Method for controlling syngas production in a system with multiple feed materials

Abstract

Two or more feed materials that possess differing syngas generation potentials are mixed in a mixer and fed as a composite feed stream into a gasifier to produce syngas. By controlling the feed rate of the mixture into the gasifier as well as the feed rates of one or more of the individual feed materials into the mixer, the syngas is produced at a target production rate, with target energy content (BTU). Potential feed materials include, but are not limited to, construction and demolition (C&D) debris, municipal solid waste (MSW), other sewage-related solids, waste tires, and other substances that contain varying levels of organic compounds capable of producing a syngas.

Description

This application is based on and claims priority to Serial No. 60/912,440, filed Apr. 18, 2007.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to syngas production methods.

2. Background of the Related Art

It is known in the prior art to provide gasification systems that convert municipal solid waste (MSW) and construction and demolition waste (C&D) into clean energy. As described in U.S. Publication No. 2006/0228294, which is representative, these systems may comprise a refractory, induction furnace that receives the feed material into a molten metal bath, wherein a mix of organic and non-organic material is treated resulting in metal recovery and efficient production of synthesis gas (syngas). The syngas can be used to fuel a combined-cycle generator to provide municipalities with clean, renewable electricity.

One of the technical objectives that must be reached to ensure commercial success of the gasification technology is to achieve a high efficiency of synthesis gas generation from the processed waste streams.

BRIEF SUMMARY OF THE INVENTION

Two or more feed materials that possess differing syngas generation potentials are mixed in a mixer and fed as a composite feed stream into a gasifier to produce syngas. By controlling the feed rate of the mixture into the gasifier as well as the feed rates of one or more of the individual feed materials into the mixer, the syngas is produced at a target production rate, with target energy content (BTU). Potential feed materials include, but are not limited to, construction and demolition (C&D) debris, municipal solid waste (MSW), other sewage-related solids, waste tires, and other substances that contain varying levels of organic compounds capable of producing a syngas.

In a representative embodiment, two or more feed materials, each preferably having a different BTU value, are mixed to create a blend, which is then fed to a gasifier. The mixture of materials having various BTU content produces a blend having a final BTU content value. Desired operating conditions are a target production rate, which typically represents a mass flow rate exiting the gasifier (or, more generally, the gasification stage), at a target energy content. According to the process, a feed rate of the mixture into the gasifier is sped up or slowed down to produce a constant or substantially constant mass flow of syngas (i.e. the target production rate), while the feed rate(s) of one or more of the individual feed materials are adjusted as necessary to maintain the target energy content. The feed rates are adjusted using one or more control signals. The control signals are generated by a controller, which derives the values of these signals by analyzing data received from components that monitor the syngas. In particular, together with temperature measurements, syngas mass flow measurements are taken in exhaust ducting from the gasifier, e.g., by means of a pitot tube or other velocity or flow measuring devices. This real-time data is then analyzed, for example, for carbon monoxide, hydrogen and/or total hydrocarbons levels, to determine the BTU content of the syngas output from the gasifier. Using the data, a controller adjusts the material feed rate(s) accordingly to attempt to maintain the syngas target production rate at the target energy content.

The foregoing has outlined some of the more pertinent features of the invention. These features should be construed to be merely illustrative. Many other beneficial results can be attained by applying the disclosed invention in a different manner or by modifying the invention as will be described.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a process flow to provide syngas at a target production rate having a target energy content according to the subject matter herein;

FIG. 2 is data processing system for use in a control system that implements the process flow shown in FIG. 1;

FIG. 3 is a representative mixing system in which the method described herein is implemented;

FIG. 4 is an embodiment where a single feedstock is added to a primary feedstock (e.g., C&D waste) to produce and maintain syngas at a target production rate and BTU value; and

FIG. 5 is another embodiment where multiple feedstocks are added to a primary feedstock to produce and maintain the syngas at the target production rate and BTU value.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

