| With the rise in energy consumption and the production of wastes due to human activities,anaerobic digestion(AD),a technology that converts waste into bio-energy,is gaining growing attention worldwide.Small and medium-sized biogas projects in rural areas of China play a significant role to improve the rural production and living conditions,leading to energy conservation and reduction of emissions.On the other hand,China is one of the world’s agricultural country,rich in biomass resources,such as crop straw and animal waste.If these wastes are not treated in time,they will cause many environmental problems,such as odor,soil and water pollution,air pollution,and so on.Therefore,the utilization of these wastes for renewable energy generation bring numerous economic,environmental and climate advantages.However,the low-temperature of digesters results in relatively low digestion efficiency and reduction of biogas output,so it is necessary to adopt heating systems to improve the digestion process efficiency in cold areas.In addition,the major problem for the low exploitation effectiveness of agricultural crop waste(ACW)for AD is that cellulose(CEL),hemicellulose(HCEL),and indigestible lignin compounds integrate together strongly forming a complex structure.This structure of organic components results in chemical and physical inhibition against microbial action,thus inhibiting the hydrolysis of organic matters in AD.Besides,high organic load,low microbial diversity,low nitrogen content(high C/N)and composition of ACWs are not conducive to AD.Therefore,the main objectives of this study are as follows:1)taking the soft biogas digester as the object,the facility form of using solar energy to increase fermentation temperature is studied on a small scale,so as to achieve the purpose of increasing biogas production in a cold climate;2)using crop straw as raw material,the mechanism of pretreatment to decompose the complex crystal structure of lignocellulose and improve the biogas yield of AD was studied;3)The effect of anaerobic co-digestion(ACoD)on biogas production from agricultural waste was studied in laboratory scale to solve the problem of low biogas generation from single material AD.In order to achieve these goals,this study has carried out relevant experimental research on the scale of small-scale and laboratory tests,respectively,in order to improve the ability of agricultural waste products to produce biogas.The main experimental results are as follows:1.The present study proposes a solar-greenhouse heating technique integrated with a north wall to heat a semi-buried low-cost soft tubular digester bag,as a solution.This experiment includes two similar semi-buried digesters,the first one(DA)was heated by the solargreenhouse integrated with a north wall,while the second(DB)was heated by only direct solar radiation(without a greenhouse),which used as a control,and both digesters were fed with cattle manure.The heating efficiency of two biogas digesters and its effect on biogas productioa was studied by continuous operation tests.The results showed that the average slurry temperature was 24.9℃ and 23.45℃ for DA and DB,respectively.Furthermore,the mean specific biogas production of DA and DB were 173 and 155.3 L/kg VS,respectively,with no significant variations in the methane content(≈62.2-62.76%).The results shown that the use of solar-greenhouse can improve the biogas output of soft biogas digester.2.This study aimed to investigate the potential of integrated solar heating techniques to raise ffie slurry temperature within a low-cost soft tubular digester and its impact on the biogas yield.Two similar digesters were used,the first one(D1)was heated by the solar greenhouse integrated with a solar water heating system and a capillary lieat exchanger,while the second(D2)was heated by only solar greenhouse,and both digesters were above ground and were fed with cattle manure.The results showed average slurry temperature of 9.5 and 4.9℃ above the mean ambient temperature for D1 and D2S respectively.Furthermore,the mean specific biogas production of D1 and D2 were 247 and 181 L/kg VS,respectively,with no significant variations in the metliane content(≈62.7%).The study indicated that using of integrated solar energy is efficient to achieve the optimum temperature for the process of biogas production roughly the most of the year.3.Anaerobic digestion of lignocellulosic biomass like agricultural crop residues has gained attention due to their excess availability in rural areas.However,anaerobic digestion of such biomass feedstock requires a pretreatment to enhance their digestibility.The present.study examined the possibility of valorizing rape straw through anaerobic digestion and the possibility of improving the biomethane yield by sole dilute acid(2%H2SO4 at 130℃ for 20 min),combined dilute acid with steam explosion(SE)(at 160,190 and 210 ℃ for 3 min),and steam explosion(at 180℃ for 5 min)combined with superfine grinding(SFG)pretreatments.To evaluate the efficiency of the pretreatment method,several analytical methods such as SEM,FTIR,XRD and fiber composition were used.The results showed that combined pretreatment could dissolve the lignocellulosic structure,which positively stimulated anaerobic methane yield.The highest cumulative CH4 yield(CMY)of 305.7 and 263.3 mL/g VS was achieved from SE at 180℃ for 5 min with SFG and H2SO4 with SE at 190℃ for 3 min,which were 77.84%and 53.17%more than untreated,respectively.This study revealed that,under optimal parameters of AD,pretreatment with SEG180 and DASE190 could significantly boost the CMY from anaerobic digestion of rape straw.4.Anaerobic co-digestion is considered to be a very effective digestion technology because it can significantly balance the nutrient of the fermentation substrate,increase the content of trace elements such as metals and increase its buffering performance,and it can overcome the problem of low gas production of traditional anaerobic digestion technology.In this study,in order to improve the biogas output,rape straw was used as the raw material,and a steam explosion combined with superfine grinding was used to pre-treatment,then anaerobic co-digestion was carried out with cattle manure(CM)at different mixture ratios(from straw:cattle 100:0 to 0:100).The experiment was carried out in a closed system under the same control conditions.The effects of combined pretreatment on the fiber structure of straw and the effect of combined anaerobic co-digestion were studied and analyzed.The results showed that combined pretreatment could dissolve the fiber structure of straw,which significantly increase the total amount of methane in anaerobic digestion.The cumulative CH4 yield(CMY)for co-digestion was improved by 11.4-59%higher than the control(CM).The highest CMY of 337.8 mL/g VS was achieved from CM:PRS40(CM:PRS,60:40),and this yield was 59%and 16.8%higher than the CM and pretreated rape straw(PRS)alone,respectively.This study revealed that,under optimal parameters of AD,pretreatment with SEG180 could significantly boost the CMY from co-digestion of cattle manure and rape straw. |
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