Simulation of Syngas Production from Lignin Using Guaiacol as a Model Compound

Lignin is an abundant component in biomass that can be used a feedstock for producing several value-added products, including biofuels. However, lignin is a complex molecule (involving in its structure three types of phenylpropane units: coumaryl, coniferyl and sinapyl), which is difficult to implement in any process simulation task. The lignin from softwood is formed mainly by coniferyl units; therefore, in this work the use of the guaiacol molecule to model softwood lignin in the simulation of the syngas process (H2 + CO) is proposed. A Gibbs reactor in ASPEN PLUS® was feed with ratios of water and guaiacol from 0.5 to 20. The pressure was varied from 0.05 to 1.01 MPa and the temperature in the range of 200–3200 °C. H2, CO, CO2, CH4, O2 and C as graphite were considered in the output stream. The pressure, temperature and ratio water/guaiacol conditions for syngas production for different H2/CO ratio are discussed. The obtained results allow to determine the operating conditions to improve the syngas production and show that C as graphite and water decomposition can be avoided. OPEN ACCESS Energies 2015, 8 6706


Introduction
Lignin is one of the most abundant biomass components.It exhibits complex structures that depending on the wood type can include a mixture of the following primary phenylpropane units: cuomaryl, coniferyl and sinapyl.In the lignin from softwood, the amount of coniferyl units is about 90%-95%, the sinapyl units from 5% to10 % and there are no coumaryl units [1].Some compounds, such as guaiacol, anisole, and diphenyl ether have been used as a representative structures of lignin for simulation and experimental studies [2][3][4].In others works, the lignin is specified as a non-conventional component using ultimate and proximate analysis [5,6].Guaiacol comes from coniferyl alcohol, and it can be used as a softwood lignin model compound to analyze biomass decomposition processes such as pyrolysis and gasification, among others.Figures 1 and 2 show the structural formulas for guaiacol and lignin [7], respectively.It should be noticed that the guaiacol structure frequently appears in the structure of lignin, so this compound represents a large portion of it and it could be considered representative of lignin.Lignin has a high potential for producing biofuels and aromatic compounds.The technology for using lignin as a raw material has not been developed much in comparison with technologies to process polysaccharides and, in all cases, lignin is a sub-product of these processes, which opens an opportunity to consider it as a feedstock to downstream process that add value to this important renewable polymer.It can be noted that carbon materials increase the possibilities of mass/energy generation [8], and syngas production is one of them.Syngas is a mixture of hydrogen and carbon monoxide (H2 + CO), which can be obtained by gasification of different materials.Currently, several options like natural gas [9], ethanol [10] and glycerol [11] have been studied to produce syngas or hydrogen in several processes [12,13].Rostrup-Nielsen proposed a syngas cycle that involves steam reforming and gasification to produce syngas from complex materials andFisher-Tropsch synthesis to obtain complex materials [14].Another interesting option is to consider biomass [15], such as lignin, as a raw material to produce energy in an integral way [16,17].
Process simulation has been used to determine the best options and operating conditions for producing biofuels [5,6,[18][19][20][21][22][23][24][25][26], particularly the software ASPEN PLUS ® (Aspen Technology, Inc., Bedford, MA, USA) has been widely used.However, the models used for lignin have oversimplified the process.To produce syngas (smaller molecules that can be obtained) from biomass the simplified models are good, but for bigger molecules (phenol, benzene, etc.) is necessary to use the lignin chemical structure or a model compound.Therefore, in this work there is proposed to use the molecule of guaiacol, which has a H/C and O/C ratio in the range reported in other studies [20], as a lignin model during the simulation of the gasification process of softwood.This will serve as a starting point that will surely culminate in the use of models very close to the actual chemical structure of lignin, so that research can be performed by process simulators for processing into intermediate products with high added value and production chain initiators, all under the concept of biorefineries.

