| As a big country in agricultural production and consumption of agricultural products,China has always paid great attention to the treatment and utilization of straw waste.At present,our country is in the key stage of upgrading the "dual carbon " target to national strategy and strengthening the pollution abatement in agricultural and rural areas.Due to the emission risks of greenhouse gases and water pollutants in straw mulching,how to realize the straw off-field utilization has become a research hotspot.Anaerobic digestion could convert biomass waste into clean energy and value-added biogas residue,so it has attracted more and more attention as an effective way to treat organic waste.Compared with conventional wet anaerobic digestion,dry anaerobic digestion has been widely used in high solid content organic waste treatment such as kitchen waste for the advantages of high organic loading rate and no secondary pollution.However,in the process of anaerobic fermentation of rice straw,the high carbon-nitrogen ratio(C/N)of the raw material and the covalently bound and entangled structure of lignocellulose would lead to a slow decomposition rate.And due to its light texture and poor fluidity of rice straw,it would also cause material stratification and surface structure,and eventually form "dead corners".It should be noted that these are the reasons for the inefficiency of rice straw dry anaerobic digestion.It’s observed that there existed insufficient organic nitrogen sources,poor mixed mass transfer effect,unclear value-added utilization methods of biogas residue and lack of comprehensive benefit evaluation during the operation of the rice straw dry anaerobic fermentation project.To solve these problems,the appropriate C/N and optimal pretreatment methods for rice straw dry anaerobic digestion under the condition of sequencing batch feeding were studied,and the fermentation law of different stirring methods under the condition of continuous feeding and discharging were also investigated.Moreover,the potential and ways of high-value utilization of biogas residues were explored.On these basis,the comprehensive evaluation of the comprehensive operation benefit of biogas project was quantified by the dynamic analysis method and the life cycle evaluation method.The main conclusions were as follows:(1)This study conducted the test of rice straw co-digestion with different amounts of pig manure on an integrated stainless-steel anaerobic digester.Results showed that the biogas production and quality tend to decrease obviously with the decrease of the proportion of pig manure,especially when the C/N ratio is higher than 50/1,the gas production efficiency decreased significantly.At the same time,the organic components of biogas residues in treatments(with C/N of 50,40,34,and 30)were substantially degraded,and the degradation rates of cellulose and hemicellulose were 8.28～13.09%and 5.19～12.81%,respectively.However,the pH of the treatment with C/N of 50 raised to 6.4 in the 33rd day,and the methane process was suppressed to some extent.(2)To shorten the hydrolysis period,further research on the influence of yellow storage pretreatment of rice straw was carried out.The neutral detergent fiber degradation rate in the dilute alkali(1%NaOH)was 13.92%,which was 7.36%and 4.67%higher than those in the water and biogas slurry treatment groups,respectively.The unit volatile solid(VS)gas production of rice straw in the dilute alkali and biogas slurry treatment groups after dry anaerobic digestion were 406 L·kg-1 and 394 L·kg-1,and the pH of biogas-preprocessing treatment dropped to the minimum point in the 9th,which was 2 to 3 days earlier than other treatment groups.Therefore,the rice straw could be co-pretreated with biogas slurry and dilute alkaline in the engineering practice to increase the efficiency of dry anaerobic digestion when C/N was 50.(3)This study adopted a horizontal plug-flow dry anaerobic reactor with a daily processing capacity of ≤15 kg·d-1.In the reactor,the frame-shaped blade mechanical stirring was improved,and the front-rear pneumatic stirring components were added.The total gas production for 40 days showed the law of mechanical stirring>front-end pneumatic stirring>whole-process pneumatic stirring,and the unit VS gas production was 441.9,389.3 L·kg-1 and 362.3 L·kg-1,respectively.The accumulation of VFAs did not occur.When it comes to the three combined optimization modes of mechanical stirring and front pneumatic stirring,the gas production of rice straw dry anaerobic digestion for 40 d increased,which was 2-8%higher than that of the single mechanical stirring methods.In particular,the digestion efficiency of mechanical stirring(4 times·d-1)and front pneumatic stirring(2 times·d-1)combines method were highest,while unit dry matter gas production rate and the organic component degradation rate were 447.2 L·kg-1,indicated that the combined mode could effectively improve the digestion efficiency.(4)The biogas residue obtained from the continuous pilot test were analyzed,it could be seen that the degradation rate of cellulose and hemicellulose reached to 14.88%and 15.06%.The organic matter content and total nutrient content are 52.86-55.33%and 2.89-3.71%,respectively.And the content of heavy metals such as As,Hg,and Pb were also lower than the safety limit,so it has a good potential for high-value fertilizer.Thorough the steps of adjusting the C/N of biogas residue,composting in high temperature,sieving after drying as well as adding beneficial microbial agents,high quality organic fertilizers were made.The related technical indicators could all meet relative standard requirements.(5)The dynamic evaluation method and life cycle evaluation were adopted to quantitatively analyze the comprehensive benefits of before and after the parameter optimization of the biogas project with taking the rice straw mulching as the benchmark.The net income of biogas project gas-fertilizer co-production could be further increased by 7.99%,and the dynamic payback period was 9.64 years.From the perspective of ecological and environmental benefits,the process optimization was more conducive to improve environmental quality of environmentally sensitive areas such as water source areas.Biogas project after parameter optimization could reduce 5.60 kg of CO2 emissions and save 2.21E3 MJ of energy per 1 t of straw processed. |
