Noncatalytic supercritical water gasification of liquid type feedstock for hydrogen production : investigation of reactor performance

Abstract

Hydrogen is considered as the promising solution for both of energy issue due to fossil fuel depletion and environmental issue due to global warming. It because hydrogen can be produced from both of renewable and nonrenewable sources and only produce air as byproduct in the case of pure hydrogen fed to Polymer Electrolyte Membrane Fuel Cell (PEMFC). The production of hydrogen by the supercritical water gasification (SCWG), a high pressure steam reforming process conducted at above the critical point of water (374°C and 22.1 MPa), has several advantages compare to the conventional gasification due to the unique physical properties of supercritical water. The low dielectric constant of SCW (2-20) allows the homogenous phase reaction of organic feedstock (as carbon source), water and the gaseous product, eliminate the mass transfer limitation due to two phase reaction system; enable the reaction to be conducted at very short res.idence time. Compare to ambient water, it also has much lower viscosity, higher diffusivity and adjustable density depends on the temperature and pressure, beneficiary for conducting faster reaction. I introduced the new reactor oesign, in this thesis named as "the second generation Supercritical Water Gasification (SCWG)," having purpose to obtain the gas-liquid flow rate stability and improve the gaseous yield. This second generation was the modification of the I" generation SCWG, which has been utilized for gasification study by the previous researcher. The modification focus was in the reactor geometry. In the I" generation SCWG, the reactor and condenser was in vertical position; allowing the feed to be fed from the top and liquid/gas flew out from the bottom. The modification done in the second generation SCWG includes titling the reactor 75° from vertical position and adding the aircooled tube. In this 2'd generation SCWG, feedstock fed from the bottom and liquid/gas flew out from the top. The apparatus performance was investigated for the noncatalytic gasification of isooctane, a model compound of gasoline. Under the similar operating condition, the second generation of SCWG could improve the hydrogen yield almost 4 times higher. Further modification to the apparatus, named as "the third generation SCWG," was done by changing the reactor material to enable the gasification experiment up to 800°C, 25 MPa and increasing the reactor volume up to 6 times larger to enable the observation at longer residence time. Again, isooctane was used as feedstock in order to compare with the previous results. Our group is the first who investigates the isooctane gasification in supercritical water. In order to compare the results with the widely studied feedstock in SCWG, glucose was chosen as the second studied feedstock. Glucose is used as a biomass model compound as it is representative of the "building block" of cellulose, the major constituents of biomass. The low concentration studies of glucose gasification were done and the results then compared with that of done by other ·researchers under similar operating condition. At high temperature (>700°C), the yield of hydrogen was higher than that of observed by other researchers. The apparatus performance was also investigated using various feedstocks from hydrocarbon with one carbon number (C1) to hydrocarbon with ten carbon number (CIO). Those feedstocks were methanol, ethanol, glycerol, glucose, isooctane and n-decane. They represent the straight, branch and aromatic hydrocarbon. Complete gasification of all feedstocks were observed under similar operating condition (25 MPa, 740°C; 10 wt%). Furthermore, the gasification of those feedstocks also studied at higher concentration of20 wt% and low temperature of 650°C. Finally the energy efficiency calculation was done to the system. The efficiencies of the experimental yields were compared to the equilibrium yields as well as the efficiencies of the system without heat recovery and with heat recovery. It can be concluded that the heat recovery could improve the efficiency largely. The other ways to improve efficiency are used the higher heating value of feedstocks and higher feedstock concentration.