and L.A.C.T.; formal analysis, H.G.M.F.G. and L.A.C.T.; investigation, H.G.M.F.G., M.A.A.M. and L.A.C.T.; resources, M.A.A.M. and L.A.C.T.; data curation, H.G.M.F.G.; writing—original draft preparation, H.G.M.F.G.; writing—review and editing, H.G.M.F.G., M.A.A.M. and L.A.C.T.; visualization, H.G.M.F.G. and L.A.C.T.; supervision, M.A.A.M. and L.A.C.T.; project administration, L.A.C.T.; funding acquisition, L.A.C.T. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data available on request because of project policies.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature and Abbreviations

Latin Symbols
BFB	Bubbling fluidized bed
CCE	Carbon Conversion Efficiency	[%]
CGE	Cold Gas Efficiency	[%]
CO:CO2	Carbon monoxide to carbon dioxide molar ratio	[molCO∙mol−1CO2]
ER	Equivalence ratio	[-]
GA	Gasification agent
GC	Gas chromatography
Gm, dry biomass	Dry biomass mass flow rate	[kgdry biomass∙s−1]
Gv, gas	Dry gas volumetric flow rate	[Nm3dry gas∙s−1]
hH2O,298K	Water enthalpy of vaporization at 298 K	[2.442 MJ∙kg−1H2O]
HHVB	Higher heating value of the dry biomass	[MJ∙kg−1dry biomass]
HPLC	High-performance liquid chromatography
H2:CO	Hydrogen to carbon monoxide molar ratio	[molH2∙mol−1CO]
H2:CO2	Hydrogen to carbon dioxide molar ratio	[molH2∙mol−1CO2]
Index i	Gaseous Carbon compound CO2, CO, CH4, C2H4, C2H6, C3H8
LHVB	Lower heating value of the dry biomass	[MJ∙kg−1dry biomass]
LHVG	Lower heating value of the dry producer gas	[MJ∙Nm−3dry gas]
MC	Molar mass of carbon	[12 kgC∙kmol−1C]
MH2	Molar mass of hydrogen	[2 kgH2∙kmol−1H2]
MH2O	Molar mass of water	[18 kgH2O∙kmol−1H2O]
NDIR	Nondispersive infrared
NL	Refers to L at normal pressure (101,325 Pa) and temperature (273 K)
Nm3	Refers to m3 at normal pressure (101,325 Pa) and temperature (273 K)
PID	Proportional-Integral-Derivative
PN	Pressure at standard conditions	[101,325 Pa]
R	Ideal Gas constant	[8314 J∙kmol−1·K−1]
RFB	Residual forest biomass
SBR	Steam-to-biomass mass ratio	[kgsteam∙kg−1biomass]
TC	Thermal conductivity
TCD	Thermal conductivity detector
TN	Temperature at standard conditions	[273.15 K]
washB	Mass fraction of ash in the biomass	[kgash∙kg−1dry biomass]
wCB	Mass fraction of carbon in the biomass	[kgC∙kg−1dry biomass]
wHB	Mass fraction of hydrogen in the biomass	[kgH∙kg−1dry biomass]
wH2OB	Mass fraction of water in the biomass	[kgH2O∙kg−1dry biomass]
wNB	Mass fraction of nitrogen in the biomass	[kgN∙kg−1dry biomass]
wOB	Mass fraction of oxygen in the biomass	[kgO∙kg−1dry biomass]
wSB	Mass fraction of sulphur in the biomass	[kgS∙kg−1dry biomass]
Ygas	Specific dry gas production or gas yield	[Nm3dry gas∙kg−1dry biomass]
yi	Molar fraction of a gaseous carbon compound measured in the dry producer gas	[moli∙mol−1dry gas]
Greek Symbols
$\Delta {\mathrm{H}}_{298\mathrm{K}}^0$	Standard enthalpy of a reaction at 298 K, 1 bar	[kJ∙mol−1]
ɛC,i	Molar fraction of carbon in gaseous compounds containing carbon (ɛC,CO2 = 1, ɛC,CO = 1, ɛC,CH4 = 1, ɛC,C2H4 = 2, ɛC,C2H6 = 2, ɛC,C3H8 = 3)	[molC∙mol−1i]
φ	O2 concentration in the dry gasification agent	[%vol.O2, dry GA]

Footnotes

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Figures and Tables

Figure 1. Schematic layout of the bench-scale 3 kWth externally heated BFB gasifier. Dashed line—Electric circuit, Continuous line—Pneumatic circuit, GENTWO—Paramagnetic online gas analyzer for O2, UCE-LAB—Electronic command unit, Micro GC Fusion—Gas chromatograph with Thermal Conductivity Detector (TCD), SICK online analyzer with nondispersive infrared (NDIR) sensor and thermal conductivity (TC) sensor (adapted from [19]).

Figure 2. Effect of steam injection on the producer gas most abundant components concentration (N2 concentration excluded) during air gasification of RFB from eucalyptus (φ = 20 %vol.O2, dry GA—Table 3 and Table 4).