Gasification of waste is a well-developed technology. According to the disclosure herein, an optimization is provided whereby two or more feed materials, preferably of varying energy (e.g., BTU) content values, are blended (mixed) and supplied to the gasifier. The syngas output from the gasifier preferably has associated therewith a “target” (or desired) production rate at a target energy content. This is desirable where, for example, the syngas is being used to operate a gas turbine or the like. Thus, for example, production rate typically is a fixed number of tons per hour (or some other temporal metric), and target energy content is some desired BTU content value at that target production rate. Using two or more feed materials, preferably of different BTU values, a mixture or blend is created in advance of the gasification stage. In particular, preferably the materials having various BTU content values are blended together for a final BTU content value. The target production rate and target energy content of the syngas are the desired operating conditions. According to the described process, the feed rate of the mixture into the gasifier is sped up or slowed down to produce a constant or substantially constant mass flow of syngas; in addition, and as necessary, the feed rate(s) of one or more of the individual feed materials (into the mixing unit) are adjusted to maintain (or attempt to maintain) the target energy content. Preferably, together with temperature measurements, syngas mass flow measurements are taken in exhaust ducting from the gasifier, e.g., by means of a pitot tube or other velocity or flow measuring devices, to calculate real-time data values. This data is then analyzed, for example, for carbon monoxide, hydrogen and total hydrocarbons levels, to determine the BTU content of the syngas. Using the data, a controller adjusts the feed rates accordingly.

FIG. 1 illustrates this basic process flow. A gasifier 100 receives a feed mixture from a mixer 102 using a feeder. The mixer 102 is supplied with at least a first feed material 101 and a second feed material 103. First feed material is fed to the mixer 102 at a fixed or adjustable rate using a feeder; second feed material is fed to the mixer 102 at a fixed or adjustable rate using a feeder. The output of gasifier 100 is syngas having a target production rate with target energy content. Monitor 104 in exhaust ducting (or other structure) measures syngas mass flow rate and analyzer 106 analyzes the CO, H₂ and other hydrocarbons to determine the energy (e.g., BTU) content of the syngas. The resulting data (perhaps with other data, such as temperature readings) is supplied to controller 108, which may be implemented in any convenient manner such as a computer, programmable logic controller (PLC), a combination thereof, or the like. The controller 108 takes the data and compares it to target production rate and target energy content. Controller 108 then generates a first control signal to adjust the feed rate of the material into the gasifier as necessary to reach and then maintain (or attempt to maintain) constant the target production rate. Controller 108 also generates second and/or third control signals and as necessary to adjust the feed rate(s) of one or both of the feed materials into the mixer 102; this operation maintains (or attempts to maintain) constant the target energy content. This control operation may be initiated at any convenient time, e.g., after a steady state of the process is achieved.

The above-described operation ensures a consistent BTU value of syngas production.

A concrete example of the process is shown in FIG. 3. This embodiment is merely representative and not limiting. The system includes a number of components including feed bins 301 and 302, sensors 303, a computer control system 304, feed mixers 305, feed controllers 306, composite mixers 307 and 309, feed controllers 308, final mixer 310, syngas production chamber 311 and syngas analyzer 312. Each feed bin 301 and 302 provides a feedstock material to its associated feed mixer 305. The feed controller 306 associated with a feed mixer controls the volume of feedstock provide to the composite mixer 307 or 309. The feed controller 308 associated with each composite mixer 307 or 309 controls the volume of combined feed (created in composite mixer 307 or 309) supplied to the final mixer 310. Final mixer 310 provides the combined materials to the syngas production chamber 311, and the output of the chamber is monitored by the syngas analyzer 312. The computer system 304 provides the overall system control.

In particular, to operate this system, sensors 303 and syngas analyzer 312 are used to monitor variation in the feed materials and the syngas production rate. The resultant data are transmitted to a computer program in computer system 304 containing pre-programmed equations that are used to adjust material input rates to achieve the desired syngas production range. In this process, preferably historic data plus real-time test results on the feed material are used to determine the syngas generation potential of each material. Preferably, each feed material is sorted and placed into separate tanks (e.g., feed bins 301) based on whether it can generate syngas above or below the target syngas production rate. The feed bins 301 hold materials that generate syngas above the target rate, and the feed bins 302 hold materials that generate syngas below the target rate. The number of bins, of course, is merely illustrative. In this embodiment, the resultant two types of feed materials are further mixed, and based on sensor data, fed to the syngas production chamber 311 to produce the target range of syngas production rate.

Referring to FIG. 3, in this embodiment, the syngas generation process is optimized by using the computer system 304 to control feed material rates, preferably as determined by real-time syngas composition data and mixing equations. The term “real-time” may also include near or “substantially” real-time data, so there is no explicit requirement that control operations be carried out instantaneously. Additionally, the computer system may also consider historical feed material analysis data, such as elemental content and organic content, with the real-time feed material analysis from the sensors 303 to further optimize the blending of the feed materials and thus the syngas production rate. Further, the analysis on the feed materials by the sensors 303 may also identify potential materials that could upset the syngas generation process, such as materials that contain an excessive level of inorganic compounds. In such case, the particular feed controller 306 might be de-actuated for a given time to ensure that such materials are not provided to the production chamber.

