Process And Mechanism Of Supercritical Water Gasification Of Algae And Phenol

The environment and energy became the most important issue as the greenhouse effectand the need of renewable energy in this century. The increased use of biomass and wastesas a renewable energy feedstock can help to reduce dependence on fossil fuels and increaseand diversify our energy supply for sustainable development.Gasification of microalgae in supercritical water (Tc=647K, Pc=22.1MPa) provides apotential way to convert the wet biomass to a fuel-rich gas containing H₂and/or CH₄, inwhich the kinetics, mechanisms and coking control are the problems facing in the field.Gasification of biomass in SCW has shown it to be an expensive process, requiring aconsiderable input of energy. Therefore, reducing the cost of energy consumption is also oneof tough problems.We tried to find an efficient way converting biomass for energy gaseous products.Algae cultivation uses land about5⁹times more efficiently than terrestrial biomass. Itshould take about30-50%land to cultivate terrestrial biomass for energy supply, meanwhileolny about5%to cultivate algae. It also offers advantages in terms of land requirements andits potential for growing on wastewater, sea. Obviously, to produce energy from algae seemmore reasonable than others biomass.The research provided sytermatic research on gasification of alga and its kinetics model.The gaseous products were mainly H₂, CO₂and CH₄, with lesser amounts of CO, C2H4, andC2H6. Higher temperatures, longer reaction times, higher water densities, and lower algaeloadings provided higher gas yields. The algae loading strongly affected the H₂yield, whichmore than tripled when the loading was reduced from15wt%to1wt%. The water densityhad little effect on the gas composition. On the basis of this observation and the completeset of experimental results, we proposed a global reaction network for algae SCWG thatincludes parallel primary pathways to each of these two types of intermediate products. Theintermediate products then produce gases. The model parameters indicate that gas yieldsincrease with temperature because higher temperatures favor production of the more easilygasified intermediate and the production of gas at the expense of char. Sensitivity analysisand reaction rate analysis indicate that steam reforming of intermediates is an importantsource of H₂, whereas direct decomposition of the intermediate species is the main source ofCO, CO₂and CH₄.Uncatalyzed SCWG of microalgae leads to a low gasification efficiency unless hightemperatures (⁵00°C) are used, the work provided systematic study of supercritical water gasification (SCWG) of real biomass (algae) with Ru/C. The catalyst loading had the mostsignificant effect on both the yields and composition of the gaseous products. Completegasification of the microalga was achieved with a catalyst loading of2g/g. Longer reactiontimes, higher catalyst loadings and water densities, and lower algae loadings provided highergas yields. This loss of activity is due, in large part, to deactivation by sulfur, which ispresent in the microalga at about0.5wt%. A simple two-step catalytic gasificationmechanism along with a step for catalyst poisoning by sulfur, led to a rate equation that wasconsistent with all of the experimental results. The presence in algae of sulfur, and perhapsother elements such as Cl that are not as prevalent in terrestrial biomass, indicates thatefficient and effective SCWG of microalgae could present new challenges in engineering andcatalyst design.Phenol and the related derivatives not only are main products in char and typical aromaticpollutants in industrial wastewater, but also are the basic unit of lignin, which is one of majorcomponents in lignocellulosic biomass. The cleavages of aromatic ring hardly occur at thetemperature around400oC. We proposed partial oxidative gasification of phenol at a lowertemperature (ranging from573to753K) in supercritical water. The results showed that O2is effective to gasification of phenol in SCW.⁷6%of phenol was gasified and2.2mol/mol of hydrogen was produced within180s with Na2CO3as catalyst at the selectedprocess conditions, a molar ratio of oxygen-to-phenol,7.5to1,723K, and24MPa. Thepartial oxidation process is a complex combination of both SCWO and SCWG, involving fourtypes of primary reactions; i.e.1) phenol oxidation,2) acid oxidation,3) acid gasification, and4) gaseous products interconversion. A lower concentration of oxygen caused a decrease inthe oxidation rates of acids and gases, resulting in conditions in which the gasification ofacids and the water-gas shift reaction were predominant. On the other hand, a higherconcentration oxygen resulted in the oxidation of acids and CO predominating.If autothermal gasification of biomass can be achieved in SCW, it will greatly reduce thecost and make the process more attractive for future development. We focused on the keyfactors upon which autothermal gasification by partial oxidation might depend. Gasificationof biomass in an autothermal process in SCW is thermodynamicallly possible, but will requirea high concentration for feedstock and a high ER. In addition, minimizing energy lossthroughout the process is also an important factor to address in order to achieve anautothermal process. It also should be noticed that it will be difficult for some wet biomass,such as sewage sludge which contains nearly85wt%water, to be gasified under autothermaloperation in SCW due to the low concentration of biomass (¹5%).