| 摘要 | 当今社会，随着化石燃料的日益枯竭和全世界对能源需求的不断提高，开发利用可持续发展、可再生能源成为研究热点。生物质能源是可再生能源之一，通过生物质转化过程，木质纤维素生物质可以通过预处理和酶解糖化产生可发酵糖，从而进一步转化为生物燃料，如生物乙醇等。然而，由于酶解糖化的效率受到木质纤维素生物质天然抗降解屏障的制约，因此，需要开发高效的预处理技术来打破其天然屏障来促进酶水解。本论文以木质纤维素生物质（玉米秸秆）为研究对象，对其进行了基于氢氧化钠的改良碱法预处理研究，以促进最终的酶解糖化效果。本论文的主要研究内容如下：
首先，对经过氢氧化钠预处理后的玉米秸秆进行了PFI（pulp refining instrument）精磨处理，考察了玉米秸秆的PFI精磨程度对酶解糖化效率和最终总糖得率的影响。结果显示，PFI精磨处理可以提高玉米秸秆的酶解糖化效率。经过PFI精磨处理后，玉米秸秆的孔隙率上升，结晶度下降。并且，玉米秸秆的打浆度与最终的总糖得率呈现出良好的线性相关性。因此，基于PFI精磨后打浆度与总糖得率的关系，本文创新性的提出了对预处理后玉米秸秆的酶解糖化效果进行在线快速预测的简单方法，这对未来工业化生产可发酵糖的在线监测和过程控制十分重要。
其次，本论文将蒽醌（AQ）和表面活性剂加入玉米秸秆的碱法预处理过程中，对玉米秸秆进行了化学品辅助的氢氧化钠预处理。对非离子表面活性剂的亲水亲油平衡值（HLB）与预处理后玉米秸秆的总糖得率之间的关系进行了研究。研究结果显示：总糖得率（y）与HLB值（x）之间存在较好的线性关系，可以用以下方程表示：y = 0.344x + 48.745, (R2 = 0.9336)。并且，使用主成分分析法（PCA）和偏最小二乘法（PLS）对玉米秸秆碱法预处理工艺变量参数进行了多变量分析，得出了变量之间存在的相互关联和影响。
再次，本论文对木质素进行在线磺化回用于预处理过程以促进酶解糖化的玉米秸秆改良碱法预处理进行了研究。将部分预处理废液中的木质素进行磺化处理，制备木质素磺酸钠（SLS），并将剩余废液在下一轮预处理中进行直接回用。结果显示，玉米秸秆经过11 wt% NaOH, 2 wt% SLS 和0.1 wt% AQ的碱法预处理后，木质素脱除率为78%，总糖得率为80%，与不加任何助剂的空白氢氧化钠预处理相比，木质素脱除率提高了6%，总糖得率提高了9%。并且发现，木质素磺酸钠的磺化度与总糖得率呈线性相关性。
然后，对改良碱法预处理玉米秸秆的酶解糖化工艺进行了优化，较佳的酶解条件为底物浓度为8wt.%，pH为4.4-4.7，纤维素酶用量为20 FPU/g-底物，β-葡萄糖苷酶的用量为5 IU/g-底物，木聚糖酶的用量为5 U/g-底物，酶解温度为50℃，酶解时间为48h。此条件下，玉米秸秆的酶水解糖化总糖得率达到80%。并且结果发现酶解过程加入表面活性剂能提高酶解效果，加入2 wt.% Tween 60，总糖得率可提高约3%。
最后，对玉米秸秆改良碱法预处理进行了中试规模的放大研究，对圆盘精磨和PFI精磨两种不同方式的精磨效果进行了对比。结果显示圆盘精磨更加低能高效。圆盘精磨的最佳运行参数如下：转数为1500 rpm, 精磨间隙为0.2 mm, 圆盘精磨时间为0.25 min, 精磨压力为0.4 Mpa，精磨物料输入量为100 kg/h。经过圆盘精磨辅助的改良碱法预处理再酶解糖化后，玉米秸秆的总糖得率达到81%，酶解液中糖浓度约为71 g/L。中试结果证明，玉米秸秆的改良碱法预处理是具有工业化潜力的预处理方法。 |
| 其他摘要 | Nowadays, with the depleting of fossil fuels and the increasing of energy requirement around the world, more attentions have been paid on the development of renewable and sustainable energy alternatives. Bio-energy is one of the sustainable energy alternatives. For lignocellulosic biomass conversion, lignocellulosic biomass can be converted by pretreatment and enzymatic saccharification to produce fermentable sugars, which can be further converted to biofuels (such as bio-ethanol). However, the efficiency of enzymatic saccharification is highly limited by the natural recalcitrance of lignocelluloses. Therefore, cost-effective pretreatment of lignocelluloses is needful to remove these obstacles to the downstream processes of biomass conversion. In this paper, modified alkaline pretreatment for improving the enzymatic saccharification of corn stover was studied. The main contents of this study were as follows:
Firstly, in this paper, corn stover was refined by a pulp refining instrument (PFI refiner) after NaOH pretreatment under varied conditions. The quantitative characterization of the influence of PFI refining on the efficiency of enzymatic hydrolysis and final total sugar yields was studied, and it was proved that the enhancement of enzymatic saccharification by PFI refining of the pretreated corn stover was largely due to the significant increment of porosity of substrates and the reduction of cellulose crystallinity. Furthermore, a linear relationship between beating degree and final total sugar yields was found, and a simple way to predict the final total sugar yields by easily testing the beating degree of PFI refined corn stover was established. Therefore, this paper provided the possibility and feasibility for easily monitoring the fermentable sugar production by the simple test of beating degree, and this will be of significant importance for the monitoring and controlling of industrial production in the future.
