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Researchers Demonstrate Potential for Co-Production of Hydrogen from Cellulosic Ethanol Byproducts Via Gasification in Supercritical Water
20 June 2009
| Concept for hydrogen gas coproduction from cellulosic ethanol byproduct streams. Credit: ACS. Click to enlarge. |
Researchers at Oregon State University have demonstrated the gasification of water-soluble biomass constituents in supercritical water in a microchannel reactor under isothermal, continuous flow condition at short residence times to produce a hydrogen-rich gas. This could potentially lead to a process for the co-production of hydrogen with certain cellulosic ethanol systems.
A paper on their work, which, according to the authors, is the also the first reported study on the gasification of xylose by supercritical water, was published online 19 June in the ACS journal Energy & Fuels.
One approach to producing cellulosic ethanol uses enzyme-catalyzed hydrolysis of the lignocellulosic biomass to produce glucose that is then fermented to produce ethanol. This process generates two byproduct streams rich in either hemicellulose or lignin.
Although the hemicellulose obtained from pretreatment of lignocellulosic biomass can be hydrolyzed to xylose and then fermented to ethanol, an alternative path to make fuel out of these byproduct streams is to combine the aqueous pretreatment stream with the solid lignin residue and then gasify the mixture. Since the residual hemicellulose and lignin are already in water, the mixture can be reformed directly to hydrogen-rich gas. Conventional steam reforming is not sufficiently reactive to convert biomass constituents to H2 and CO2. However, a growing body of literature has demonstrated the potential of supercritical water (374 °C and 221 bar) to reform biomass constituents to a compressed H2-rich gas.
—Goodwin and Rorrer (2009)
Aaron Goodwin and Gregory Rorrer used a mixture of xylose (the principal sugar in hemicellulose) and phenol (the principal moiety in lignin) dissolved in water to serve as a model feedstock for supercritical water gasification of aqueous hemicellulose and lignin from lignocellulose pretreatment.
The team used microchannel reactors with inner diameters of less than 1.0 mm. The microchannel reactors offer high rates of heat transfer to endothermic reforming reactions. They found that:
The carbon in xylose was completely reformed to H2-rich gas (62% H2, 34% CO2) by supercritical water at 250 bar. Hydrogen gas yields of 8.2 ±0.6 mol H2 mol-1 xylose where achieved within a one second residence time.
Phenol was difficult to gasify, and the activation energy for phenol conversion was 264 ± 20 kJ mol-1. However, when a mixture of 1.6 mol phenol/mol xylose was gasified at 750 °C and 250 bar, the apparent rate constant of phenol conversion increased from 0.66 ± 0.03 to 2.8 ± 0.3 s-1. Although the H2 gas yield was 2.9 mol H2 mol-1 phenol + xylose (41% H2 in product gas), if the 19% CH4 in the product gas was also reformed, the yield increased to 8.5 ± 0.6 mol H2 mol-1 mixture.
Resources
Aaron K. Goodwin and Gregory L. Rorrer (2009) Conversion of Xylose and Xylose-Phenol Mixtures to Hydrogen-Rich Gas by Supercritical Water in an Isothermal Microtube Flow Reactor. Energy Fuels, Article ASAP doi: 10.1021/ef900227u
June 20, 2009 in Cellulosic ethanol, Hydrogen Production | Permalink | Comments (5) | TrackBack (0)
Comments
Posted by: ejj | June 20, 2009 at 08:26 AM
[quote]A supercritical fluid is any substance at a temperature and pressure above its critical point. It can diffuse through solids like a gas, and dissolve materials like a liquid. Additionally, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties to be "tuned". Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes. Carbon dioxide and water are the most commonly used supercritical fluids, being used for decaffeination and power generation respectively.
In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions (temperature, pressure and sometimes composition) at which a phase boundary ceases to exist. The term "critical point" is sometimes used to specifically denote the vapor-liquid critical point of a material. The vapor-liquid critical point denotes the conditions above which distinct liquid and gas phases do not exist.[end quote]
Posted by: ai_vin | June 20, 2009 at 09:41 AM
It they could turn the CO2 and H2 into renewable methane or methanol, that would really be something.
Posted by: SJC | June 20, 2009 at 11:00 AM
Wet-air-oxidation or Wet-Oxygen-Oxidation is a used process that can convert organic materials to simpler materials in water below 374 degrees Celsius. The processes can be controlled in temperature and oxygen concentration to determine what the major products will be. The process was originally developed to make artificial vanilla out of wood. Its main use right now is to remove organic chemicals from caustic solutions. Another main use is to regenerate activated charcoal for water purefication in the PACT process. It was once used to reuse clay from special papers as well.
Acetic acid is a principal residue of the process if the temperatures are kept low. Other simple carbolixic acids are also produced. These can be anaerobically fermented into methane.
Simple organic molecules of many types can be anaerobically digested into methane very rapidly. And at the high prices for propane it might be best for US farmers and others to set up large permanent digesters that digest finely ground corn, other grains and potatoes. The methane produced can be separated by various processes from CO2 to use in devices that burn natural gas. The initial digestion process is complete within a day, and is not slow like the old manure digestors which are still useful but far less productive.
WAO is perhaps the fastest and cheapest way of hydrolysing cellulose to simple sugars. It is just a higher pressure version of a pressure cooker.
Liquid water does not exist above 374 degrees even at very high pressures, and this H2O can dissolve organic materials but salts are not dissolved. The H2O is very reactive at these high tempertatures, and all organic chemicals are quickly converted to CO2 and H2O if enough oxygen is present.
Supercritical oxidation can quickly decontaminate soils, but most hydrocarbons can be removed from soils by feeding a mix of butane and air through the soil to build up a large mass of organisms that thrive on hydrocarbons.
In the background is the fact that there is not very much biomass to convert to fuel compared the the fuel already being used. But any organic materials wasted by humans should be converted to fuel or carbon to enhance productivity of soils.
There have been various commercial efforts to make food out of petroleum or natural gas when gas was cheaper than food. The fact is that there is no biomass that cannot be converted into food. Wood has been and can be used to make beer which is a calorie containing food and the yeast has vitamins and protein. ..HG..
Posted by: Henry Gibson | June 20, 2009 at 06:15 PM
ai_vin: Thanks - I figured there was some scientific definition...I am 10+ years from my days of organic chemistry & general chemistry in college!
Posted by: ejj | June 20, 2009 at 07:21 PM
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"Supercritical" water = high pressure, high temperature water....though it sounds like water that is about to detonate and blow up the world!