CO₂ Absorption Enhancement Mechanism And Process Of Renewable CO₂ Absorbents Derived From Biogas Slurry

Bio-natural gas can be acquired from biogas through biogas upgrading.The application of bio-natural gas in a large scale not only contributes to alleviating the shortage of natural gas,but also achieves the CO₂ near zero emissions during the life cycle of the conversion and utilization of energy.The key problem for biogas upgrading is CO₂ separation with low-cost.As one of the promising technologies,the key issue for CO₂ chemical absorption process is to reduce CO₂ regeneration heat comsuption sharply.Using the ―once-through‖ absorption process without covering the regeneration stage may lead to the great reduction of CO₂ regeneration energy consumption.However,in the once-through process,absorbents are not recycled and once-through used.Evidently,the typical chemical commodities can not fully satisfy the requirements of the process due to the high price and limited supply of solvents.Thus,it is of importance to screen the novel absorbents with low cost and large quantity.Clearly,determining how to use these CO₂-rich solvents should be taken into consideration as well.Hence,the weak alkaline biogas slurry generated from the anaerobic digestion process using biomass as the substrate was put forward to act as the ―once-through‖ absorbent for CO₂ absorption in this study.Additionally,CO₂-rich biogas slurry was applied into agriculture through irrigation.Theoretically,it is anticipated to realize a low-cost CO₂ capture and safe carbon storage by applying biogas slurry into ―once-through‖ CO₂ absorption process.However,the CO₂ absorption capacity of biogas slurry is too low,suggesting that its absorption performance should be enhanced greatly on the premise of ensuring the low phytotoxcity of biogas slurry.Thus,to solve these key issues,CO₂ absorption performance enhancement mechanism of biogas slurry was firstly investigated in this study.Additionally,CO₂ absorption charactersitics of the renewable ammonia aqueous solution recovered from biogas slurry were focused.Finally,a new process combining ammonia recovery and CO₂ absorption was analyzed.The main conclusions are as following: free ammonia in biogas slurry and CO₂.Adding the biomass ash into biogas slurry can increase the CO₂ absorption capacity of biogas slurry,reduce its COD content,turbidity and keep the low phytotoxicity of CO₂-rich biogas slurry.With the addition of 200 g/L cotton straw ash,CO₂ absorption capacity of biogas slurry increases to 0.156 mol/L,which is 6 times of the raw biogas slurry.Ascribed to the reduction of ammonium nitrogen concentration,the vacuum process for concentrating biogas slurry can not increase the CO₂ absorption capacity of biogas slurry.However,it can provide a new concept for enhancing the CO₂ absorption capacity greatly in the next step work by using the renewable aqueous ammonia solution recovered from biogas slurry in vacuum concentration process.Theoretically,CO₂ absorption capacity of biogas slurry can be enhanced greatly through adopting the renewable aqueous ammonia solution as CO₂ absorbent.（1）Research on the enhancement mechanism of CO₂ absorption performance of biogas slurry.Effects of vacuum regeration,exogenous biomass ash addition and vacuum concentration on CO₂ absorption performance of biogas slurry and its phytotoxicity were experimented.The results showed that the vacuum regeneration process can greatly elevate the CO₂ absorption capacity of biogas slurry to 0.125mol/L（3.18 times of the raw biogas slurry）by stripping the dissolved CO₂ in biogas slurry and retain its ammonium nitrogen concentration under the conditions of 77 °C and 40 k Pa.During the study of vacuum regeneration of biogas slurry,it was found that the difference between the activation energy of CO₂ regeneration from biogas slurry and from ammonium bicarbonate solution is insignificant,demonstrating that the reaction mechanism of CO₂ regeneration from biogas slurry is mainly the decomposition of ammonium bicarbonate.So,it can be concluded that absorption mechanism between biogas slurry and CO₂ is the chemical reaction between the（2）Renewable aqueous ammonia solution from biogas slurry as CO₂ absorbent.In order to solve the problems of low CO₂ absorption performance of biogas slurry,a new concept was put forward in the study to recover the renewable ammonia from biogas slurry to act as CO₂ absorbent.Renewable ammonia can be recovered and enriched from biogas slurry by vacuum membrane distillation.Thus,in order to investigate the feasibility of this concept,the ammonia removal performance,recovery characteristics of renewable aqueous ammonia with high concentration and the reaction kinetics between the renewable aqueous ammonia and CO₂ should be explored.Results showed that ammonia separation performance is dominated by the initial p H of biogas slurry,and both mass transfer coefficient of ammonia and ammonia sepereation factor can be elevated by increasing the initial p H of biogas slurry.It should be noted that the water flux has an important impact on the overall ammonia mass transfer coefficient.And an interesting ―S‖ curve logistic model can well fit the relationship between the water flux and the overall ammonia mass transfer coefficient,suggesting