The process begins after each feed material has been sorted by its syngas generation potential into individual feed bins 301 and 302. Each feed material is then fed to a feed mixer 305. The feed mixers 305 could be rotary dryers, traditional mixing tanks, rotating drums or any other device capable of mixing each feed material to produce a consistent composition. Materials in feed bin 301, which have a syngas generation potential above the target syngas production rate, preferably are fed by computer system 304 to an “above” composite mixer 307; while materials in feed bin 302, which have a syngas generation potential below the target syngas production rate, preferably are fed by the computer system 4 to a “below” composite mixer 309. Preferably, materials from the composite mixer 307 and composite mixer 309 are then fed at specific rates as determined by the computer system 304 into a final mixer 310 prior to being fed to the syngas production chamber 311. A representative production chamber 311 is of the type described in U.S. Publication No. 2006/0228294, or as described in U.S. Pat. No. 5,571,486. The particular production chamber 311 is not a limitation of the present invention.

The size of the mixing tanks, the material feed rates, and the residence/mixing time of each material are ultimately determined by the target range of syngas production rate. For example, a narrow target range will require larger tanks and longer mixing times.

A feed controller 306, controlled by the computer system 304, sets the feed rate of each feed mixer 305 by adjusting the operating parameters of the physical dispensing device. A dispensing device could be a screw drive, a conveyor system, or any other mechanical means of moving feed material from the feed mixers 305 to the composite mixers 307 and 309. Also, it is assumed that simple level sensors are used to ensure that the dispensing devices that move materials from the feed bins 301 and 302 to the feed mixers 305 operate in a manner that, in a preferred embodiment, ensure each feed mixer 305 remains full at all (or substantially all) times.

A sensor 303 monitors each feed stream. These sensors may include, but are not limited to, devices that measure secondary radiation such as a CMOS or CCD image sensors plus a source of primary radiation including white or infrared light. Feed material sensor data is then sent to the computer system 304. This data may be used to detect variation in the composition of the feed stream. Preferably, this information is used by the computer system 4 as an adjustment factor in determining the feed rates to the composite mixers 307 and 309, and final mixer 310.

Immediately after exiting the syngas production chamber 311, and after any required cooling, a syngas analyzer 312 determines the syngas production rate. Production data include, but is not limited to, the determination of volumetric flow rate, hydrogen gas concentration, and carbon monoxide concentration. Potential methods for rapid syngas analysis include, but are not limited to, Raman Spectroscopy and GC Mass Spectroscopy (GCMS). Data from the syngas analyzer 312 is sent to the computer system 304.

After receiving continuous real-time data from the feed controllers 306 and 308, the material sensors 303, and the syngas analyzer 312, the computer system 304 then relates the target production rate and target energy content data with the real-time feed rate and composition data. The computer system 304 then executes a computer program based, for example, on the equations presented below, to maintain the syngas at the target production rate and target energy content. As noted above, typically the target production rate is controlled by adjusting the feed rate of the material into the gasifier, whereas the target energy content typically is controlled by having the computer system 304 inform each feed controller 306 and 308 to dispense the appropriate amount of the feed materials into the composite mixers 307, 309, and 310.

Representative mixing calculations are now described. In particular, it can be shown via an energy balance on the syngas production chamber 311 that the energy potential (E) of the feed material entering the production chamber 311 must be equal to the energy generated (R) by the combustion reaction within the chamber. If X is the mass feed rate to the chamber 311 and H is the energy potential per mass unit of the feed material, it can be shown the incoming potential energy rate (E) is equal to the product of X and H, which is equal to the energy R generated by the production chamber:

E (energy/time)=X (mass/time)×H (energy/mass)=R (energy/time)  (1)

Because a constant mass feed rate to the chamber 311 and a steady state process is assumed, and because it is also assumed the energy potential of the feed materials to the composite mixers (307, 309) is constant, the cumulative mass feed rate of the mass streams exiting the composite mixing tanks (307, 309) must be equal to mass feed rate entering the chamber. Thus, if HL is the “below-target” energy potential of the feed stream from the composite mixer 307 and HH is the “above-target” energy potential of the composite mixer 309 and X_b and X_a are the respective mass rates exiting the composite mixers 307 and 309, it can be shown that:

(HL (energy/mass)×X_a (mass/time))+(HH (energy/mass)×X_b (mass/time))=R(energy/time)  (2)

A target energy generation level can be represented via the following variables:

• • R_t=target energy generation rate
  • R_l=lower limit energy generation rate, and
  • R_u=upper limit energy generation rate.

