Under Pressure Underground: Gravity Pressure Vessels Convert Waste into Biofuels

By Peter Hurrell and Zbigniew "Zig" Resiak | June 02, 2008
The first attempt at commercializing a process for ethanol from cellulose occurred in Germany in 1898 and involved the use of dilute acid to hydrolyze the cellulose to glucose. A similar process is in use today. Cellullose molecules are polymer chains of different forms of cellulose bound together with lignin. The process works by de-polymerizing the lignocellulose, freeing the celluloses from the lignin, which are then hydrolyzed to the simpler sugars for fermentation to alcohol. The process uses acid as a catalyst. Dilute acid may be used under high heat and pressure, or concentrated acid can be used at lower temperatures and pressure. The mixture must be neutralized and cleaned, and yeast fermentation is used to produce alcohol.

Many chemical processes work better using subcritical or superheated water under pressure. These conditions have been used in the chemical and food industry for more than 180 years. Examples include dilute-acid hydrolysis of celluloses and starch to saccharides, the extraction of instant coffee, extracting indigo dye from woad, and treating wastewater sludge through wet-air oxidation.

In all of these applications, the process used has generally remained a batch procedure where the water is pumped into a pressure tank and a heat exchanger. After treatment the resulting liquid is returned through the heat exchanger, which pre-heats the inflow which moves through a pressure-regulating valve before being released to normal pressure. The process depends on energy- intensive mechanical and electrical pumps and pressure tanks. It has mostly been used for small-scale production since the mixing requirements and the need to add chemicals while maintaining temperature and pressure limits the potential for scale-up.

In 1967 James Titmas modified the process with the aim of making the best use of the pressure and heat from the subcritical water process. His goal was to convert biomass to useful materials using wet oxidation, pyrolysis and hydrolysis. To accomplish this, he placed the pressure vessel below ground in a borehole. Using gravity and heat from the process minimizes the amount of energy needed and creates continuous flow. Another advantage is that being underground creates an environment for efficient thermal insulation, a small plant footprint, and improved health and safety. To obtain the natural pressure needed to maintain the temperature in subcritical water, the reactor has to be placed no more than 7,200 feet underground.

This is within the capabilities and expertise of the oil drilling industry. The accuracy and skill of drilling wells vertically, straight and lining them to preserve water aquifers is also well proven. The technique has been proven in use. The U.S. EPA and Bow Valley Energy used a 4,200-feet deep vertical-tube reactor based on a 1982 patent for the wet-air oxidation of sewage sludge with heat recovery at Longmont, Colo. After modification, parts of this plant were moved to Apeldoorn, Netherlands, where it was used to treat sewage sludge from 1992 to 2004. The plant out-performed its design expectations. In its later years the use of Taylor bubble and heat recovery was abandoned in favor of product recovery following the Titmas approach.

The gravity pressure vessel provides a simple way of making the subcritical water process continuous. It uses the heat released from the controlled wet oxidation of process contaminants to drive the water flow, much in the same way as an autogenic thermal airlift pump. This greatly increases production capacity because the gravity pressure vessel works as a continuous, linear, plug flow reactor with high internal heat and pressure recovery and no moving parts. This makes the process easy to control and scale-up without the need for multiple arrays of pumps, pressure tanks or complex controls.

The gravity pressure vessel is comprised of a long steel pipe, shaped like a test tube, of a fixed diameter between 12 and 24 inches. The annulus of an open-ended steel pipe creates updraft and is suspended within the test tube. This updraft protrudes above the test tube and descends to within a few feet of its concave bottom. Small bore steel pipes are suspended in the updraft to inject steam and chemicals, for temperature control, cathodic protection and cleaning. The diameter of the tube and updraft pipes is governed by hydraulics of the supercritical water and the need for a self-cleansing velocity as well as the small bore pipes.

The entire gravity pressure vessel is freely suspended inside a steel-lined borehole, which is cemented into the ground. A pressure cap is placed over the space between the gravity pressure vessel and the borehole and a vacuum is applied to the void between the enclosed space to form a thermal barrier between it and the borehole. Through the top of the gravity pressure vessel the pipes connecting to the gravity pressure vessel include a feed solution to the annulus formed between the updraft and the test tube. A pipe is used for discharging the treated solution from the updraft with the smaller pipes at the top. The process defines the depth of the gravity pressure vessel.

Wet-Air Oxidation of Sludge
Wet-air oxidization of sludge should be carried out at a depth of 6,000 feet. Sludge at 3 percent to 6 percent dry solids passes down the outer annulus and oxygen is injected near the bottom. Oxidation is rapid, raising the temperature to 600 degrees Fahrenheit. The treated material rises through the updraft to the outlet for final treatment, degassing and heat capture. As it rises, it passes heat through the updraft to pre-heat the descending sludge prior to oxidation. The process achieves more than 95 percent destruction of biological/chemical oxygen demand and neutralizes all inorganic material.

Since sludge can be processed as a liquid, it can be taken directly from sewage treatment works. There is no need for expensive drying as required for other processes such as incineration. The process is self-sufficient in energy and even generates a surplus, which can be converted into electricity.

