Combined production of bioethanol and biogas from wheat straw

Due to the problems with environment and green house gases more biofuels as bioethanol and biogas needs to be produced to reduce the problems. Wheat straw as raw material is a good choice for ethanol and biogas production since wheat is the most produced crop in Sweden and the whole Europe and it isn’t competitive to food. In this study three different options for pretreatment of wheat straw, comprising steam pretreatment with water, acetic acid or phosphoric acid, were investigated and the product was then investigated in two different process alternatives with simultaneous saccharification and fermentation (SSF) to ethanol and anaerobic digestion (AD) to biogas. In the first alternative pretreated slurry was used in the SSF and the filtered stillage was used in the AD. In the second alternative washed cake was used in the SSF and the hydrolysate was used in the AD. When slurry, from the steam pretreatment with phosphoric acid as impregnation material, was used in the SSF, the highest ethanol yield, 83% of theoretical, was achieved. The highest methane yield, 754 ml CH4/g VSfed , was achieved when using filtered stillage, where the wheat straw had been pretreated with acetic acid. The overall best yield was obtained when using phosphoric acid as impregnation material, the whole slurry to SSF and the filtered stillage to AD. The recovered energy in ethanol and methane compared to that in the wheat straw, based on higher heating values, was for this process configuration 60%. Introduction The problems with the environment and green house emissions have led to that the European Parliament has set goals that have to be achieved in year 2020. The green house gases have to be reduced by 20 % and at least 10% of the fuel has to be biofuel. [1] To achieve this goal an investment in bioethanol and biogas can be a solution. Using wheat straw as raw material is a good choice since wheat is the most produced crop in Sweden and the whole Europe and it isn’t competitive to food. [2] The wheat straw consists of three different types of main constituents, cellulose, hemicellulose and lignin. Cellulose consists of tightly packed glucose chains while hemicellulose consists of a branched network, where xylose is dominating. [3] Since hemicellulose consists of mostly pentose sugars and the most techniques for ethanol production are improved for hexoses, it is desirable to separate the hemicellulose and the cellulose and have an alternative usage for the hemicelluloses, like biogas production. The production of ethanol from lignocellulosic material consists of five main steps, pretreatment, enzymatic hydrolysis of cellulose, fermentation of hexoses, separation and effluent treatment. [4] Steam explosion is a pretreatment method, where the material is treated with saturated steam at high pressure during a time period from seconds to several minutes, after which the material is suddenly depressurised. The hemicellulose is solubilized and the cellulose is exposed, which enhances the enzyme accessibility during hydrolysis. When the hemicellulose is solubilized, acetyl groups are cleaved off and forms acetic acid. The main disadvantages of steam explosion are the partially sugar degradation to 5-hydroxymethylfurfural (HMF) and furfural and the generation of other toxic compounds, which could have a negative effect on the following hydrolysis and fermentation steps. The steam explosion can be combined with dilute acids. The most common used acid, sulphuric acid, has though a negative effect on the anaerobic digestion (AD) in case the stillage is used in AD, leading to less produced methane. Using organic acids has a potential in high hydrolysis yields and lower degradation products compared to sulphuric acid. In addition to that, organic acids have the advantage that they can be used as raw material in the anaerobic digestion, resulting in a higher biogas production. [5, 6] Phosphoric acid, which is not an organic acid, might be a good acid to use, since phosphor is needed as nutrient in the anaerobic digestion. During enzymatic hydrolysis the cellulose is converted into glucose monomers, which are fermented to ethanol during the fermentation. By combining these steps in simultaneous saccharification and fermentation (SSF), a shorter residence time is needed and also problem with glucose inhibition in the hydrolysis step is avoided. [7] Biogas is produced by anaerobic digestion (AD), which is a complex microbiological process with different kind of microorganisms which have to collaborate. AD is divided in four steps, hydrolysis, fermentation, anaerobic oxidation and methanogenesis. In the first step fats, proteins and carbohydrates are converted into monomers and oligomers, which are fermented to organic acids, alcohols and ammonia. In the third step, acetates, hydrogen and carbon dioxide are formed. And in the last step the acetate is used to form methane and carbon dioxide. The last step is also the limiting step since the methanogens grows slowest. [6] Depending on the substrate, the biogas potential is better or worse. Monomer sugars are easy to digest, but since the methanogens have a slow growth the digestion of the fatty acids are