Glycerin, a Biodiesel By-Product with Potentiality to Produce Hydrogen by Steam Gasification

This work investigates the possibility of providing a use to one of the major byproducts generated during biodiesel processing: glycerin. In particular, the steam gasification of water/glycerin mixtures is studied, analysing the influence of temperature (range 600–900  ̋C), inlet flow rate (0.5–3 mL ̈min ́1) and water/glycerin ratio (6–12 wt/wt, %) on the gas composition (H2, CO, CH4 and CO2), higher heating value, and generated power. In general, a more diluted water/glycerin mixture is more interesting in order to provide a higher fraction of hydrogen in the gas produced, although it also involves a decrease in the power obtained. Higher temperatures cause a greater contribution of water gas and water gas shift reactions in all cases, thus increasing the H2 proportion of the gas. Finally, a greater inlet flow rate increases gas production, but decreases the hydrogen proportion.


Introduction
Nowadays, the finite duration of fossil resources, the increasing energy demand and heterogeneous distribution of fossil fuel reserves is generating more and more geopolitical conflicts, which make evident the non-sustainability of the current worldwide energy model. Likewise, the environmental problems associated to the exploitation of fossil fuels are a question of global concern. In this frame, the International Energy Agency (IEA) defines as a primordial target the achievement of a substantial bioenergy contribution to future global energy demands by accelerating the production and use of cost-competitive bioenergy on a sustainable basis, thus providing increased security of supply whilst reducing greenhouse gas emissions from energy use [1].
Especially in the context of transport, biofuels have gained prominence during last years, and they are seen as a near-term alternative. Biodiesel is a non-toxic and biodegradable fuel that can be produced from a variety of crops. Moreover, numerous studies have been carried out using vegetable oils as raw materials [2][3][4]. The literature also shows works in which unconventional raw materials, as oil obtained from algae, bacteria, mushrooms and microalgae are used [5,6].
Biodiesel is obtained by direct transesterification of vegetable oils and tallows; this process involves the alcohol (glycerol) displacement of the triglyceride structure, by means of the incorporation of another short alcohol chain (methanol or ethanol). This causes the separation of the three fatty acid molecules forming the original triglyceride, which remain as methyl or Energies 2015, 8,[12765][12766][12767][12768][12769][12770][12771][12772][12773][12774][12775] a real application of the by-product, which would otherwise become a harmful residue and would have to be discarded, adding complexity to the process. The obtaining of a high calorific value gas from the gasification of glycerin can be useful to provide part of the energy needed in the biodiesel production or can be stored for further applications.

Materials
Glycerin from the transesterification of vegetable oils was provided by the biodiesel manufacturing plant of Bioenergética Extremeña, located in Valdetorres (Extremadura, Spain). It was used as it results from biodiesel production, without any further treatment, and characterized in terms of its chemical composition, which was made according to standard technical specifications, as described in Table 1. In addition, glycerin heating value was determined by a calorimetric bomb (mod. 1351, Parr Instrument Compant, Moline, IL, USA). Moreover, the thermal behavior of glycerin was analyzed using a thermobalance (Setsys Evolution, SETARAM, Caluire, France) under an air flow rate of 100 cm 3¨m in´1. An initial mass of 15.0˘0.1 mg was used, employing a heating rate of 20˝C¨min´1. The analyses were made in the temperature range 25-800˝C.

