M. Dolores Bermejo

High glucose selectivity in pressurized water hydrolysis of cellulose using ultra-fast reactors

A new reactor was developed for the selective hydrolysis of cellulose. In this study, the glucose selectivity obtained from cellulose was improved by using ultra-fast reactions in which a selective medium was combined with an effective residence time control. A selective production of glucose, fructose and cellobiose (50%) or total mono-oligo saccharides (>96%) was obtained from the cellulose in a reaction time of 0.03 s. Total cellulose conversion was achieved with a 5-hydroxymethylfural concentration lower than 5 ppm in a novel micro-reactor. Reducing the residence time from minutes to milliseconds opens the possibility of moving from the conventional m(3) to cm(3) reactor volumes.

Governing Chemistry of Cellulose Hydrolysis in Supercritical Water

At extremely low reaction times (0.02 s), cellulose was hydrolyzed in supercritical water (T=400 °C and P=25 MPa) to obtain a sugar yield higher than 95 wt%, whereas the 5-hydroxymethylfurfural (5-HMF) yield was lower than 0.01 wt %. If the reaction time was increased to 1 s, the main product was glycolaldehyde (60 wt%). Independently of the reaction time, the yield of 5-HMF was always lower than 0.01 wt%. To evaluate the reaction mechanism of biomass hydrolysis in pressurized water, several parameters (temperature, pressure, reaction time, and reaction medium) were studied for different biomasses (cellulose, glucose, fructose, and wheat bran). It was found that the H(+) and OH(-) ion concentration in the reaction medium as a result of water dissociation is the determining factor in the selectivity. The reaction of glucose isomerization to fructose and the further dehydration to 5-HMF are highly dependent on the ion concentration. By an increase in the pOH/pH value, these reactions were minimized to allow control of 5-HMF production. Under these conditions, the retroaldol condensation pathway was enhanced, instead of the isomerization/dehydration pathway.

Experimental study of the supercritical water oxidation of recalcitrant compounds under hydrothermal flames using tubular reactors.

The hydrothermal flame is a new method of combustion that takes place in supercritical water oxidation reactions when the temperature is higher than the autoignition temperature. In these conditions, waste can be completely mineralized in residence times of milliseconds without the formation of by-products typical of conventional combustion. The object of this work is to study the hydrothermal flame formation in aqueous streams with high concentrations of recalcitrant compounds: an industrial waste with a high concentration of acetic acid and various concentrated solutions of ammonia. A tubular reactor with a residence time of 0.7 s was used. Oxygen was used as the oxidant and isopropyl alcohol (IPA) as co-fuel to reach the operation temperature required. The increase of IPA concentrations in the feeds resulted in a better TOC removal. For mixtures containing acetic acid, 99% elimination of TOC was achieved at temperatures higher than 750 °C. In the case of mixtures containing ammonia, TOC removals reached 99% while maximum total nitrogen removals were never higher than 94%, even for reaction temperatures higher than 710 °C. Ignition was observed at concentrations as high as 6% wt NH(3) with 2% wt IPA while at IPA concentrations below 2% wt IPA, the ammonia did not ignite.

Energetic approach of biomass hydrolysis in supercritical water.

Cellulose hydrolysis can be performed in supercritical water with a high selectivity of soluble sugars. The process produces high-pressure steam that can be integrated, from an energy point of view, with the whole biomass treating process. This work investigates the integration of biomass hydrolysis reactors with commercial combined heat and power (CHP) schemes, with special attention to reactor outlet streams. The innovation developed in this work allows adequate energy integration possibilities for heating and compression by using high temperature of the flue gases and direct shaft work from the turbine. The integration of biomass hydrolysis with a CHP process allows the selective conversion of biomass into sugars with low heat requirements. Integrating these two processes, the CHP scheme yield is enhanced around 10% by injecting water in the gas turbine. Furthermore, the hydrolysis reactor can be held at 400°C and 23 MPa using only the gas turbine outlet streams.

Supercritical Water Oxidation (SCWO) of Solid, Liquid and Gaseous Fuels for Energy Generation

The SCWO (Supercritical Water Oxidation) process is well known for being able to destroy any kind of compound without producing prejudicial byproducts. This fact together with the high potential for energy production (because the high pressure high temperature effluent generated in the process) makes it a good candidate for generating energy from bio-fuels, especially those which valorization by conventional combustion can be problematic. In this work, different literature energetic studies of the SCWO process are analyzed. When comparing the heat produced by direct expansion of the effluent and by indirect heating steam generation it is observed that when direct expansion is used, the energetic efficiency is much higher than when the effluent is used to heat an auxiliary fluid of a Rankine or Brayton cycle. Nevertheless, the production of energy by direct expansion of the SCWO is not technically available in the short term. In any case, obtaining a high temperature effluent it is a key point for optimizing energy utilization. To do so, reactors in which the effluent is not diluted or reactor working at hydrothermal flame regime are desirable. Also the lay-out of the plant is important for energy utilization and factors as preheating scheme must be thoroughly studied. In addition to all of this, SCWO process has the additional advantage of the possibility of CO2 sequestration.

Reactors for Supercritical Water Oxidation Processes

Supercritical water oxidation (SCWO) process can utilize different reactors to reduce the operation problems related with solids precipitation and corrosion. Tubular reactors are one of the most frequently used but its use is limited by plugging problems. Industrial plants operate with two parallel reactors. While one is under operation, the other is being cleaned to avoid obstruction due to salt precipitation. Transpiring wall reactor avoids plugging problems but dilution water reduces effluent temperature, reducing in this way the energy quality content of the effluent. Vessel reactors are under application to minimize corrosion by the control the wall temperature and the materials of construction. The implementation of the hydrothermal flame as internal heat source reduces the residence time and opens the way to reduce the reactor volume. Other reactors offer specific solution for some SCWO processes.