Now, because the actual energy (R_a) generated by the production chamber 311 can be accurately deduced from composition measurements made by the syngas analyzer 312, the variation in the energy generation rate (R_v) from the target level can be established:

R_v=R_a−R_t  (3)

Consider if R_v is positive. This indicates the feed rate X_a to composite mixer A (which has the lower energy potential HL) must be increased and the feed rate X_b to composite mixer B (which has the higher energy potential HH) must be decreased by an equivalent mass rate to decrease the overall variation R_v.

Ultimately, to produce an energy generation rate within a target range, preferably calculations that employ differential equations are iterated by the computer system. 304. Also, prior to executing these calculations an initial design should be established, based upon a target energy production range for specific mass feed rate that specifies the volume of each mixing tank. For example, for a given mass feed rate, larger mixing tanks produce longer residence times for a given feed material, which decreases variations in material concentration over time (which subsequently decreases the rate of variation of energy generation by the production chamber 311).

The following is a sample calculation.

Using applied differential equations and assuming perfect mixing due to the relatively minute change in the overall composition within each mixer caused by the addition of new feed material, it can be shown that the time rate of change for a given feed material, A is given by the following equation:

dA/dt=rate of amount gained−rate of amount lost  (4)

Because the volume of any mixing container is known, i.e., constant, a differential equation can be created for each mixing vessel that can render a solution for the mass of A present in a mixing vessel at any given time. For example, if A is entering a mixing vessel at 10 pounds per minute and there is 5 pounds of A in the tank initially:

dA/dt=10−A/5  (5)

Solving this equation, it can be shown that:

A=50−25(e−t/5)  (6)

Thus for the given feed rate, a set time can be entered to determine the mass of A present in the system. This mass value can then be multiplied by the syngas generation density, i.e., the amount of syngas generated by unit of mass of A, to calculate the syngas production rate for A.

The computer system 304 can execute similar calculations for each feed material to determine its contribution to the overall syngas production rate and then adjust each feed rate via the controller modules 306 to optimize the target syngas production rate based upon the sensor data. For example, if the syngas analyzer 312 reports a syngas production rate that is below the target range, the feed rate of the “above-target” materials can be increased and the feed rate of the “below-target” materials can be decreased to keep the syngas production rate within the target range.

FIG. 2 illustrates a representative computer system 304. A data processing system 200 suitable for storing and/or executing program code will include at least one processor 202 coupled directly or indirectly to memory elements through a system bus 205. The memory elements can include local memory 204 employed during actual execution of the program code, bulk storage 206, and cache memories 208 that provide temporary storage of at least some program code to reduce the number of times code must be retrieved from bulk storage during execution. Input/output or I/O devices (including but not limited to keyboards 210, displays 212, pointing devices 214, etc.) can be coupled to the system either directly or through intervening I/O controllers 216. Network adapters 218 may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or devices through intervening private or public networks 220.

The computer may be connected to another computer or system over a network, such as wide area network (WAN), local area network (LAN), protected network (e.g., VPN), a dedicated network, or some combination thereof. More generally, the various system components illustrated in FIG. 3 may be controlled with any collection of one or more autonomous computers (together with their associated software, systems, protocols and techniques) linked by a network or networks. The control system calculations comprise a set of preferably software-based functions (e.g., applications, processes, execution threads, or the like) or firmware-based functions that provide the described mixing method.

FIG. 4 illustrates a more simplified embodiment where only a single feedstock Product 2 is added (blended or mixed) to a primary feedstock, Product 1, in this case construction and demolition waste (C&D). In the drawing the Product 1 feeder is illustrated by reference number 400 and the Product 2 feeder is illustrated as reference number 402. The materials are combined in blender/mixer 404 and provided to gasifier 406. The output syngas 410 is analyzed to provide a gas analysis 412, which is then provided to the computer system 414 to provide the one or more feedback control signals 416 and/or 418 to the respective feeders.

In this example, the C&D waste is being processed in a facility that may include several stages (not shown): C&D handling and sorting, C&D pre-processing, C&D debris post-processing, gasification, and, optionally, post-gasification/energy generation. These stages may be carried out in a single building, facility or enclosure, or in co-located processing facilities. Thus, for example, the handling and sorting, and pre-processing stages are performed in a first enclosure, while the post-processing and gasification stages are carried out in a second, nearby building, facility or enclosure. Preferably, the C&D processing takes place in a continuous or partially-continuous manner as bulk debris is received at the processing facility. A representative end-to-end system of this type is described in Ser. No. 12/021,987, filed Jan. 29, 2008, the disclosure of which is incorporated herein by reference.