Steam Gasification Experiments
The runs were performed under continuous regime, using a bench-scale experimental set-up like the one shown in Figure 1.
The gasifier, with an inner diameter of 4 cm (outer diameter of 4.3 cm) and total height of 75 cm was placed inside an electrical furnace, which provided the heat for reactions. The furnace was well insulated to prevent major heat losses and a thermocouple was placed inside the furnace, in close contact with the reactor walls, to monitor the temperature in the reaction medium. Nitrogen (100 cm 3¨m in´1) was fed to the reactor during the heating up and cooling down periods. Once the furnace had achieved the target temperature, the glycerin, diluted in water, was fed into the gasifier and the process was initiated. Previous to experimentation, the influence of water/glycerin ratio showed that these conditions were optimal [20]. The inlet steam was produced in a coil by another electrical furnace and its flow rate was controlled by water pump (M312, Gilson, Middleton, WI, USA) supplying water to the generator. Several inlet water/glycerin ratios were studied (6:1; 9:1, 12:1), also varying the mixture flow rates (0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 g¨min´1). Moreover, the reactor temperature was varied in the range 600-900˝C. The runs were made once; the suitability of the Energies 2015, 8,[12765][12766][12767][12768][12769][12770][12771][12772][12773][12774][12775] procedure can be supported by previous studies on pyrolysis and gasification made by the research group [21][22][23][24][25].
The gas produced was made to pass through a quenching system (glass receivers covered by ice), where the tars and condensable high molecular weight hydrocarbons were collected. The composition of the gas produced (H 2 , CO, CO 2 and CH 4 ) was analyzed by gas chromatography (GC), using two gas chromatographs (HRGC 4000, KONIK, Miami, FL, USA). Both instruments are identical, although their columns are fed with different carrier gases: He and N 2 . In the first case CO, CO 2 and CH 4 are monitored, while H 2 is analyzed using N 2 as carrier, in order to avoid interferences due to the similar thermal conductivity of H 2 and He. Both the fed inert gas (nitrogen) and the produced syngas flow rate were measured by appropriately calibrated flow meters. Once analyzed, the gas produced was transported by a pipe and properly evacuated outdoor.  Table 1 shows the chemical analysis of the glycerin feedstock and the analytical method specifications used to perform each analysis. As it can be observed, the glycerin had many impurities, which may be detrimental for its energetic utilization. It is necessary to emphasize the high presence of sulphur and potassium, with 40,000 mg¨kg´1 and 18,000 mg¨kg´1, respectively, and the presence of phosphorus and sodium, in proportions of 100 mg¨kg´1 and 130 mg¨kg´1 respectively. Some of these compounds may be harmful for the environment and also damage the experimental installation. For example, and standing out as one of the most toxic pollutants resulting from combustion and gasification processes, organosulphur compounds are converted to H 2 S. This gas has been related to fatal diseases for animals and humans, and it also causes stress cracking in metallic installations, reducing the lifetime of processing and handling equipment. Previous studies made by the authors on the thermal degradation of glycerin have confirmed the emission of H 2 S [26]. This gas can be successfully reduced by adsorption on selectively functionalized activated carbons as well as other synthetic materials such as zeolites or more costly and sophisticated technologies such as membrane separation [27].

Glycerin Characterization
On the other hand, the presence of sodium and potassium can form low melting point eutectics in the gasification bed, which is also a matter of concern [28]. These metals can be removed by chemical pretreatments of glycerin: for instance, Carmona et al. [29] have recently studied the elimination of sodium by ion exchange processes on a strong acid resin. Our research group is also investigating the removal of Na and K by dehydration of glycerin, which results in the precipitation of their respective sulphates.
The higher heating value of the glycerin was 3300 kcal¨kg´1, which is lower than that of pure glycerin [30], as it can be expected due to its content in water and other impurities.
In Figure 2 the weight loss against temperature (TG) and its derivative (DTG) have been plotted. As it can be inferred, glycerin thermal degradation occurs in several different stages. This decomposition profile can be associated with the chemical characteristics of glycerin: first, there is a slight weight loss up to 115˝C, which becomes more marked in the range 115-350˝C, when most of the glycerin has been pyrolyzed. At temperatures higher than 350˝C, the TG curve only shows a residual mass proportion equal to 2%; which is similar to the ash content of the material. The DTG curve shows two peaks centered at 135˝C and 295˝C, being the latter much wider, in accordance to a more persistent weight loss. The first decomposition stage can be associated with the release of water (glycerin is hygroscopic), as well as some low-temperature volatiles, such as methanol, the co-reactant in the transesterification reaction [31]. The second and larger decomposition stage can be associated the degradation of impurities, such as fatty acid methyl esters as well as residues from previous degradation reactions [32]. The thermal instability of glycerin has been previously observed by other authors, who improved this property by blending glycerin with different plasticizers [33].
Energies 2015, 88, page-page 5 handling equipment. Previous studies made by the authors on the thermal degradation of glycerin have confirmed the emission of H2S [26]. This gas can be successfully reduced by adsorption on selectively functionalized activated carbons as well as other synthetic materials such as zeolites or more costly and sophisticated technologies such as membrane separation [27].
On the other hand, the presence of sodium and potassium can form low melting point eutectics in the gasification bed, which is also a matter of concern [28]. These metals can be removed by chemical pretreatments of glycerin: for instance, Carmona et al. [29] have recently studied the elimination of sodium by ion exchange processes on a strong acid resin. Our research group is also investigating the removal of Na and K by dehydration of glycerin, which results in the precipitation of their respective sulphates.
The higher heating value of the glycerin was 3300 kcal•kg −1 , which is lower than that of pure glycerin [30], as it can be expected due to its content in water and other impurities.
In Figure 2 the weight loss against temperature (TG) and its derivative (DTG) have been plotted. As it can be inferred, glycerin thermal degradation occurs in several different stages. This decomposition profile can be associated with the chemical characteristics of glycerin: first, there is a slight weight loss up to 115 °C, which becomes more marked in the range 115-350 °C, when most of the glycerin has been pyrolyzed. At temperatures higher than 350 °C, the TG curve only shows a residual mass proportion equal to 2%; which is similar to the ash content of the material. The DTG curve shows two peaks centered at 135 °C and 295 °C, being the latter much wider, in accordance to a more persistent weight loss. The first decomposition stage can be associated with the release of water (glycerin is hygroscopic), as well as some low-temperature volatiles, such as methanol, the co-reactant in the transesterification reaction [31]. The second and larger decomposition stage can be associated the degradation of impurities, such as fatty acid methyl esters as well as residues from previous degradation reactions [32]. The thermal instability of glycerin has been previously observed by other authors, who improved this property by blending glycerin with different plasticizers [33].

