BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to a pyrolysis process, and more particularly to a pyrolysis process for gasifying biomass into a synthetic gas or “syngas” using plasma torch technology.
II. Discussion of the Prior Art
The ability to harness and efficiently use the energy present in renewable resources like biomass has long been a goal of those who supply energy for society. One method by which this can be done is through extraction of fuel gas from biomass. Biomass is composed primarily of hydrogen, carbon and oxygen, with hydrogen comprising approximately 45-50% of the biomass composition. The extraction of hydrogen and other fuel gases from biomass requires heating the biomass to temperatures at which the biomass breaks down into various constituents, including hydrogen, which can be purified, separated, compressed and stored as a clean fuel. This approach, known as pyrolysis, requires energy from some source to raise the temperature of the biomass to cause it to dissociate, theoretically releasing the hydrogen and other fuel gas components. The ultimate goal of the pyrolysis process is to generate synthetic gas from the feedstock, and to do so economically. The purity of the extracted syngas depends on several design and operating variables incorporated in various pyrolysis and extraction technologies.
Traditional autothermal gasifiers combust a portion of the biomass feedstock, using that heat to pyrolyze the remainder—break it down to oils and smaller gaseous compounds. Most traditional gasifiers require substantial pre-processing of the feedstock with shredding and palletizing commonly utilized. Another major drawback of this traditional gasification technology is the limited ability to completely dissociate the feedstock constituents into fuel gas in the lower-temperature gasification zone, and to control the distributed temperature in the overall process. There are often significant amounts of dilution gases (nitrogen, carbon dioxide, and water) and problematic higher carbon-chain organic compounds (tars) to deal with in the downstream subsystem of traditional gasifiers.
- SUMMARY OF THE INVENTION
Therefore, a more effective and efficient method of extracting synthetic gas from biomass is desired.
The present invention provides for a renewable synthetic gas plasma pyrolysis process as an integrated, clean, non-combustion heat source to gasify a wide variety of biomass into hydrogen and carbon monoxide using plasma torch technology. A method is used where biomass is converted into syngas by feeding biomass into a reaction chamber, directing the flame of a plasma torch across the face of the biomass in the chamber, and extracting hydrogen and carbon monoxide rich gas from the reaction chamber.
Advantages of the system include a uniform and controllable process temperature, plasma torch/feedstock close-coupling, a smaller and simpler system than conventional gasifiers, and feedstock improved handling via processing as-delivered whole bales of biomass such as corn stover, wheat straw or switch grass, for example. Additionally, a significant advantage of the system is the unique pyrolysis chamber with plasma torch and oxidant integration for uniform, controllable gasification temperature to insure total dissociation of the organic feedstock and the ability to use different plasma gases to augment the recovery of syngas.
DESCRIPTION OF THE DRAWINGS
These and other objects, features, and advantages of the present invention will become readily apparent to those skilled in the art through a review of the following detailed description in conjunction with the claims and accompanying drawings in which like numerals in several views refer to the same corresponding parts.
FIG. 1 is a diagram of a baled, biomass gasification concept of the pyrolysis process of the present invention; and
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 2 is a diagram of the pyrolysis process system.
The present invention represents broadly applicable improvements for pyrolysis process systems. The embodiments herein are intended to be taken as representative of those in which the invention may be incorporated and are not intended to be limiting.
Referring first to FIG. 1, there is shown a diagram of the biomass gasification concept of the pyrolysis process of the present invention. The invention is described using corn stover as the biomass, but other agricultural and lignocellulosic materials can be processed as well. The assembly itself is indicated generally by numeral 10. It includes a plasma reaction chamber 12, a plasma torch 14, a feedstock supply inlet 16, and an air/steam supply inlet 18, and a gas outlet 20.