Secondly, AQ and surfactant was added in the alkaline pretreatment as a pretreatment additive. The relationship between hydrophile-lipophile balance (HLB) value of nonionic surfactants and the final total sugar yields of corn stover pretreated by NaOH pretreatment assisted with AQ and nonionic surfactants was studied. The results showed that the total sugar yields (y) as a function of HLB value (x) of nonionic surfactants can be expressed by the following equation: y = 0.344x + 48.745, (R2 = 0.9336). Moreover, in this work, the process parameters were characterized by two multivariate analysis methods of Principle Component Analysis (PCA) and Partial Least Square (PLS) to investigate the detail relationships of the key process variables of alkaline pretreatment of corn stover, and the results showed pretreatment system was multivariate and the variables were inter-relted to different extents.
Thirdly, a modified alkali pretreatment process was studied, based on the on-line production and utilization of lignosulfonate for improving enzymatic saccharification of corn stover. Sodium lignosulfonate (SLS) produced by directly sulfonating lignin in spent liquor separated from alkali pretreatment was reused in the next cycle of alkali pretreatment. The results showed that, by using 11 wt% NaOH, 2 wt% SLS and 0.1 wt% anthraquinone (AQ) in this pretreatment, delignification rate and final total sugar yields could reach about 78% and 80% respectively, which were about 6% and 9% higher compared to the blank conventional NaOH pretreatment under the same conditions. In addition, sulfonation degree of SLS had linearly impact on the final total sugar yields.
Then, the enzymatic hydrolysis of modified alkali pretreated corn stover was optimized, and the optimal enzymatic hydrolysis conditions were: solid content 8%, pH 4.4-4.7, cellulase dosage 20 FPU/g-substrate, β-glucosidase dose 5 IU/g- substrate, xylanase dosage 5 U/g-substrate, enzymatic hydrolysis temperature 50 ºC and enzymatic hydrolysis duration 48 h. Under such conditions, about 80% final total sugar yields could be obtained.
Finally, the pilot scale experiment of modified alkaline pretreatment was studied. Comparable results could be obtained either by MAP + PFI beating or by MAP + disc refining, but disc refining was more cost-effective. The optimum operation parameters were as follows: 1500 rpm (speed); 0.2 mm (gap); 0.25min (time); 0.4 Mpa (steam pressure); 100 kg/h (input). After MAP + disc refining + enzymatic hydrolysis, 81% final total sugar yields could be obtained and the sugar concentration of final solution was about 71 g/L. The modified alkaline pretreatment of corn stover was proved to be a promising pretreatment method which would be potential for the future industrial production. |
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