Influence of Water/Glycerin Inlet Rate
The effect of water/glycerin inlet rate was studied for all temperatures and water/glycerin ratios. However, as the tendencies found in all cases were similar, and for the sake of brevity, only the mixture of water/glycerin (9/1) and the temperature of 900 °C is presented here. Figure 3 shows the evolution of the concentration of the produced gases for this experimental series.

Influence of Water/Glycerin Inlet Rate
The effect of water/glycerin inlet rate was studied for all temperatures and water/glycerin ratios. However, as the tendencies found in all cases were similar, and for the sake of brevity, only the mixture of water/glycerin (9/1) and the temperature of 900˝C is presented here. Figure 3 shows the evolution of the concentration of the produced gases for this experimental series. From the experimental data, the values of H2 molar fraction, highest heating value of the gas (HHV, kJ•N•m −3 ), derived power (kJ•min −1 ), and energy (kJ) per mL of glycerin, were calculated and are given in Table 2. Also, from the data of power and flow rate, the energy obtained per mL of water/glycerin mixture was obtained: Power kJ min 1 Flow rate mL min ⁄ E n e r g y per mL of mixture kJ mL ⁄ From this value and considering the water/biomass ratio, the parameter "energy per mL of glycerin" can be calculated, as: Energy per mL of glycerin Energy per mL of mixture kJ mL of mixture ⁄ Ratio mL of mixture/mL of glycerin (2) From Figure 3, it can be seen that as the inlet flow rate increases, there is an increase in the production of all the gases (this effect is not seen in the 2.0-2.5 range). Therefore, the power also gets greater. Regarding hydrogen, in general, its molar fraction decreases as the flow rate increases, being this effect more marked for lower rates (see Table 2). The slight decrease on the HHV is related to the lower proportion of hydrogen.
It is interesting to highlight that the molar fraction of CO2 as well as that of CH4 exhibited greater values for higher inlet flow rates. The reactions involved in hydrogen production by steam reforming of glycerin (C3H8O3) can be described by Equations (3)-(8): From the experimental data, the values of H 2 molar fraction, highest heating value of the gas (HHV, kJ¨N¨m´3), derived power (kJ¨min´1), and energy (kJ) per mL of glycerin, were calculated and are given in Table 2. Also, from the data of power and flow rate, the energy obtained per mL of water/glycerin mixture was obtained:

Power pkJ{minq¨1
Flow rate pmL{minq " Energy per mL of mixture pkJ{mLq From this value and considering the water/biomass ratio, the parameter "energy per mL of glycerin" can be calculated, as:

