## 1 Introduction  
The production of biofuels from renewable resources, such as agricultural crops, woody and herbaceous biomass, and waste materials, are critical in sustaining the world's economy and responding to global climate change. [1] Corn stover (CS), as one of these renewable resources, is a valuable feed-stock for the production of bio-products due to its easy renewability, low cost and abundance in the nature [2; 3; 4]. It is one of the most abundant agricultural wastes; as of 2015, its production reached 224.6 million tons only in China [5]. CS can be transformed to biogas by anaerobic digestion, which can potentially replace fossil fuels in heat and power generation [6]. Conversion of biomass to biogas involves a series of biological processes in the anaerobic digestion. Hydrolytic bacterium hydrolyzes the biodegradable material of biomass at first, and then methanogens generate biogas from hydrolyzed material under anaerobic conditions [7]. Material biodegradability is the most important factor in anaerobic digestion, while numerous factors can affect the efficiency of digestion [6; 8]. Lignin is found to be one of main reasons which result in the difficulty in anaerobic digestion of biomass like CS; there is a strong negative correlation with conversion rate of polysaccharides [9; 10]. Lignin is a complex aromatic polymer present in the cell wall of CS that prevents microbial action into the interior of lignocellulosicoses [11]. An effective pretreatment step prior to biogas conversion is necessarily required to reduce resistance by altering the chemical and structural features of the biomass [12]. Therefore, pretreatment of lignin is very important to produce biogas from biomass. The purpose of pretreatment process is not only to separate lignin from cellulose and hemicellulose of lignocellulosicoses, but also simultaneously reduces cellulose crystallinity and increases the porosity of the biomass [13]. Sodium hydroxide (NaOH) pretreatment has been reported as one of the most effective chemical methods for improving biomass biodegradability [12; 14; 15], and is advantageous since it is conducted at a milder temperature compared to other pretreatment technologies (including physical, chemical, biological, and so on) [1; 16; 17; 18; 19; 14; 13]. Compared to acid pretreatment, the solubility of cellulose is not increased to lesser degree with the alkali pretreatment [1; 18; 20]. Saponification reactions of intermolecular ester bonds cross-linking cellulose and hemicelluloses or lignin occur in the lignocellulosic materials during the NaOH pretreatment process [21]. It decreases the crystallinity of cellulose and degree of sugar polymerization by disrupting the lignin structure [22]. The increased substrate porosity and carbohydrate exposure can make biomass more accessible to cellulolytic microorganisms compared to raw materials [23], resulting in improved biogas production. For example, the degradation rate of lignin and biogas yield of CS could be improved by 46.2% and 48.5%, respectively, after the pretreatment of CS with 7.5% NaOH [24]. The pretreatment of CS with NaOH combined with methanol significantly enhanced the enzyme accessibility of CS, resulting in 97.5% glucan and 55.0% lignin removal efficiency, respectively [25]. When CS was pretreated with 2% NaOH combined with 78 mg/L ozone at 80 \({}^{\circ}\)C, the enzymatic hydrolysis degree of the cellulose in CS could be improved, achieving the maximum lignin removal rate of 88.9% [26]. However, NaOH pretreatment alone is inefficient for large-scale applications mainly due to its cost and strong alkalinity. The considerable loss of cellulose and hemicellulose was also reported during NaOH pretreatment [5]. Although 86.1% lignin was removed by NaOH pretreatment, 93.8% hemicellulose that could be converted to sugar was lost at the same time [27]. Comparing with NaOH, CaO is considered a cheaper pretreatment alternative in the market; it can provide a certain alkalinity as calcium hydroxide (Ca(OH)\({}_{2}\)) while reacting with water [20].