Sequential parametric optimization of methane production from different sources of forest raw material

Abstract

The increase in environmental problems and the shortage of fossil fuels have led to the need for action in the development of sustainable and renewable fuels. Methane is produced through anaerobic digestion of organic materials and is a biofuel with very promising characteristics. The success in using methane as a biofuel has resulted in the operation of several commercial-scale plants and the need to exploit novel materials to be used. Forest biomass can serve as an excellent candidate for use as raw material for anaerobic digestion. During this work, both hardwood and softwood species—which are representative of the forests of Sweden—were used for the production of methane. Initially, when untreated forest materials were used for the anaerobic digestion, the yields obtained were very low, even with the addition of enzymes, reaching a maximum of only 40 mL CH4/g VS when birch was used. When hydrothermal pretreatment was applied, the enzymatic digestibility improved up to 6.7 times relative to that without pretreatment, and the yield of methane reached up to 254 mL CH4/g VS. Then the effect of chemical/enzymatic detoxification was examined, where laccase treatment improved the methane yield from the more harshly pretreated materials while it had no effect on the more mildly pretreated material. Finally, addition of cellulolytic enzymes during the digestion improved the methane yields from spruce and pine, whereas for birch separate saccharification was more beneficial. To achieve high yields in spruce 30 filter paper units (FPU)/g was necessary, whereas 15 FPU/g was enough when pine and birch were used. During this work, the highest methane yields obtained from pine and birch were 179.9 mL CH4/g VS and 304.8 mL CH4/g VS, respectively. For mildly and severely pretreated spruce, the methane yields reached 259.4 mL CH4/g VS and 276.3 mL CH4/g VS, respectively. We have shown that forest material can serve as raw material for efficient production of methane. The initially low yields from the untreated materials were significantly improved by the introduction of a hydrothermal pretreatment. Moreover, enzymatic detoxification was beneficial, but mainly for severely pretreated materials. Finally, enzymatic saccharification increased the methane yields even further.

High-Titer Methane from Organosolv-Pretreated Spruce and Birch

“…The highest yield obtained from birch was 284.2 mL/g VS. This yield was close to that obtained in our previous study (305 mL CH 4 /g VS) from birch that was given hydothermal pretreatment with additional SO 2 as a catalyst and cellulolytic enzymes [52]. One major difference was that during our previous work, the whole pretreated slurry was used, which also contained sugars from hemicellulose, whereas in the current work only the cellulose-rich fiber fraction of the pretreatment was used.…”

“…The improvement in methane yields when cellulolytic enzymes were included can be attributed to the fact that the enzymes lead to better solubilization of cellulose. The hydrolysis of carbohydrates to monomeric sugars is considered to be the rate-limiting step in anaerobic digestion [13,52]. The same positive effect of enzymes on methane yields was also found in our previous work with hydrothermally pretreated spruce [52], and in some other studies in the literature.…”

“…The hydrolysis of carbohydrates to monomeric sugars is considered to be the rate-limiting step in anaerobic digestion [13,52]. The same positive effect of enzymes on methane yields was also found in our previous work with hydrothermally pretreated spruce [52], and in some other studies in the literature. For example, Sonakya et al [53] used wheat grain and treated it with Tizyme Trizyme?…”

“…This was also evidenced by the higher biomass solubilization during the pretreatment experiments with an increase from 25%-30% to 50%-55% when the acid catalyst was included [46]. The highest yield of methane obtained during this part of the work was 185 mL CH 4 /g VS; which in comparison with the biogas production from hydrothermally treated spruce without the use of enzymes [52], the biogas production yield from organosolv treated spruce was between approximately 26% and 94% higher (depending on the severity of the hydrothermally treated spruce). Moreover, in the presence of the acid catalyst, the lignin concentration increased from 31.7% w/w to 37.8% w/w [46] when the duration of pretreatment was increased, and this increase in lignin content could affect the methane yield, as lignin "protects" the cellulose from microbial attacks.…”

“…This was also evidenced by the higher biomass solubilization during the pretreatment experiments with an increase from 25%-30% to 50%-55% when the acid catalyst was included [46]. The highest yield of methane obtained during this part of the work was 185 mL CH4/g VS; which in comparison with the biogas production from hydrothermally treated spruce without the use of enzymes [52], the biogas production yield from organosolv treated spruce was between approximately 26% and 94% higher (depending on the severity of the hydrothermally treated spruce).…”