Energy per mL of glycerin "
Energy per mL of mixture pkJ{mL of mixtureq Ratio pmL of mixture{mL of glycerinq (2) From Figure 3, it can be seen that as the inlet flow rate increases, there is an increase in the production of all the gases (this effect is not seen in the 2.0-2.5 range). Therefore, the power also gets greater. Regarding hydrogen, in general, its molar fraction decreases as the flow rate increases, being this effect more marked for lower rates (see Table 2). The slight decrease on the HHV is related to the lower proportion of hydrogen.
It is interesting to highlight that the molar fraction of CO 2 as well as that of CH 4 exhibited greater values for higher inlet flow rates. The reactions involved in hydrogen production by steam reforming of glycerin (C 3 H 8 O 3 ) can be described by Equations (3)-(8): C`CO 2 Ø CO (8) All the reactions are favored by low pressure, so the process is usually carried out at atmospheric pressure. If we take into account the abovementioned chemical equilibria, we could attribute the results obtained in this series to the participation of the methanation reaction (Equation (7)), as well as the displacement of the water gas shift reaction towards the production of CO 2 (Equation (6)).

Influence of Temperature
The molar production of the gases analyzed during the glycerin steam gasification processes was investigated under different temperature conditions (600-900˝C). Again, the experiments were made for all the possible experimental conditions of inlet flow rate and water/glycerin ratio. However, in order to avoid excessive graphical information and because the effect was found to be similar for all cases, we present in Figure 4 the results corresponding to an inlet flow rate of 3 mL min´1 and a water/glycerin ratio of 9/1. 2C + 2H2O ↔ CO2 + CH4 (7) C + CO2 ↔ CO (8) All the reactions are favored by low pressure, so the process is usually carried out at atmospheric pressure. If we take into account the abovementioned chemical equilibria, we could attribute the results obtained in this series to the participation of the methanation reaction (Equation (7)), as well as the displacement of the water gas shift reaction towards the production of CO2 (Equation (6)).

Influence of Temperature
The molar production of the gases analyzed during the glycerin steam gasification processes was investigated under different temperature conditions (600-900 °C). Again, the experiments were made for all the possible experimental conditions of inlet flow rate and water/glycerin ratio. However, in order to avoid excessive graphical information and because the effect was found to be similar for all cases, we present in Figure 4 the results corresponding to an inlet flow rate of 3 mL min −1 and a water/glycerin ratio of 9/1. From the experimental data, we calculated the previously described characteristic parameters, which are listed in Table 3. From the gas profiles collected in Figure 4, as well as Table 3, one can infer several conclusions; firstly, the molar production of H2 and CO2 shows a gradual increase as the temperature gets greater. Also, the evolution of CO shows a defined increase in the 600-700 °C temperature range but then decreases slightly for higher temperatures. CH4 presents the same tendency than CO, although the changes are very slight. Regarding the molar fraction of hydrogen (Table 3), it is improved at 900 °C as well as the corresponding power. However, the reduction of CH4 and CO cause a decrease on the heating value of the gas. From the experimental data, we calculated the previously described characteristic parameters, which are listed in Table 3. From the gas profiles collected in Figure 4, as well as Table 3, one can infer several conclusions; firstly, the molar production of H 2 and CO 2 shows a gradual increase as the temperature gets greater. Also, the evolution of CO shows a defined increase in the 600-700˝C temperature range but then decreases slightly for higher temperatures. CH 4 presents the same tendency than CO, although the changes are very slight. Regarding the molar fraction of hydrogen (Table 3), it is improved at 900˝C as well as the corresponding power. However, the reduction of CH 4 and CO cause a decrease on the heating value of the gas. The literatures show abundant studies on the effect of temperature during steam gasification of biomass [21,22,34]. Changes in this variable within relatively narrow limits can produce significant shifts of the equilibrium composition towards either the starting materials or the end products [35]. In general, an increase in hydrogen production is always found as temperature is raised, which is mainly attributed to the participation of water gas reaction (Equation (5)). The water gas equilibrium shift (Equation (6)) might also be present; in this case, the decrease in CO would support this hypothesis.

