Lime Pretreatment
Lime is the highly caustic mineral material obtained by heating or calcination of limestone; during this process quick lime or calcium oxide (Cao) is formed, and through subsequent addition of water, the less caustic but still strongly alkaline slaked lime or hydrated lime (calcium hydroxide (Ca(OH)2) is produced. Slurry of slaked lime is used in the biomass pretreatment process. The lime pretreatment is carried out by slurrying the lime with water, spraying it on to the biomass material, and then storing the material in a pile for a period of hours to weeks. The particle size of the biomass is typically 10 mm or less. Lime pretreatment processes can be carried out at lower temperatures compared to other pretreatment technologies. Elevated temperatures can be used to reduce contact time from days to hours; for example, 3 h at 85°C for wheat straw, and 13 h at 100°C for corn stover have been used in the lime pretreatment method. Unlike acid-catalyzed pretreatments, some of the alkali is converted to irrecoverable salts or incorporated as salts into the biomass by the pretreatment reactions, and this is a definite disadvantage in the lime pretreatment process. Lime pretreatment works well on soft biomass forms such as rice straw [190], switchgrass [189], sugarcane bagasse [191], corn stover [192], sweet sorghum [193], and bermuda grass [194].
One advantage of lime pretreatment is that the cost of materials required to pretreat a given quantity of biomass is the lowest among alkaline treatments. Though the lime pretreatment is energy intensive, lime can be recovered as insoluble calcium carbonate CaCO3 by precipitation with CO2 after solid-liquid separation [191]. The calcium carbonate can then be converted to quicklime (CaO) using well established lime kiln technology. Another possibility is to utilize the carbon dioxide produced in the fermentation process to capture lime. As a further advancement in this direction, Park and coworkers developed a novel lime-pretreatment process (CaCCO process) that did not require a solid — liquid-separation step [195]. This process also used carbon dioxide neutralization of lime after pretreatment, resulting in a final pH of about 6. In the technique of Park and coworkers, calcium carbonate produced by the process was kept in the reaction vessel and no significant inhibitory effects on enzymatic saccharification and fermentation were observed. In the CaCCO process, solubilized carbohydrates such as xylan, starch, and sucrose were also kept in the vessel, enabling high recoveries of monomeric sugars. During these experiments, simultaneous saccharification and fermentation (SSF) of lime-pretreated rice straw with 10% loading (grams of rice straw/grams of water) using Saccharomyces cerevisiae and Pichia stipitis yielded 19.1 g L -1 ethanol, which was calculated as 74% of the theoretical yield from glucose and xylose in biomass. Thus, this process represents an effective pretreatment method for rice straw with recycling of lime [195].
The same group later improved the CaCCO process by optimization of the room temperature lime pretreatment [196]. They found that keeping lime/dry-biomass ratio at 0.2 (w/w) and leaving room temperature lime-pretreatment for 7 days resulted in an effect on the enzymatic saccharification of cellulose and xylan equivalent to that of the pretreatment at 120°C for 1 h. Furthermore, they noted that sucrose, starch, and P-1,3-1,4-glucan, which could be often detected in rice straw, were mostly stable under the room temperature lime pretreatment condition. Thus, the improved CaCCO process, now called the energy saving room temperature CaCCO process, could preserve/pretreat the feedstock at room temperature in the wet form with minimum loss of carbohydrates [190].