Materials and Methods
A Gibbs reactor was implemented in the software ASPEN PLUS ® for the syngas production, which was fed with water (w) and guaiacol (g).The ratio water/guaiacol (w/g) was varied from 0.25 to 20, four levels of pressure were tested (0.05, 0.1, 0.51 and 1.01 MPa) and the analyzed temperature range was from 200 to 3200 °C.We propose Reaction (1) as the main gasification reaction: The following parallel reactions are also involved, in general, these describe the main pathways to obtain the products proposed: The Gibbs free energy is a thermodynamic variable used to identify the chemical equilibrium in a system, its global minimization allows identifying the amount of substances present in the process.If the system contains n components, the molar fraction of component i is xi; the change of Gibbs free energy, gsys, in the system is defined in Equation ( 11): (11) If Equation ( 11) is derived with respect to and if only two components are present (a and b), the change of the free energy of the system with respect to each component is obtained by Equation ( 12): Here, ° corresponds to the standard Gibbs free energy for the pure component a, R is the universal gas constant and T is the temperature for the system.The minimum free energy of a system is found when Equation ( 12) is equal to zero and by solving the system it is possible to find the equilibrium composition [27], which gives the products and the amount of each one that is present.

Results and Discussion
The effects of pressure, temperature and water/guaiacol ratio (w/g) on the syngas production were studied in this work.The best operating conditions that improve the H2 and CO (syngas) production for gasification were determined.This process was analyzed at 0.05, 0.1, 0.51 and 1.01 MPa, whereby it was found that the syngas production decreases when the pressure increases and increases with temperature until a maximum is reached and then it drops.The best yields were obtained at the pressures of 0.05 and 0.1 MPa, where the maximum H2 productions were 1300 mol/h at 0.05 MPa and 1289 mol/h at 0.1 MPa the Figure 3a,b show the results.For 0.51 MPa the production was 1109 and 930 mol/h for 1.01 MPa, the yield were lower at same temperature (700 °C).It should be noticed in Figure 3a,b that the H2 production exhibits a maximum at around 700 °C, which can be explained by the gasification process, and another one around 3200 °C that confirms the thermal decomposition of water, as it was reported previously [28][29][30][31] and is proposed in Reaction (10). Figure 3c,d are for the case of carbon monoxide production at pressures of 0.05 and 0.1 MPa and exhibit the same highest value (700 mol/h) at both pressures.From these results, the pressure of 0.1 MPa is the one selected for the analysis, which corresponds to the atmospheric pressure.The ratio w/g that allows obtaining the highest values for hydrogen production is from 15 to 20.On the other hand, the oxygen production increases at high temperatures (2200 to 3200 °C), which is shown in Figure 4a; this is consistent with the water decomposition area, Reaction (10).Additionally, the carbon formation as graphite [10,11], Reactions ( 6)-( 8), has to be avoided because it can cause troubles in the processing, methane is necessary in Reaction (8) and at high temperature methane is not present (Figure 5c); therefore, according with the results shown in Figure 4b, the operating conditions have to be chosen as 0.1 MPa, 15-20 w/g ratio and 300-1200 °C as in this zone carbon as graphite is absent.The hydrogen production increases proportionally with the w/g ratio and the temperature up to 700 °C and until 1000 °C.The maximum hydrogen production was obtained at 700 °C and w/g ratio of 20, Figure 5a shows that when the temperature continues increasing the hydrogen production drops.This can be because the Reaction ( 3) is inverted to reach equilibrium contributing to the consumption of carbon dioxide as shown in Figure 5c and on the other hand Reaction ( 8) is not present because methane is absent up to 700 °C as shown in Figure 5d.It should be noted that other processes for hydrogen production have been reported [11] using temperatures of around 900-1000 K (627-727 °C) and also the pyrolysis process for hydrogen production is carried out around 700 °C [13,18].The carbon monoxide production increases proportionally with the temperature and inversely with the w/g ratio.For the temperature range from 300 to 1200 °C, the maximum CO production can be reached at 1200 °C and w/g ratios from 15 to 18 (see Figure 5b).It is important to note that the conditions under which the CO production is maximum are different for the hydrogen production, so the H2/CO mole ratio in syngas has to be observed as a response variable; this will be discussed later.
The CO2 production exhibits a maximum at about 600 °C and then it decreases inversely proportional to the temperature; meanwhile, it is directly proportional to the w/g ratio (see Figure 5c).These results are consistent with the literature that establishes that at about 550 °C the steam reforming process yields the maximum CO2 production [10,12]; for this analysis, the maximum production is obtained at 500-600 °C and with w/g ratios of 16 to 20.The proposed Reactions (2), ( 3) and ( 9) produce CO2, but Reaction ( 9) is present when carbon is in the system as graphite, and as Figure 4b has shown, this is possible for w/g ratios lower than 5 and in this case the w/g ratio is 20.
Figure 5d shows that at low temperatures the production of methane is high so Reactions (4) and (5) are present.Methane is the only substance that has hydrogen in its structure, so if the hydrogen production increases the methane one decreases; the maximum production of hydrogen is observed in Figure 5a at 700 °C, where the production of methane is close to zero [10,12] as shown in Figure 5d.As the Reactions (1)-( 3) and ( 8) have hydrogen as product it can be proposed that these reactions are present until 700 °C, and even that Reaction (10), the water decomposition reaction present at the highest temperature has hydrogen as a product.Additionally, it is possible to see that the methane production is independent of the w/g ratio, as the methane production at low and high w/g ratio values is the same.
The maximum water consumption is at temperatures of 600-700 °C and a w/g ratio of 20, which is shown in Figure 6a; this evidence supports the assumption that the H2 and CO2 production is carried out by the Reactions (1)-( 3), which are present under these conditions.Figure 6b shows the other area in which the water consumption increases due to the water decomposition.The productions of H2, CO, CO2, CH4, O2 and C in the guaiacol gasification at different temperature conditions are shown in Figure 7a,b.If producing syngas (CO + H2) is the target, it is important to convert all the carbon to CO and to minimize the CH4 formation [17]; this condition is reached at a pressure of 0.1 MPa, a w/g ratio of 20 and temperatures of 700-1200 °C (see Figure 7).In Figure 7b, it can be observed that the water decomposition is starting at 2700 °C, and at 3200 °C the oxygen presence and hydrogen increase are clear.Syngas is the raw material for several processes including the Fisher-Tropsch synthesis, where there are different building blocks according with the H2/CO ratio, as air gasification it is lower or equal to 1, a value higher than 1 is obtained with steam gasification and it is ideal for liquid hydrocarbons, a common target ratio is 2 [14,17].Depending on the operating temperature, different H2/CO ratios are obtained for the syngas process (see Figure 8), from 800 °C the H2/CO ratios are from 2 to 3.5, which is similar to those obtained with natural gas technologies [9].If it is decided to maximize the H2/CO ratio, the optimum temperature is 700 °C, pressure 0.1 MPa and ratio w/g 20; the energy required for this conditions is 350.87 GJ/h.