Influence of Water/Glycerin Ratio
In this experimental series, the water/glycerin ratio was studied in the range 6/1, 9/1, 12/1, under all the experimental combinations of temperature and inlet flow rate. Figure 5 shows the results corresponding to a temperature of 900˝C, and an inlet flow rate of 3 mL min´1. From the experimental data, we calculated the characteristic parameters previously described for this series, which are listed in Table 4.  The literatures show abundant studies on the effect of temperature during steam gasification of biomass [21,22,34]. Changes in this variable within relatively narrow limits can produce significant shifts of the equilibrium composition towards either the starting materials or the end products [35]. In general, an increase in hydrogen production is always found as temperature is raised, which is mainly attributed to the participation of water gas reaction (Equation (5)). The water gas equilibrium shift (Equation (6)) might also be present; in this case, the decrease in CO would support this hypothesis.

Influence of Water/Glycerin Ratio
In this experimental series, the water/glycerin ratio was studied in the range 6/1, 9/1, 12/1, under all the experimental combinations of temperature and inlet flow rate. Figure 5 shows the results corresponding to a temperature of 900 °C, and an inlet flow rate of 3 mL min −1 . From the experimental data, we calculated the characteristic parameters previously described for this series, which are listed in Table 4.  From the analysis of the evolution of gas production for different water/glycerin ratios, under all the conditions analyzed, the following tendencies are inferred: in the first place, there is an increase in the total production of gases as the proportion of glycerin in the inlet mixture is raised, and on the hydrogen produced per mL of glycerin fed.  From the analysis of the evolution of gas production for different water/glycerin ratios, under all the conditions analyzed, the following tendencies are inferred: in the first place, there is an increase in the total production of gases as the proportion of glycerin in the inlet mixture is raised, and on the hydrogen produced per mL of glycerin fed.
This tendency can be related to the fact that an excess in water produces the displacement of Equation (3) towards the products and is consistent with other works which obtained the highest quantity of hydrogen with excess water at all temperatures [13]. Some authors [18] have obtained a cut point temperature showing a decrease in H 2 production for higher water/glycerin ratios. However, in our case, the hydrogen production was favoured with this parameter throughout the whole range studied. It is also noticeable that this effect is more marked at greater temperatures, which is consistent with the endothermicity of equilibrium in Equation (3).
Finally, it is outstanding that the parameter "energy per mL of glycerin" does not follow the trend of the power, as it happened for Tables 2 and 3. While less glycerin dilution causes enhanced gasification, the consideration of the amount of glycerin that is really used in the process can offer a more realistic identification of optimal conditions, if, for example, the aim is to use a lower quantity of raw material.
From the results obtained in this work it can be stated that steam gasification of glycerin can yield a gas with an energy content close to 10 MJ/Nm 3 . The literature shows that this value is similar or higher [23,36] than that provided by previous studies on the steam gasification of biomass sources. Taking into account that biomass feedstock needs previous grinding and drying conditioning processing, the interest of the present work can be highlighted, since the glycerin here is used just as it is generated in the biodiesel production process.
On the other hand, the authors are concerned in relation to the energy consumption of the process, and, in this frame, future works will be devoted to study the possibility of installing a heat interchanger at the reactor exit, and thus take advantage of the fumes' heat, which would be used to provide part of the energy needed to produce steam.

Conclusions
In this work, the glycerin generated during a biodiesel manufacturing process was used as a feedstock for hydrogen production, in order to valorize this byproduct, which, due to its low purity, cannot be used for the traditional manufacture of other materials. Different variables influencing the process were studied (temperature, water/glycerin ratio and inlet flow rate) in order to optimize the process.
The results obtained allow us to obtain the following conclusions: (1) Increasing the inlet mixture flow rate is beneficial in order to produce a greater amount of gas and higher power, although it is detrimental if the final goal is to obtain a hydrogen-rich gas. (2) The addition of water to crude glycerine can be interesting because it provides a greater glycerin reforming. In addition, it moves the equilibria water gas and water gas shift towards the production of hydrogen. (3) Using higher temperatures is interesting for providing a greater fraction of hydrogen, although it also involves a decrease in the heating value of the gas. (4) Further research will be devoted to improve the system energy efficiency by studying the incorporation of a heat recovery system and thus taking advantage of the physical exergy of the gas. Also, further studies will address pretreatments of the raw material as well as treatments of the gases in order to mitigate this environmental problem.