进料浓度对玉米秸秆与牛粪全混式厌氧发酵特征影响研究

目前,中国大多数沼气工程均采用农作物秸秆与畜禽粪便为主要原料,但对于实际沼气工程的工艺控制,尤其是对于沼气工程进料混配及发酵浓度等控制工艺仍缺少参考和支撑。该研究采用玉米秸秆与牛粪原料,在中温条件下,利用纯牛粪、纯秸秆以及秸秆与牛粪干物质比(S:CM)=1:1、1:3、3:1条件的混合原料,按照不同干物质浓度(total solid,TS)=3%、6%、8%,梯次启动CSTR反应器,系统探讨物料配比与发酵浓度对反应器产气量、甲烷体积分数、pH值、氧化还原电位(oxidation-reductionpotential,ORP)、挥发酸(volatilefattyacids,VFAs)含量等运行特征的影响。结果表明,正常运行时,纯秸秆与纯牛粪条件下厌氧发酵产气效果显著低于混合原料,且所有条件下反应器的产气量基本都随着发酵浓度升高而升高。在发酵浓度为8%条件下,S:CM=3:1和1:1的反应器容积产气率在运行130和150 d后分别达到峰值0.78和0.76 L/(L·d)。随着反应器的持续运行,170 d后各反应器的产气率与pH值降低,而VFAs与ORP升高。S:CM=3:1和1:1的反应器容积产气率降低至0.6 L/(L·d),pH值降低至6.5左右。这主要是由于恒定的搅拌功率条件下,随着反应器内TS升高,搅拌转速降低,进而在反应器内产生搅拌死区与浮渣等问题,导致反应器内局部酸化,造成系统整体失稳。在启动期间,所有5个条件下反应器内ORP呈缓慢上升趋势。运行172 d后,S:CM=1:1条件下ORP迅速增加至高于–300 mV。总体而言,厌氧系统中VFA浓度随着进料中秸秆比例增加而增加,且丙酸积累发生并变得更加严重。这也在一定程度上表明,与采用秸秆与粪便混合原料厌氧发酵相比,采用纯秸秆原料厌氧发酵生产沼气厌氧反应器的运行稳定性较差。

Effect of feed concentration on continuous stirred anaerobic fermentation characteristics with mixed corn straw and cow manures

At present, most biogas projects in China use crop straw and animal manures as the main raw materials, however, there is still a lack of reference and support for the process control of the actual biogas project, especially for the control of the feed mix of biogas plants. In this study, two kinds of common materials, corn stalk and cow dung, were used as the influent with different mixed ratio under the medium temperature conditions. The mixed ratio of straw and cow manure (S:CM, as TS ratio) was adjusted to 1:1, 1:3, 3:1, while the reactors with straw and cow manure were as the controls. The CSTR started at the condition of TS=3%, then the TS was increased to 6% and 8% sequentially. The effects of different feeds on the operation characteristics of biogas engineering such as gas production, methane content, pH value, ORP (oxidation reduction potential), volatile acids were systematically discussed. The results showed that the biogas production of the reactors with straw or cow manure was significantly lower than that with mixture. And the biogas production rate increased with the increasing of TS. Under the conditions of TS=8%, the reactor with influent of S:CM=3:1 got the peak biogas production rate of 0.78 L/(L·d) after 130 d running, and that with influent of S:CM=1:1 got its peak biogas production of 0.76 L/(L·d) after 150 d running. As the TS in the reactor was increased to 8%, the pH values in the five reactors did not change significantly after increasing the concentration, indicating that there was still a certain buffering capacity in the reactor. Then, with the continuous operation of the reactor, the pH value in the reactor gradually decreased on the 170th day, and the pH value in the five reactors decreased to about 6.5 in 180 days. The pH value decrease led to a decrease in the gas production rate of the reactors. The biogas production rate decreased to 0.6 L/(L·d) with a decreased pH value to 6.5. This was mainly because under the constant stirring power condition, as the concentration of the materials in the reactor increased, the stirring speed was reduced, and problems such as stirring dead zone and scum were occurred in the reactors, resulting in acidification in the reactor. This in turn caused the system to be unstable overall. During the startup, the ORP of all the five reactors showed a slowly rising trend. After 172 d running, the ORP in the reactor with S:CM=1:1 rapidly increased to above -300 mV. Overall, the concentration of VFAs in the anaerobic system increased with the increasing proportion of straw in the feed-in materials. Moreover, the accumulation of propionic acid occurred and became more serious as the proportion of straw in the feed-in material increased. This result showed that the biogas plants with more straw might ran at more unstable conditions, compared with those feeding in mixture of straw and manure.