Conclusions
The syngas production process from water and lignin modeled as guaiacol based on the Gibbs free energy was proposed for the first time in this work.The effects of pressure, temperature and w/g ratio were studied.Pressure values above 0.1 MPa decrease the syngas production and lower values do not significantly increase its production.The w/g ratio that lets one obtain higher hydrogen production was from 15 to 20.Temperatures in the range of 300-1200 °C allow obtaining different amounts of H2, CO, CO2, CH4 without water decomposition and in the absence of carbon as graphite.The ratio w/g 20, 0.1 MPa and temperature from 700-1200 °C enables the production of syngas with H2/CO ratios from 4.4 to 2.3.The water decomposition region was identified at temperatures from 2200 to 3200 °C.The maximum hydrogen production was observed at 700 °C and at this point the methane was practically absent.If water was the limiting reactant at temperatures higher than 1200 °C, graphite formation was observed.This work contributes to the knowledge needed to use biomass in an integrated way for the production of biofuels and high molecular weight compounds.

Figure 1 .
Figure 1.The structure of guaiacol, the lignin model compound used in this study.

Figure 4 .
Figure 4. Production of (a) oxygen and (b) carbon as graphite at 0.1 MPa.

Figure 8 .
Figure 8.The H2/CO ratio in syngas value at 0.1 MPa, ratio w/g 20 and different temperatures.