Thermochemical Liquefaction Characteristics Of Typical Biomass In Sub-and Supercritical Organic Solvent

The shortage of fossil fuel and the environmental pollution produced by the utilization of fossil fuel have further enhanced the development of biofuels.Biomass is the primary feedstock for the production of renewable liquid biofuels.In recent years,the technology of thermochemical liquefaction has been wildly applied to producing bio-oil from biomass.Biomass liquefaction is a low-temperature and high pressure thermochemical process during which biomass is broken down into fragments of small molecules in water or another suitable solvent.These light fragments,which are unstable and reactive,can then repolymerize into oily compounds with various ranges of molecular weights.During the liquefaction of biomass,the relative interactions among molecules of biomass and that of solvent play a key role in determining the distributions and compositions of liquefaction products.Using water as a reaction medium instead of organic solvents is advantageous from the environmental and economical points of view but also is more importantly disadvantageous due to its high critical point.The drawbacks of utilizing water as the solvent for liquefaction of biomass,however,include lower yields of the water-insoluble products,which typically have a higher heating value compared with the water-soluble products with a lower heating value,and a higher oxygen content in the liquid products,resulting in low heating values for the obtained liquid products.To enhance the yields of liquid products with lower oxygen contents,and thus higher heating values,organic solvents,such as methanol,ethanol,acetone,1,4-dioxane and so on,have been utilized instead of water.On the basis of the present research on thermochemical characteristics of typical biomass in sub/super-critical organic solvents and its deficiency,we carried out six areas of research work.The first section describes the degradation behaviors of sewage sludge by liquefaction in different organic solvents(methanol,ethanol and acetone).The solvent efficiency in terms of conversion yield was found to be:methanol>ethanol>acetone.However,higher bio-oil yield could be obtained with acetone as liquefaction solvent.The chemical properties of bio-oil products were significantly affected by the type of solvent used for the liquefaction process.Methanol and ethanol yield major ester compounds,while acetone favors the formation of ketone and N-containing compounds.In addition,some phenolic compounds,alcohols and very few hydrocarbons were also qualified.The caloric value of bio-oil produced using ethanol is up to 38.42 MJ/kg,higher than other bio-oil products produced using methanol and acetone.Moreover,the bio-oils are found to contain a considerable fraction of light components using thermogravimetric analysis(TGA),and the content of low-boiling-point(bp<350℃)compounds are ranging from 78.13%to 85.76%of the weight for above three bio-oils.The second section describes the different degradation behaviors of lignocellulosic(biomass rice straw),microalgae(Spirulina)and sewage sludge by liquefaction.Although the content of organic materials in sewage sludge was the lowest among above three biomasses,the yield of bio-oil from sewage sludge reached up to 39.5±1.16%，higher than those from rice straw(21.1±0.93%)and Spirulina(34.5± 1.31%).Besides,the bio-oils from sewage sludge had the highest heating value(36.14 MJ/kg).However,Spirulina showed the highest conversion(79.7±1.02%).GC-MS analysis results indicated that the major compounds in the bio-oil from rice straw were phenolic compounds.In case of sewage sludge and Spirulina,esters were the main compositions.The volatility distribution of hydrocarbons was evaluated by C-NPgram(Carbon-Normal Paraffin gram).It was showed that the majority of hydrocarbons from sewage sludge and Spirulina were distributed at C17 and C20,whereas they were distributed at C8 in rice straw-derived bio-oil.The types of feedstocks have a significant effect on biomass liquefaction.The third section optimizes the liquefaction parameters of rice husk in sub-/super-critical ethanol.The influences of reaction parameters on the yields of liquefaction products were investigated.Liquefaction experiments were performed at various reaction temperatures(T,240～360℃),solid-liquid ratios(R1,5～15%),and solvent filling ratios(R2,10～30%)with or without catalyst.Two types of catalysts were involved,including iron-based catalysts(FeSO4 and FeS)and alkali metal compounds(NaCO3 and NaOH).The dosage of catalyst was also optimized.Without catalyst,the bio-oil yields ranged from 11.8%to 24.2%,depending on T,R1 and R2.And the bio-oil yields increased firstly and then decreased with increasing T and R2,while the bio-oil yields continuously declined with increasing R1.NaOH was certified to be an ideal catalyst for rice husk liquefaction and the optimal dosage was approximately 10%.The obtained bio-oils had much higher caloric values of 20.9-24.8 MJ/kg compared to 14.9 MJ/kg for the crude rice husk sample.Without catalyst,the main components of bio-oil were phenolic compounds.In the case of NaOH as catalyst,long-chain alkanes were the major compositions of bio-oil.The fourth section optimizes the liquefaction parameters of Spirulina in sub-/super-critical ethanol.The influences of reaction parameters on the yields of liquefaction products were investigated.Liquefaction experiments were performed at various reaction temperatures(T,280～380℃),solid-liquid ratios(R1,0.025～0.125g/mL),and solvent filling ratios(R2,10～30%)with or without catalyst.Two types of catalysts were involved,including iron-based catalysts(FeSO4 and FeS)and alkali metal compounds(NaCO3 and NaOH).The dosage of catalyst was also optimized.Without catalyst,the bio-oil yields were in the range of 35.4%and 45.3%depending on the changes of T,R1 and R2.And the bio-oil yields increased generally with increasing T and R2,while the bio-oil yields reduced with increasing R1.The FeS catalyst was certified to be an ideal catalyst for the liquefaction of Spirulina microalgae.The optimal dosage of catalyst(FeS)was ranging from 5～7%.The bio-oils have much higher heating values than the crude Spirulina sample and fatty acid ethyl ester compounds were dominant in the bio-oils,irrespective of whether catalyst was used.The fifth section qualitatively describes the bioavailability and eco-toxicity of heavy metals in sewage sludge(SS)and liquefaction residues(LR)of SS.The total content of heavy metals in LR nearly doubled.High residual rates of heavy metals(about 80%)indicated that the heavy metals in SS were concentrated into LR after liquefaction.The comparisons of sequential extraction results between SS and LR showed that after liquefaction,the mobile and easily available heavy metal fractions(acid soluble/exchangeable and reducible fractions)were mainly transformed into the relatively stable heavy metal fractions(oxidizable and residual fractions).The bioavailability and eco-toxicity of heavy metals in LR were relieved,though the total concentrations of heavy metals increased.The sixth section quantitatively evaluates the potential ecological risk and pollution degrees of heavy metals in LR versus SS.The leaching rates(R1)of heavy metals in LR were much lower than those in SS,revealing that the mobility/leachability of heavy metals was well suppressed after liquefaction.Geo-accumulation index（Igeo)indicated that the liquefaction process significantly weakened the contamination degrees of heavy metals.Potential ecological risk index(RI)demonstrated that overall risks caused by heavy metals were obviously lowered from very high risk in SS to low risk in LR.According to the risk assessment code(RAC),each tested heavy metal had no or low risk to the environments after liquefaction.In a word,the pollution hazards of heavy metals in LR were markedly mitigated.