Water Footprint And Carbon Balance In The Cultivation,Fermentation,and Energy Utilization Processes Of Industrial Biogas Crops

As a substitue for natural gas,“industrial biogas”,which has been produced through large-scale refinement,can be economically transported to its demanding areas through a natural gas pipeline network or pressured tank.In recent years,industrial biogas has been rapidly developed all over the world to deal with energy crisis and to reduce green-house gas emissions.As an important raw material of industrial biogas,“industrial biogas crops”are more recently widely planted,which require not only high conversion efficiency but also good environmental effects.Carbon balance and water footprint can reflect the carbon conversion and consumption of water resources and are thus used as the main environmental impact factors to evaluate the sustainability of industrial biogas and to screen the industrial biogas crops.Energy production with industrial biogas crops mainly consist of three stages,i.e.crop cultivation,anaerobic fermentation,and biogas utilization.During the crop cultivation,crops absorb CO₂ from the atmosphere and convert it into biomass;and at the same time,the soil respiration release CO₂ into the atmosphere,which has been generally ignored in the carbon balance analysis of industrial biogas.To investigate the carbon balance and water footprint of industrial biogas crops,five land use types were selected as experimental testing sites to gown different crop energy plants,including two maize varieties?Zea mays cv.Quchen 9 and Yunrui 88?,hybrid pennisetum?Penniseturm americanum×P.purpureum?,alfalfa?Medicago sativa?and non-cropping bare land as the control.During the Spring season of 2012 and 2013 a random block designed field test was carried out in the experimental farm of Yunnan Agricultural University?25°18'N and 102°45'E,1,930 m above the seas level?.Each land type had a 30 m² size with three replicates each.Soil respiration?R_s?was assessed by a combination of the alkali absorption method and static chamber with an infrared CO₂analyzer.Soil water content was measured with a soil profile moisture meter.Meteorological data were collected with an auto-meteorological station.The energy production and the carbon conversion from the crop straw were measured with anaerobic fermentation experiments at 35?.The carbon emission reduction of the crops was compared with a standard coal.The agro-ecosystem carbon balance analysis was combined with the life cycle assessment?LCA?theory to determine the carbon balance in the energy production and utilization processes of industrial biogas crops.The water resource consumption was determined with the combination of the water footprint?WF?analysis with the LCA.The generated results as follows would provide a theoretical basis for a better selection of an industrial biogas crop,an efficient utilization of water resources,and mitigation of greenhouse gas emissions.1)Significant differences in the above-ground biomass,biogas production,and energy production were observed between the four crops.All these three mentioned values were highest in the hybrid pennisetum,but lowest in the alfalfa.During the observation period,their mean biomass decreased in this following order:hybrid pennisetum?29.00 t/hm²?>maize Quchen 9?25.68 t/hm²?>maize Yunrui 88?23.34 t/hm²?>alfalfa?11.51 t/hm²?.No significant differences in the mean biomass could be observed between the two breeds of maize,while significant differences at the 0.01 level appeared between any two crops.The mean biomethane gas production decreased in the following order:hybrid pennisetum?10,502 m³/hm²?>maize Quchen 9?7,776 m³/hm²?>maize Yunrui 88?6,304 m³/hm²?>alfalfa?3,594 m³/hm²?,and energy production was 356,264,214,and 122 GJ/hm²,respectively.Significant differences in the mean biomethane gas and energy production were observed between the two breeds of maize or at the 0.01 level between any two crops.2)The WF?water footprint?of biomass production?WF_M?and the WF of energy production?WF_E?were lowest for the hybrid pennisetum,highest for alfalfa during the cultivation,fermentation,and energy utilization processes.The WF_M values of the maize Quchen 9,maize Yunrui 88,hybrid pennisetum and alfalfa were 0.222,0.243,0.205,and0.360 m³/kg,whilst their WF values of energy production?WF_E?were 21.58,26.60,16.75,and 33.98 m³/GJ,respectively.3)Among the four crops the R_s vales was highest in the alfalfa land,least in both the maizes,the hybrid pennisetum and the bare land and lowest hybrid pennisetum land.The correlations between R_s and atmosphere temperature,soil temperature and soil water content differed with crop species,but all of them were significantly related to soil water content at 10 cm soil depth.The R_s decreased in the following order:alfalfa land?5,460kg/hm²?