Feedstock inlet 16 is generally a sealed hydraulic ram feed system comprising a structure in which whole corn stover bales 22 or other biomass is passed and continuously feed into the system at a controlled rate. The whole corn stover bales 22 or other biomass are transferred from the feedstock inlet 16 to the plasma reaction chamber 12. As the bales 22 enter the chamber they enter a high enthalpy plasma coupling zone 24 and are met with a plasma torch 14 directed in a “close coupled” fashion onto the reacting interface of the continuously-fed whole corn stover bale 22. Discharged from the plasma torch 14 is a highly energetic jet of ionized plasma-gas species that is continuously moved/articulated across the exposed face of the bale 22. This torch movement, or rastorizing follows a prescribed pattern via a preset control parameter strategy implemented using a programmed digital controller coupled to servomotor drives for optimum coverage (both time and space) of the moving bale and its exposed interface with the plasma reaction zone 24. If additional oxygen is required for carbon gasification, it may be supplied from inlet 18. Oxygen, air, or steam may be injected from inlet 18 toward the face of the bale 22 to assist the pyrolysis process and to prevent the buildup of char. Steam provides additional oxidant, but more importantly, serves as a process temperature control feature.
When using a plasma oxygen or oxygen-enriched air torch 14, temperatures within the plasma jet cause dissociation of the diatomic gases into monatomic gas ions that are highly reactive. This high enthalpy plasma jet containing highly reactive monatomic gases can be focused directly on the feedstock, resulting in rapid dissociation of the feedstock and accelerated chemical reactions. There is also the added benefit of significant ultra-violet (UV) radiation from the plasma jet that is known to assist in breaking carbon bonds. The plasma jet also acts as a bulk gas heater, maintaining a desired reaction temperature in the volume of the gasifier vessel, as the torch power is turned up or down.
The directed high enthalpy coupling of monatomic plasma gas 26 (oxygen or oxygen-enriched air) and the bio feedstock 22, with controlled introduction of additional oxygen (or oxygen enriched air) results in rapid dissociation of feedstock and formation of a hydrogen and carbon monoxide rich gas. This, in effect, combines the advantages of some autogasifier energy release plus the thermal and UV “boosting’ effect of the plasma gas.
The present invention's use of plasma torches as a clean non-combustion heat source to gasify biomass simplifies the process and produces a pyrolysis gas containing a much higher percentage of hydrogen than in traditional gasification processors.
For example, thermo/chemical model was tested using basic constituents of corn stover input feedstock, together with a 90% oxygen plasma. The pyrolysis gas exiting the pyrolysis chamber contained approximately 48% hydrogen (by volume) and 48% carbon monoxide (by volume) and was relatively insensitive to the form of biomass feedstock (i.e. very similar results for wood chips/pellets, switch grass, etc).
In general, the present invention drastically simplifies the biomass gasification process by eliminating or minimizing up-stream and down-stream components of a commercial plant. The purity of the syngas produced by the process virtually eliminates the need for catalytic crackers for tars produced by traditional gasification plants. The near-total elimination of nitrogen in the gas simplifies the gas separation requirements. Both of these accomplishments significantly simplify the down-stream plant requirements. Minimizing feedstock costs can also be addressed with the use of a feed system that uses whole bales (as received) from commercial baling systems currently in use.
The present invention provides several advantages over traditional gasification methods. First, the process provides a uniform and controllable process temperature. The process uses one or more plasma torches to heat the feedstock and evolving gases directly, providing the temperatures necessary for complete dissociation of feedstock compounds without tars and partially dissociated hydrocarbons. The process temperatures can be easily controlled by controlling torch power input to match the feed rate. Exit average process gas temperatures of 1000-1500 degrees centigrade are easily achievable and maintained, assuring maximum fuel gas recovery with minimal tar components. There is no heat integration problem that is typical with partial-combustion gasification systems. In commercial-scale systems, multiple, multimegawatt torches may be employed within a single reactor to achieve uniform feedstock and gas heating. The torches' position can be independently controlled as a function of time within the furnace (insertion depth and angular position) via flexible control algorithms to achieve the most efficient, time-averaged feedstock conversion. Processing requires no combustion, consequentially producing a pyrolysis gas low in nitrogen and carbon dioxide.
