6.5.1 Bioethanol Production

The heterogeneous nature of lignocellulosic biomass feedstock used for production of bioethanol makes the treatment process challenging. An efficient pretreatment to maximize enzymatic hydrolysis efficiency is necessary to reduce economics of the total process. Besides, there is a limitation on the use of acid and alkali in conventional high temperature and high-pressure pretreatment method due to its high-energy input. Alternative heating techniques are sought to reduce energy input at the same time increase the total process efficiency. MW pretreatment could be a good alternative because it can reduce the pretreatment time at higher temperature, and for this reason its use for bioethanol production has been extensively investigated as summarized in Table 6.2.

Binod et al. [13] have reported that MW treatment of sugarcane bagasse with 1 % NaOH at 600 W for 4 min followed by enzymatic hydrolysis gave reducing sugar yield of 0.665 g/g dry biomass. Combining MW-alkali-acid treatments with 1 %

NaOH followed by 1 % sulfuric acid resulted in an increase in reducing sugar yield to 0.83 g/g dry biomass. MW-alkali treatment at 450 W for 5 min resulted in almost 90 % of lignin removal from the bagasse. From the results, they found that combined MW-alkali-acid treatment for short duration enhanced the fermentable sugar yield.

The positive effects of the synergy between alkali and MW irradiation had also been confirmed by the works of Keshwani et al. [14] on hydrolysis of switchgrass. They reported that pretreatment using MW irradiation at lower power levels resulted in more efficient enzymatic hydrolysis. The application of MW irradiation for 10 min at 250 W to switchgrass immersed in 3 % NaOH (w/v) produced the highest yields of reducing sugars. The finding suggests that combined MW and alkali is a promising pretreatment method to enhance enzymatic hydrolysis of switchgrass.

On the contrary, instead of alkali, Balcu et al. [15] proposed the use of acid in combination with MW pretreatment. They found that elevated temperatures close to 180 °C are not necessary for better conversion of lignocellulosic biomass into sugars. They suggested the use of 0.82 % aqueous solution of sulfuric acid, getting very good yield even at low temperature of 140 °C.

Moreover, a two-stage MW pretreatment method, which includes MW-alkali pretreatment for hemicelluloses extraction and MW-glycerine pretreatment for lignin extraction of corn stalk as proposed by Zou et al. [16], seems to be more promising. They reported that MW-alkali pretreatment is suitable for hemicelluloses extraction with the following suitable treatment conditions: liquid-to-solid ratio of 20 mL/g, alkali consumption of 150 wt%, treatment time of 10 min, MW power of 116 W/g and the particle size of 40-80 mesh. Using MW and pure glycerine, lignin can be extracted at optimal treatment time of 30 min, and MW power of 66.7 W/g.

The use of glycerine as a solvent has also been investigated by Intanakul et al. [17] for an improved enzymatic hydrolysis of lignocellulosic wastes by MW pretreatment under atmospheric pressure. The benefits of using glycerine as a solvent include no high-pressure build-up even if the temperature reaches 200 °C. Their results showed that with the pretreatment, more than twice the amount of reducing sugars could be produced from enzyme saccharification compared with no pretreatment at all. Unlike the steam explosion process which requires high-pressure and subsequent pressure release, this technique provides some advantages regarding high temperature and high pressure handling.

The technique was also applied to pretreatment of green coconut fiber for bioethanol production [18]. Prior to MW irradiation for 10 min at 250W, pretreatment using alkaline hydrogen peroxide gave higher yield of reducing sugar (35.98 mg/g) and high ethanol yield (1.16 g/g) compared to alkaline sodium peroxide.

Optimization of the method as applied to pretreatment of wheat straw for ethanol production has also been investigated using various techniques such as orthogonal design (L9(34)) [19]. This optimization technique was applied by investigating the effects of four factors including the ratio of biomass to NaOH solution, pretreatment time, MW power, and the concentration of NaOH solution with three different levels on the chemical composition, cellulose/hemicellulose recoveries, and ethanol con­centration. Results showed that pretreatment with the ratio of biomass to liquid at 80g/kg, the NaOH concentration of 10kg/m3, and the MW power of 1000 W for 15 min was the optimal condition. They obtained ethanol yield of 148.93 g/kg wheat straw at this optimum condition, much higher than that from the untreated material which was only 26.78 g/kg.

The use of ionic liquids (IL) in combination with MW pretreatment was also reported [20]. In this method, MW irradiation enhances the solubility of cellulose in IL while decreasing the degree of polymerization. This results into improved cellulose hydrolysis. Results showed a 50-fold increase in the rate of enzymatic hydrolysis of cotton cellulose when MW irradiation is used in combination with IL dissolution pretreatment at 110 °C, about four times better than when only IL was used for dissolution.

Development of a continuous process, although a very challenging approach for the use of MW irradiation, could be the most economical and efficient for large — scale commercial production of bioethanol. The group of Mitani et al. [21] from Kyoto University (Japan) attempted to develop a prototype for a continuous MW pretreatment system for bioethanol production from woody biomass as shown in schematic diagram in Fig. 6.2. A ceramic pipe was set in a metal vessel, and a mixture of woody biomass, water, and solvents flows through the pipe. MW propagates in the internal space of the metal vessel, and it is absorbed by the mixture since MW can penetrate through the ceramic pipe. In the present system, the mixture flows through a metal pipe, and it is irradiated with MW at T-junction metal pipe sections. The gray arrows and white arrows in Fig. 6.2 show the mixture flow and the MW irradiation direction, respectively. MW frequency of the present pretreatment system is 2.45 GHz-band, which is the same as that of a MW oven. The diameter of the metal pipe is 75 mm.

Metal

pipe

unit

Performing MW pretreatment of a mixture consisting of 70 g of Japanese cedar sapwood chips and 770 g of solvents (ethylene glycol:phosphoric acid = 95:5), about

45.9 % of the total saccharide from woody biomass can be obtained as compared to only 43.6 % with the conventional heating. However, the energy consumption was quite higher at 552 kJ compared to only 498 kJ with the conventional heating. The amount of bioethanol that can be produced from this experiment was estimated to be 14.8 g of bioethanol corresponding to an energy of about 439 kJ.