Characterizing Wheat Straw for Bio-Fuel Applications

Biomass, such as wheat straw, is an inexpensive natural biopolymer found in abundance. It is rich in cellulose and hemicellulose, which can be transformed into bio-fuel. A critical stage in the production of bio-ethanol from wheat straw, is pre-treatment that should facilitate economic feasibility and efficient conversion into bio-fuel.

It is expected that this pre-treatment will increase the number of accessible chemical sites of the straw to improve convertibility in the hydrolysis and fermentation steps that follow. Dynamic vapour sorption (DVS) was used for evaluating the wheat straw pre-treatment, and UV for evaluating the sugar yield.

A schematic view of bio-fuel production is shown in Figure 1. Steps involved include; biomass handling, biomass pre-treatment, cellulose hydrolysis, glucose fermentation and ethanol recovery. For ethanol production, pentose from hemicellulose can also be used without hydrolysis.

Pre-treatment of the lignocellulosic biomass is a common feature of the enzymatic hydrolysis step, and this results in a more efficient reaction. It is an important step as it has direct impact on the subsequent yield of enzymatic hydrolysis to produce glucose, and alcohol fermentation process in the production of bio-fuel.

This particular pre-treatment is used to disturb the microfilbrils’ crystalline structure to release cellulose and hemicellulose polymer chains, and alter amorphous content and open up pores in the biomass to increase the accessible site of straw to enzymatic activity, which may be shown by an increase in specific surface area, hydrophilicity and specific surface area etc.

Figure 2 shows the pre-treatment function. Using the dynamic vapour sorption technique these surface modifications can be characterized quantitatively, in terms of the sorption and desorption behaviour.

Figure 1. Schematic of bio-fuel production

Figure 2. Schematic of pre-treatment

Experimental Procedure

A novel twin-screw extrusion technology was used for pre-treatment of raw wheat straw (RWS). This new technology has been reported to enable pre-treatment of lignocelluloses biomass and offers rapid heat transfer, high shear, rapid and effective mixing and feasibility to combine with other pre-treatment – all in one continuous process. Table 1 shows the pre-treatment conditions, and an illustration of the extruder is shown in Figure 3.

Table 1. Pre-treatment conditions of wheat straw

Figure 3. Illustration of the twin-screw extruder

Sample RWS is raw wheat straw without having undergone any pre-treatment. It is used as a control sample to be compared with the pre-treated sample (WCB10009). A co-rotating twin-screw extruder with H2O and alkaline (NaOH) at 50°C was used for extrusion of the pre- treated sample.

Using the SMS dynamic vapour sorption instrument (DVS), water sorption measurements on the sample were performed. In order to determine when equilibrium is reached, the instrument was run in dm/dt mode (mass variation over time variation) during measurements. At each RH segment, a fixed dm/dt value was chosen. This criterion enables the DVS software to ascertain automatically when equilibrium is reached, and complete a relative humidity step.

The relative humidity set point moves to the next programmed level when the rate of change of mass lies below this threshold over a specified time period. The SMS DVS Analysis Suite version 6.1.1.2 and SMS DVS Analysis Suite version 6.1.1.2 (Advanced) macros were used to perform the analysis.

Results

The dynamic water sorption behaviour on the RWS sample at 25°C can be seen in Figure 4. It can be observed that at lower relative humidity (RH < 50%), equilibrium is established at a much faster rate than at higher RH values. The low uptake seen at lower RH is because of surface sorption. Sorption develops from the surface to the bulk at higher RH, hence the higher uptake.

Figure 4. Dynamics of water sorption in raw wheat straw at 25°C

Figure 5 shows the corresponding isotherms exhibiting a mixed Type II/III behaviour, seen by considerable uptake and low initial sorption at an increased RH. Type II behavior shows a surface monolayer sorption mechanism that makes BET surface calculation feasible through the water sorption measurement.