that there is an optimal water flux to get the high ammonia mass transfer coefficient and high ammonia separation factor simultaneously.The ammonia concentration of the renewable aqueous ammonia solution recoved from biogas slurry is affected greatly by the operating parameters and operation time in ammonia recovery process.In this study,when the initial total ammonia nitrogen（TAN）concentration of raw biogas slurry was 0.07-0.28 mol/L,the ammonia concentration of the renewable aqueous ammonia is 0.4-2.4 mol/L,and its p H values range from 10.7 to 11.5.It means that free ammonia is the main active composition in the renewable ammonia.The CO₂ absorption capacity of renewable aqueous ammonia is about 0.987 mol-CO₂/mol-NH3.Besides the inorganic carbon（mainly CO₂）,volatile fatty acids mainly including ethanol,acetic acid,propionic acid and butyric acid are the primary impurities in the renewable auqoues ammonia solution,in which their total concentrations are about 0.026 mol-C/L.（3）Kinetics of CO₂ absorption by renewable ammonia aqueous solution.CO₂ absorption rates of renewable aqueous ammonia and unloaded aqueous ammonia were both investigated in a wetted-wall column.Results showed that the second-order reaction constant（k2）of CO₂ absorption of 2 mol-N/L renewable auqoues ammonia at 25 °C is about 4306 L/（mol·s）,which is slightly lower than that of aqueous ammonia（4772 L/（mol·s））due to the negative effect of VFAs.The average overall CO₂ mass transfer coefficients of renewable aqueous ammonia are lower by about 4.67%-17.31% than that of aqueous unloaded ammonia,especially for these cases that the TAN concentration is less than 1 mol-N/L.Furthermore,increasing the concentration of VFAs and CO₂ loading in the renewable auqoues ammonia leads to the rapid decline of the overall mass transfer coefficient of CO₂,in which butyric acid has the greatest negative impact on absorption,followed by acetic acid,ethanol and then CO₂.（4）CO₂ absorption performance of biogas slurry-based renewable absorbents in the packed column and membrane contactor.After the study on CO₂ absorption characteristics of biogas slurry-based renewable CO₂ absorbents,their CO₂ absorption performance were investigated in the real packed column and membrane contactor.In the packed comlumn,the renewable ammonia,monoalkanolamine（MEA）and Na OH aqueous solutions were tested at 8 oC and atmospheric pressure.Results showd that CO₂ absorption performance of renewable ammonia aqueous solution is superior to Na OH,but inferior to MEA.When the ammonia concentration is 0.5,1 and 2 mol-N/L,the volumetric fraction of CH₄ in the outlet biogas reaches about 90%,95% and 98%,respectively.The results imply that the renewable aqueous ammonia solution can be used directly in the typical CO₂ absorber.In a hydrophobic polypropylene hollow fiber membrane contactor,the higher biogas upgrading performance can be achieved at 200 m L/min biogas flowrate and 50 m L/min solvent flowrate when using 0.3 mol-N/L renewable ammonia aqueous solution as the solvent.For this case,the volumetric fraction of CH₄ can be increased to about 90%.It should be noted that the operating parameters and membrane contactor parameters have great influence on CO₂ removal performance from biogas.Thus,the selection of membrane module parameters,including the length of module,the number of hollow fibers,biogas flow rates and absorption performance should be carefully considered when using the membrane contactor and low concentration renewable absorbents for biogas upgrading.（5）Research on the new process combining ammonia recovery from biogas slurry and CO₂ absorption.Combining the advantages of CO₂ absorption using biogas slurry and renewable aqueous ammonia,and the merits of the application of CO₂ rich absorbents in agriculture,a new process combining ammonia recovery from biogas slurry and biogas upgrading was put forward in this study.The recovered renewable aqueous ammonia is used for CO₂ absorption to realize simultaneous biogas upgrading and ammonium fertilizer generation.After ammonia recovery,the phytotoxicity of the treated biogas slurry reduced significantly,which means the improved applicability in agricultural irrigation.It is well known that CO₂ emissions are inevitable for heat and electricity production.And heat and electricity are required for ammonia recovery from biogas slurry.So,there are indirect CO₂ emissions for the renewable ammonia production.However,the utilization of renewable ammonia in once-through can fulfill CO₂ capture from biogas.So,when the renewable ammonia aqueous solution with concentration higher than 10 g-N/L was adopted in a once-through absorption process,the net carbon negative emission can be achieved since CO₂ release in the ammonia recovery stage is lower than CO₂ absorption amount by ammonia based on the assumption that CO₂ emissions in heat and electricity production are lower than the world average values.The results showed that the novel concept using the biogas slurry-based renewable absorbent and the corresponding CO₂ absorption process is beneficial to reduce the cost of CO₂ capture from biogas and the safty CO₂ storage.Additionally,the useful results can also be beneficial to nutrients recovery from biogas slurry and the development of carbon negative emission technology.