>Quchen 9 land?3,313 kg/hm²?>Yunrui 88 land?3,271 kg/hm²?>hybrid pennisetum land?2,960 kg/hm²?>bare land?2,377 kg/hm²?.No significant differences in the R_s were observed among lands to grow Maize Quchen 9,Maize Yunrui 88 and hybrid pennisetum,whilst significant differences at the 0.01 level were showed between the three crops lands,alfalfa land,and bare land.The R_s of Maize Quchen 9 and Maize Yunrui 88significantly correlated with the atmosphere temperature,whilst the bare land and the two maize crops significantly correlated with soil temperature.The R_s of all the four crops lands and bare land significantly correlated with soil water,and the correlation coefficient was highest in hybrid pennisetum and was lowest in Maize Quchen 9.4)The carbon balance in the industrial biogas crop agro-ecosystem might be the CO₂sink carbon or carbon neutral.The net ecosystem production decreased as follows:hybrid pennisetum?9724 kg/hm²?>Quchen 9 maize?8715 kg/hm²?>Yunrui 88 maize?8531kg/hm²?>alfalfa?-0.3 kg/hm²?.No significant differences in the NEP were observed between the two maize crops,while significant differences at the 0.01 level were showed between any two crops.To produce 1.0 kg of biomass,the R_s of Maize Quchen 9,Maize Yunrui 88,hybrid pennisetum,and alfalfa were 0.119,0.125,0.090,and 0.429 kg,respectively.The carbon emissions from the bare land were deducted,while the R_s were0.034,0.034,0.018,and 0.242 kg,respectively.These results indicated that the hybrid pennisetum had a better effect to reduce greenhouse gas emissions than maize and alfalfa and was more preferential to be popularized and cultivated.5)The cultivation,fermentation,and energy utilization processes of these industrial biogas crops could be a carbon weak sink or a carbon source depending on the particular species.Comparing with the standard coal to get the same calorific value,the carbon reduction was highest in hybrid pennisetum,and lowest in alfalfa.Generally,the crop root was not used as a raw material for the industrial biogas.Regarding the crop root and biogas residue as the parts that were not degradated,a total of 696,1727,and 194 kg/hm² carbon were fixed by the maize Quchen 9,maize Yunrui 88,and hybrid Pennisetum,which acted as a CO₂ sink,while 3685 kg/hm² carbon was emissioned from alfalfa field that acted as a CO₂ source.Considering the long-term effect of industrial biogas on the environment,biogas residue and crop root could be degraded completely,and the carbon inside would eventually entered the atmosphere.As a result,the soil respiration would be the"only"carbon source in the industrial crop biogas energy utilization process.The ratio of the carbon of biomethane gas from the crop straw per hetare to the carbon of soil respiration is expressed as?_c and used to evaluate the efficiency of R_s in industrial biogas crop energy utilization process.The higher?_c,the better the emission reduction benefits of biogas crop energy utilization could be.The?_c decreased in the following order:hybrid pennisetum?2.07?>Quchen 9 maize?1.30?>Yunrui 88 maize?1.11?>alfalfa?0.36?.6)Compared with the standard coal getting the same calorific value,the carbon reduction in the cultivation,fermentation,and energy utilization processes of the industrial biogas crops decreased as hybrid pennisetum?37303 kg/hm²?>Quchen 9 maize?26895kg/hm²?>Yunrui 88 maize?23682 kg/hm²?>alfalfa?8280 kg/hm²?.In summary,the WF_M and the WF_E were lowest in the hybrid pennisetum,while the cultivation,fermentation,and energy utilization processes were highest in alfalfa among the four industrial biogas crops.The soil respiration was the main carbon source in the processes.The carbon balance in the industrial biogas crops agro-ecosystem acted as the CO₂ sink carbon or carbon neutral.The cultivation,fermentation,and energy utilization processes of the tested industrial biogas crops could be a carbon weak sink or a carbon source depending on a particular species.Considering the long-term effect of the industrial biogas on environment,the soil respiration could be the"only"carbon source in the industrial crop biogas energy utilization process.Compared with the standard coal getting the same calorific value,the carbon reduction was highest in hybrid pennisetum and lowest in alfalfa.For a comprehensive consideration of the crops'WF and carbon balance,the hybrid pennisetum demonstrated a higher biogas productionand CO₂ emission reduction,but a lower WF and could thus be the most preferential crop among these tested four industrial biogas crops in Central Yunnan,though the maize Quchen 9 showed an obvious advantage over the maize Yunrui 88.