Figure 3. Effect of steam addition on the CO, CO2, H2, and CH4 yield during air gasification of RFB from eucalyptus (φ = 20 %vol.O2, dry GA—Table 3 and Table 4).

Figure 4. Effect of steam addition on the LHVG and performance parameters during air gasification of RFB from eucalyptus (φ = 20 %vol.O2, dry GA—Table 3 and Table 4).

Figure 5. Effect of steam addition on the H2:CO and CO:CO2 molar ratios during air gasification of RFB from eucalyptus (φ = 20 %vol.O2, dry GA—Table 3 and Table 4).

Figure 6. Effect of O2 concentration (φ) in the gasification agent, on the producer gas most abundant components concentration (N2 concentration excluded) during air gasification of RFB from eucalyptus (SBR = 0.0 kgsteam∙kg−1biomass—Table 3 and Table 4).

Figure 7. Effect of O2 concentration (φ) in gasification agent on the CO, CO2, H2, and CH4 yield during air gasification of RFB from eucalyptus (SBR = 0.0 kgsteam∙kg−1biomass—Table 3 and Table 4).

Figure 8. Effect of O2 concentration (φ) in gasification agent on the LHVG and performance parameters during air gasification of RFB from eucalyptus (SBR = 0.0 kgsteam∙kg−1biomass—Table 3 and Table 4).

Figure 9. Effect of O2 concentration (φ) in gasification agent on the H2:CO and CO:CO2 molar ratios during air gasification of RFB from eucalyptus (SBR = 0.0 kgsteam∙kg−1biomass—Table 3 and Table 4).

Table 1

Main biomass gasification reactions [10,14,15].

	Reaction	$\mathbf{\Delta }{\mathbf{H}}_{298\mathbf{K}}^0$ [kJ·mol−1]	Equation
Heterogenous reactions
Devolatilization	$\mathrm{Biomass}+\mathrm{heat} \to \mathrm{Permament} \mathrm{gas}+\mathrm{Tar}+\mathrm{Char}$	endothermic	(R1)
Water gas	${\mathrm{C}}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\mathrm{CO}$	131	(R2)
Char-steam	${\mathrm{C}}_{\left(\mathrm{s}\right)}+2{\mathrm{H}}_2\mathrm{O} \to { \mathrm{CO}}_2+2{\mathrm{H}}_2$	90	(R3)
	${\mathrm{C}}_{\left(\mathrm{s}\right)}+{ \mathrm{H}}_2\mathrm{O} \to { ½\mathrm{CH}}_4+{½\mathrm{CO}}_2$	8	(R4)
Boudouard	${\mathrm{C}}_{\left(\mathrm{s}\right)}+{\mathrm{CO}}_2\to 2\mathrm{CO}$	173	(R5)
Methanation	${\mathrm{C}}_{\left(\mathrm{s}\right)}+2{\mathrm{H}}_2\to {\mathrm{CH}}_4$	−75	(R6)
Partial oxidation	${\mathrm{C}}_{\left(\mathrm{s}\right)}+{½\mathrm{O}}_2\to \mathrm{CO}$	−111	(R7)
Complete oxidation	${\mathrm{C}}_{\left(\mathrm{s}\right)}+{\mathrm{O}}_2\to {\mathrm{CO}}_2$	−394	(R8)
Homogenous reactions
Water gas-shift	$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}_2+{\mathrm{CO}}_2$	−41	(R9)
Steam reforming	${\mathrm{CH}}_4+{ \mathrm{H}}_2\mathrm{O} \rightleftharpoons \mathrm{CO}+3{\mathrm{H}}_2$	206	(R10)
	${\mathrm{CH}}_4+2{\mathrm{H}}_2\mathrm{O} \rightleftharpoons { \mathrm{CO}}_2+4{\mathrm{H}}_2$	165	(R11)
	${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}+{ \mathrm{nH}}_2\mathrm{O} \rightleftharpoons \mathrm{nCO}+\left(\mathrm{n}+\frac{\mathrm{m}}{2}\right){\mathrm{H}}_2$	endothermic	(R12)
Dry (CO2) reforming	${\mathrm{CH}}_4+{ \mathrm{CO}}_2 \rightleftharpoons 2\mathrm{CO}+2{\mathrm{H}}_2$	247	(R13)
	${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}+{ \mathrm{nCO}}_2 \rightleftharpoons 2\mathrm{nCO}+\left(\frac{\mathrm{m}}{2}\right){\mathrm{H}}_2$	endothermic	(R14)
CO oxidation	$\mathrm{CO}+{½\mathrm{O}}_2\to {\mathrm{CO}}_2$	−283	(R15)
H2 oxidation	${\mathrm{H}}_2+{½\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}$	−242	(R16)

Table 2

Chemical characterization of the RFB from eucalyptus.