Second, plasma torches have great adaptability and performance. Because the plasma torches can run on different plasma gases, air is often selected for use, based on cost and availability. However, plasma torches can also run on other gases, including oxygen, nitrogen, carbon monoxide, carbon dioxide, or mixtures of these gases, including enriched air (up to 90% oxygen content). Plasma torch thermal efficiency and consumable components (i.e. electrodes and outer-housing assemblies) can be affected by both the plasma gas used as well as the reactor chamber chemistry. Plasma torches may take the form described in U.S. Pat. Nos. 4,549,065; 4,559,439; 4,587,397 and 5,214,264 the teachings of which are incorporated by reference.
Third the present invention provides a smaller and simple system. Because the system is capable of higher operating temperatures without the formation of tars, and because there is no requirement for combustion air, the processing vessel and other downstream subsystems are smaller and less expensive than traditional systems. Vapor conditioning and gas cleanup for the system is simplified due to the absence of tars, nitrogen and carbon dioxide in the pyrolysis gas. No further cracking or cleaning of tars is required. Downstream gas cleanup equipment is smaller, with lower capital and operating costs due to a cleaner and lower overall gas volume. Gas cleanup systems focus primarily on removal of particulate. When hydrogen production is to be maximized, water-gas-shift catalytic processes (using steam to shift carbon monoxide to carbon dioxide with a byproduct of hydrogen) are also simplified with the present invention because there is no requirement for tar-cracking catalysts.
Fourth, the system provides improved feedstock handling and feedstock flexibility. To minimize the cost of feedstock handling, a proof-of-concept system has been designed to accept baled feedstock with no pre-processing. There are several ongoing studies directed toward the collection of agricultural biomass in the field in various bale configurations and sizes. Bales tend to vary in dimensions due to moisture, temperature and baling equipment specifications. The bale feed system is designed to accommodate those variables. Loading and feeding the bales of biomass as they come from the field greatly simplifies feedstock handling. Bales can be stored on site and loaded directly into the feed system with no additional processing. With feed rate control, temperature control and controlled oxidant injection in the gasification furnace being controlled variables, the conversion process is adaptable to different biomass feedstock. Experience and model projections show hydrogen production from pyrolysis of wood (forest slash and mill waste), corn stover, switch grass, and wheat straw varies by only 3-4%. The primary difference in using different feedstock is the up-front feedstock handling and preparation, and with a system designed to feed bales with no pre-processing, multiple agricultural feedstock can be processed.
Fifth, the design is readily scaleable. The combined information from thermochemical equilibrium analyses and bench scale data serves to define the pilot-scale performance. Elements of efficient feedstock/plasma coupling in the high-enthalpy zone of the plasma/bale interface will need to transfer to the larger, pilot scale design. Use of the process thermochemical model together with proprietary, fluid dynamic process reactor simulation models can be used to help to identify reactor design parameters to define/control residence time so that scale-up problems can be minimized.
The key to the foregoing advantages of the described system is the unique pyrolysis chamber with plasma torch and oxidant integration for uniform controllable gasification temperature to insure total dissociation of the organic feedstock and the ability to use different plasma gasses to augment the production of synthetic gas.
With reference to FIG. 2, a diagram of the pyrolysis process system is seen, whereby the operation of the pyrolysis process can be described as follows. First, biomass, such as whole corn stover bales 22 are fed into the feedstock inlet as described in the previous discussion of FIG. 1. The corn stover bales 22 are urged into the plasma reaction chamber 12 together with oxygen fed through inlet 18. The bales 22 are then met with a highly energetic jet of ionized plasma-gas species which is exerted from plasma torch 14 and is continually moved across the exposed face of the bales 22. A pyrolysis gas is produced containing a high percentage of hydrogen and carbon monoxide. This gas exits the outlet 20 and travels into the ejector/scrubber system 28, many forms of which are known in the art. Waste water produced is separated subjected to a waste water treatment system 30 here as well. The pyrolysis gas next passes through a draft fan 32 and through the on-line gas analyzer 34 and flow meter 36. Last the gas is either delivered to a commercial gas separation system 38 or to the thermal oxidizer for startup/shutdown 40.
This invention has been defined herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment details and operating procedures, can be accomplished without departing from the scope of the invention itself.