limited, which can result in a lower alkanity. Stillage from ethanol production can have high protein content since almost all sugar has been fermented, which can result in inhibition by ammonia. Combining stillage with sugar like not fermentable pentoses from wheat straw can maybe improve the biogas production. [6] In this study, three different soaking materials were tested combined with the steam pretreatment, water, 1 % acetic acid and 0.4 % phosphoric acid. The pretreated slurry was then investigated in two different process configurations, see Figures 1 and 2. In the first configuration, the SSF was done on the whole slurry from the pretreatment. The slurry after SSF was stripped so the ethanol was separated from the slurry. The rest product was filtered and the liquid part was taken to the AD. In the second configuration, a separation was made after the pretreatment and the liquid part was taken for the AD and the solid part to the SSF. Figure 1 Schematic flowsheet of the first process configuration Figure 2 Schematic flowsheet of the second process configuration Materials and Methods Raw material Wheat straw was provided as full length straw in bales and had a total solid content (TS) of 92%. The wheat straw was grinded in a hammer mill with a 20 mm insertion and sieved to obtain 1-5 cm long pieces. Steam pretreatment The straw was treated in a soaking step with water, 1 % acetic acid and 0.4 % phosphoric acid, respectively, at room temperature and with the residence time of 30 min and was then pressed in a tincture press with a pressure of 270 bar to a TS (total solid content) of 45-50 %. The last step in the pretreatment was steam explosion with 400 g dry material in a 10-litre reactor at 190°C for 10 min. SSF A 2-litre fermentor (Infors AG) was used during the SSF and it was sterilized at 120°C for 20 minutes. A total amount of 1kg was filled in the fermentation flasks. The substrate was diluted to a water insoluble content (WIS) of 7.5%. Nutrients as (NH4)2HPO4 and MgSO4*7H2O with the concentration of 0.5 and 0.025 g l -1 respectively, Celluclast 1.5L and Novozyme 188L at 15 FPU/g WIS and 25 IU/g WIS were added. Everything was stirred by hand, except the material that had been pretreated with phosphoric acid, to ease the mechanical stirring before the material was fed to the fermentor. When the material was placed in the fermentor, the pH was adjusted by hand with 10 % sodium hydroxide, to pH 4.8 and then the baker’s yeast, saccharomyces cerevisiae, at 5 g l -1 was added. In the case of using phosphoric acid pretreated material, 5 % ammonia was used for pH adjustment. The ammonia was added before the material was fed to the fermentor to get a rough adjustment. The pH was adjusted during the run with a buffer of 10 % sodium hydroxide or 5 % ammonia and 10 % sulphuric acid. The temperature was set to 35°C and the residence time was set to about 140 hours. Rotary evaporation The slurry after SSF was evaporated in a rotary evaporation with a water bath temperature at 80°C and was performed until about one deciliter had passed the cooling system and collected in a round bottom flask. AD The AD was performed in a 1220 respectively 1070 ml reactors from BlueSens. The volume of produced biogas was measured with a milligas counter and the methane concentration was measured with IR sensor head and measuring cap. The equipments for the biogas production were calibrated for half an hour. The pH of the substrate was adjusted to pH 7 by hand with ammonia. The reactor was filled with a total amount of 500 ml. The ratio of volatile solids between the substrate and the sludge was set to 1:1. The substrate was added undiluted. Anaerobic sludge was taken from Domsjö waste water treatment plant and was delivered from Domsjö Industries. It was stored in plastic cans at room temperature. The environment for the sludge was totally anaerobic. The reactor was filled with nitrogen gas during one minute to make the atmosphere anaerobic. A low stirring rate was set for a homogenous environment. The temperature for the water bath was set to 37°C. The experiment was running between one and three weeks till the gas production was leveled out. After the experiment was ended the slurry was centrifuged at 3500 rpm for 10 min to separate the solid part with the liquid part. Analysis TS were determined by drying the sample in the oven at 105°C for at least 20 hours. VS were determined by ashing the 105°C dried sample in an ash oven at 550°C for two hours. The composition of straw, the WIS and the hydrolysate were determined according to NREL procedures. [8, 9] HPLC analysis was made using Shimadzu HPLC equipped with an RI detector. All samples were filtered through 0.20 μm cellulose acetate filter. Hydrogen column was used to separate galactose, lactic acid, glycerol, formic acid, acetic acid, ethanol, HMF and furfural. The oven temperature was set to 50°C. Lead column was used to separate monomeric sugars. The oven temperature for the lead colon was set to 85°C. Results and discussion Composition of wheat straw The composition of dry wheat straw is shown in Table 1. The ash content is very low while it should be somewhere around 5 %. Table 1 Composition of wheat straw in g per 100 g dry material