	Eucalyptus
Proximate analysis [%m/m.db]
Moisture (a)	7.7
Ash	3.3
Volatile matter	79.0
Fixed Carbon	17.7
Ultimate analysis [%m/m.db]
C	50.9
H	6.1
N	0.9
O	38.8
Ash	3.3
LHVB [MJ ∙kg−1dry biomass]	19.3

^(a) air-dried (as received). db—dry basis.

Table 3

Operating conditions of the experiments performed in this work.

Experiment	#1	#2	#3	#4	#5	#6	#7
Gm, biomass [kg biomass∙h−1] a	0.204	0.204	0.204	0.204	0.255	0.306	0.407
Gv,air [NLair∙min−1] b	4.00	4.00	4.00	4.00	3.75	3.50	3.00
SBR [kgsteam∙kg−1biomass] a	0.0	0.2	0.4	0.6	0.0	0.0	0.0
Gv,liquid H2O [mL H2O∙min−1]	0.00	0.68	1.36	2.04	0.00	0.00	0.00
φ [%vol.O2, dry GA] c	20	20	20	20	25	30	40
Gv,O2 [NLO2∙min−1]	0.000	0.000	0.000	0.000	0.250	0.500	1.000

^a—air-dried biomass (moisture content: 7.7 %m/m, Table 2). ^b—dry synthetic air (20 %vol. of O₂ in N₂). ^c—GA—gasification agent. NL—Litre at standard temperature and pressure (273 K and 101,325 Pa).

Table 4

Summary of results obtained in this work.

Experiment	#1	#2	#3	#4	#5	#6	#7
ER [-]	0.25	0.25	0.25	0.25	0.25	0.25	0.25
SBR [kgsteam∙kg−1biomass]	0.0	0.2	0.4	0.6	0.0	0.0	0.0
φ [%vol.O2, dry GA]	20	20	20	20	25	30	40
Average producer gas composition [%vol.dry gas]
CO	12.90 ± 0.14	11.24 ± 0.12	10.07 ± 0.11	8.97 ± 0.09	15.51 ± 0.16	18.11 ± 0.18	22.22 ± 0.23
CO2	16.21 ± 0.17	17.60 ± 0.18	18.21 ± 0.19	18.99 ± 0.20	18.71 ± 0.19	21.02 ± 0.20	24.41 ± 0.20
H2	8.72 ± 0.13	11.56 ± 0.19	12.93 ± 0.19	14.64 ± 0.22	10.32 ± 0.16	11.93 ± 0.18	12.91 ± 0.19
CH4	3.12 ± 0.06	2.98 ± 0.05	2.83 ± 0.05	2.72 ± 0.05	3.51 ± 0.07	3.81 ± 0.07	4.62 ± 0.09
C2H4	1.32 ± 0.05	1.35 ± 0.03	1.22 ± 0.02	1.23 ± 0.03	1.43 ± 0.03	1.52 ± 0.04	1.82 ± 0.03
C2H6	0.20 ± 0.05	0.19 ± 0.02	0.19 ± 0.01	0.18 ± 0.02	0.33 ± 0.01	0.32 ± 0.02	0.32 ± 0.01
C3H8	0.20 ± 0.01	0.25 ± 0.03	0.23 ± 0.02	0.21 ± 0.04	0.31 ± 0.03	0.33 ± 0.02	0.42 ± 0.05
N2	57.42 ± 1.10	55.21 ± 1.11	54.85 ± 1.05	53.67 ± 1.06	50.02 ± 1.01	43.04 ± 1.08	33.52 ± 1.09
Yield [gi∙kg−1dry biomass]
CO	288.60	261.50	235.64	214.72	299.27	316.90	322.85
CO2	569.53	643.18	669.91	714.01	567.36	577.77	557.62
H2	13.90	19.21	21.62	25.01	14.20	14.88	13.40
CH4	39.63	39.61	37.91	37.18	38.62	38.02	38.23
C2H4	29.08	31.33	28.51	29.53	27.03	26.26	26.18
C2H6	4.79	4.85	4.66	4.54	6.21	5.63	4.67
C3H8	7.03	9.02	8.34	8.02	9.10	8.25	9.14
Performance parameters
LHVG [MJ∙Nm−3dry gas]	4.77	4.89	4.74	4.73	5.60	6.30	7.50
Ygas [Nm3dry gas∙kg−1dry biomass]	1.79	1.86	1.87	1.92	1.50	1.40	1.20
CGE [%]	44.20	47.16	45.96	46.94	45.00	45.90	45.10
CCE [%]	67.44	69.80	68.20	68.79	68.20	70.00	69.20
H2:CO [molH2∙mol−1CO]	0.67	1.03	1.28	1.63	0.67	0.66	0.58
H2:CO2 [molH2∙mol−1CO2]	0.54	0.66	0.71	0.77	0.55	0.57	0.53
CO:CO2 [molCO∙mol−1CO2]	0.80	0.64	0.55	0.47	0.83	0.86	0.91

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