Dilute-Acid Hydrolysis of Biomass
Dilute-acid hydrolysis of cellulose to sugars requires a 1,600- to 2,000-feet deep gravity pressure vessel. The biomass mash containing 8 percent to 12 percent dry solids flows down the outer annulus and steam is injected at the bottom to initiate a temperature rise. Oxygen is added at the entry to the updraft to burn off dissolved lignin. Acid is then added. As the cellulose disassociates to saccharides (sugars), the temperature rises to 460 degrees Fahrenheit. An alkali is injected, immediately neutralizing the acid. Once autogenic thermal balance is established, the steam supply is cut. Heat from the rising saccharide solution passes through the updraft to pre-heat the cellulose mixture that is descending in the outer annulus. Using the gravity pressure vessel increases the efficiency of converting biomass to sugars by two- to three-fold, greatly enhancing the potential of producing ethanol for biofuels and other applications.

Most ethanol today is made from crops rich in sugar and starch, raising concerns about elevated food prices and fuels inflation. Using a gravity pressure vessel in subcritical water to convert non-food biomass to ethanol is an important part of the solution.

Ethanol can be made profitably from a wide range of biomass sources including non-food crops. Using municipal solid waste as a raw material has the added advantage of being a steady source of biomass throughout the year, unaffected by seasons, climate, disease or international pricing cartels.

The gravity pressure vessel process can assist the household waste industry because it changes a waste material that currently incurs a cost to treat to a raw material that can
create an income from treatment. As biomass represents approximately 60 percent of municipal solid waste in the United States (66 percent in the European Union and 87 percent and more in Asia), it is profitable to convert to ethanol. Sewage sludge, which contains approximately 30 percent biomass, can also be treated and converted in the plant.

Municipal solid waste-to-ethanol facilities work in three identifiable stages. The first is preparing the biomass by shredding, settlement in water to remove inert materials, maceration and thickening. The second is to treat the biomass with supercritical water, and passing it through a settlement tank and molecular sieves to clean it. In a third stage, the contained saccharides are converted to ethanol. The process plant and equipment used are standard to the wastewater industry and are enclosed and covered. There are no airborne emissions from the treatment of waste. Dioxins cannot be produced since the working temperature is low. Smells and particulates are avoided. Water from the process is recycled and any residual will be treated for discharge to inland waterways. The carbon dioxide produced can be used as the acid in the hydrolysis reaction with the rest available for sale or sequestration.

Using municipal solid waste to make ethanol betters all existing and projected environmental targets for treatment. It eliminates landfilling and cuts out the greenhouse gases that would otherwise be emitted from landfill or from the treatment process. The process is entirely carbon negative and qualifies for carbon credits. Ethanol made from municipal solid waste offers major benefits toward biofuels substitution targets in any country without affecting the food economy.

A municipal solid waste-to-ethanol plant is affordable. Its capital cost can be significantly less than 40 percent of an equivalent incineration plant, and is simple and more economical to operate and maintain. The income from municipal solid waste tipping fees and/or the sale of ethanol can finance the design, construction, operation and maintenance of a plant within a few years without fees increasing above current landfill charges.

In Conclusion
Superheated (subcritical) water is an environmentally benign solvent that has many applications. Until recently it has been used in a batch process, but the gravity pressure vessel makes it possible to turn this process into a continuous or linear process.

Gravity pressure vessels also find their use in the wet-air oxidation of sewage sludge, which produces surplus energy, but the quantity of sludge that can be treated in a stand-alone treatment facility is limited to the larger urban areas or regional centers.

A particularly interesting application of the gravity pressure vessel is in the conversion of biomass to saccharides in order to make ethanol fuels, using dilute-acid hydrolysis. This process can be economical using a wide range of biomass materials, including non-food crops and waste such as municipal solid waste. While the yield of ethanol from some materials may be higher than municipal solid waste, this can be offset with a tipping fee. The process also promises an environmentally friendly solution for municipal waste and an alternative to landfill and incineration. GeneSyst International Inc., which has developed and patented a gravity pressure vessel reactor with the aim of transforming municipal solid waste to ethanol, calculates that a comparable municipal solid waste-to-ethanol plant can cost less than 40 percent than an equivalent thermal destruction plant.

The process is not dependent on food crops such as wheat and corn, but takes commercial advantage of industrial waste with a high cellulose content such as paper and wood, municipal solid waste (after separation of the recyclable materials), sewage sludge and other cellulose materials that would otherwise be disposed of.

The ethanol produced is an effective use of bioenergy resources, in terms of both greenhouse-gas emissions and monetary value, which takes on the wider environmental impacts, and contributes to sustainable emission reductions needed to fulfill a low carbon economy.

Peter Hurrell is managing director of GeneSyst International Inc. U.K. and Ireland. Reach him at hurrellconsult@aol.com. Zbigniew "Zig" Resiak is the program director for Indiana Ethanol Power. Reach him at zresiak@rwa.com or (317) 780-7249.