Figure 5. Isotherm of water sorption on raw wheat straw at 25°C

As biomass pretreatment is an aqueous thermal process, water sorption is preferred. The bulk sorption is further verified by the hysteresis of the desorption. Figure 6 shows the BET plot indicating a specific surface area of 173.45m2/g.

Figure 6. BET plot of water sorption on raw wheat straw at 25°C

Maintaining the same experimental conditions, the water sorption measurement was performed on the pre-treated sample (WCB10009). Figure 7 shows a superimposed plot of the isotherms of the two samples. A mixed Type II/III behaviour, can be seen from all the isotherms.

Figure 7. A superimposed plot of the isotherms of the pre-treated and untreated samples

In terms of total uptake at 95% RH, the pre-treated sample (WCB10009) has an uptake of 59.62%, a much higher value than 24.76% of the untreated sample (RWS). This indicates an open-up of additional accessible sites to water molecules, which will benefit the hydrolysis process.

Reducing cellulose crystallinity is one strategy employed for the increase of enzymatic convertibility for fermentable sugars for bioethanol production as hydrolysis rate of the amorphous area of the cellulose is faster than the crystalline region. The crystalline cellulose surface and amorphous region can be penetrated by most reactants, including water. Hence, the amount of water uptake is indicative of the amorphous content of biomass.

Hysteresis is not observed in the water sorption behaviour on the pre-treated sample unlike the untreated sample. This is because of the pore swelling in the biomass, especially in the amorphous region of the cellulose and hemicellulose, hence less capillary force.

Table 2 shows the results of BET specific surface measurements on the two samples. The specific surface area is slightly increased due to pre-treatment enabling more accessible sites to hydrolytic reaction.

UV spectrum study conducted on the raw and pre-treated wheat straws show that subsequent to pre-treatment, glucose recovery in straw increases by at least 30 times. Figure 8 shows the comparison of glucose yield of raw wheat straw and pre-treated wheat straw.

Table 2. Summary of BET specific surface area of the two samples

Figure 8. Yield of glucose recovery after enzymatic hydrolysis

To support the surface area data on the straw samples, the isotherms were fitted using the SMS Isotherm Suite to the Young and Nelson Isotherm model containing broad range of theoretical and semi-empirical equations to fit sorption isotherms.

The isotherms are then deconvoluted to monolayer and multilayer, so that the Young and Nelson model produces a term related to the strength of vapour interaction with the surface (E term), amount of vapour on the surface (A term) and amount of vapour in the bulk (B term).

The Young and Nelson component plots for samples RWS and WCB10009 are shown in Figures 9 and 10 respectively. Table 3 shows the Young and Nelson parameters for the samples and also show that the surface interactions between the treated sample WCB 10009 and water molecules are significantly higher than the interactions of the untreated RWS sample and water.

The increase in the accessible site of straw to enzymatic activity is shown by the increase in sorption properties, Young and Nelson constant E and specific surface area. The increase in the BET surface area value and the slight increase in the Young and Nelson parameter A for sample WCB 10009 are consistent with each other.

Table 3. BET specific surface area and Young and Nelson parameters for samples RWS and WCB 10009

Figure 9. Young and Nelson component plots for the untreated RWS sample

Figure 10. Young and Nelson component plots for the treated WCB 10009 sample

Conclusion

Using the SMS DVS instrument, two wheat straw samples were characterized. It can be seen from the results that the specific surface area, uptake of the water sorption, the strength of vapour interaction with the surface, and amorphous content of the pre-treated sample are more than those of the untreated raw wheat straw sample.

This proves that there are more accessible sites for the following enzymatic hydrolysis process when the sample is pre-treated with the twin screw extrusion technique. The impact of this on biofuel production yield is substantial. The HPLC study has shown that glucose recovery in straw increases by at least 30 times.

This information has been sourced, reviewed and adapted from materials provided by Surface Measurement Systems Ltd.

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