As noted above, an object of having multiple feeds is to equalize the BTU content of the feed materials to the gasifier to produce a constant or substantially constant BTU gas output. In this example, construction and demolition wastes (C&D), which have been appropriately sorted and dried, are provided as the main feed component to the gasifier. Because it is a waste material, the incoming BTU content ranges from approximately 5,000-7,000 BTU/lb; thus, for a constant system feed rate, the energy content of the output gas would vary percentage-wise equally. Preferably, the product syngas has a content of approximately 325 BTU/lb. To produce a constant BTU output, it is thus necessary to add a higher BTU content material. In this example, this higher BTU content material (Product 2) is waste rubber (e.g., chrome rubber), which has a consistent content of more than 10,000 BTU/lb. The rubber is blended or mixed with the C&D waste in blender/mixer 404. The blend ratio may be set volumetrically, although this may not be an optimal approach. Thus, preferably, the system uses one or more of GCMS, infrared and other analytical equipment to measure for hydrogen, carbon monoxide, methane and other hydrocarbons, as well as for mass flow. The results of the analysis 412 are fed to a combination computer/PLC system 414, which utilizes the analytical data in conjunction with mass flow and energy content of the various species to determine a real-time (or near real-time) syngas energy value. This energy value when compared to the desired value enables the computer system 414 to produce a signal 418 to speed up or slow down the high BTU feed stock or, in the case of a higher desired mass flow, to enable the computer system 414 to produce a signal 416 to slow down the primary feed stock (and perhaps the rubber feeder as well) while maintaining BTU content. These output signals are produced in real-time (or substantially near real-time) to minimize energy fluctuations in the syngas. Preferably, materials are fed to the system with gravimetric feeders 400 and 402.

Of course, the particular type of waste material that is added to the primary feed will vary depending on the primary feed characteristics and BTU content, the availability of other feed stocks, as well as the energy content of those additional materials. Thus, for example, in appropriate circumstances municipal solid waste (MSW) may be used as an additive, as its energy content (approximately 4,000-5,000 BTU/lb) varies more than most other waste streams. Most areas of the world produce MSW, so it may be a convenient additive. Of course, higher BTU content material availability will vary considerably depending on location.

FIG. 5 shows another embodiment. The system utilized here is an expansion of that shown in FIG. 4. Here, one or materials of lower BTU content are fed with one or more high energy content materials, such as waste plastics, paper, rubber, or sludge to produce a constant output gas. In this example there are four materials (500, 502, 504 and 506) although this is not a limitation. The mentioning of specific high energy wastes is not to be inclusive, but only an example of such feed stocks. Preferably, the system described here also incorporates component availability and switches automatically from one high energy product to another as needed.

While the above describes a particular order of operations performed by certain embodiments of the invention, it should be understood that such order is exemplary, as alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, or the like. References in the specification to a given embodiment indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Further, while given components of the system have been described separately, one of ordinary skill will appreciate that some of the functions may be combined or shared in given systems, machines, devices, processes, instructions, program sequences, code portions, and the like.

Further, although an embodiment of the invention has been described in the context of an “above-target” and “below-target” materials, there may be multiple such levels (such as below, intermediate, above, or the like) or even just one level.

As used herein, the phrase “target production rate” should not be construed as being limited to a single value, as a “rate” may include a range of acceptable values (typically, the mass flow rate). Also, the word “maintain” in the phrase “maintain production rate” does not require that the associated production rate or energy content be exactly equal to a given value. Also, the word “mixed” or “mixing” may be considered synonymous with “blend” or “blending.”

The number and organization of the feed bins and feed mixers shown in FIG. 3 is also merely representative of the general concept shown in FIG. 1, and the present invention should be deemed to cover all such embodiments, however configured.

Having described our invention, what we now claim is set forth below.

Claims

1. A method of producing syngas using a gasifier, comprising:

establishing a target production rate and a target energy content for syngas output from the gasifier;

providing at least first and second feed materials as a mixture to the gasifier;

monitoring syngas being produced by the gasifier; and

based on data obtained by the monitoring step, adjusting a feed rate of the mixture to attempt to maintain the target production rate, and adjusting a feed rate of at least one of the first and second feed materials to attempt to maintain the target energy content.

2. The method as described in claim 1 where the first and second materials each have different energy content.

3. The method as described in claim 1 further including:

analyzing data generated by the monitoring step to identify levels of carbon monoxide, hydrogen and total hydrocarbons in the syngas; and

determining BTU content of the syngas.

4. The method as described in claim 1 where the first feed material is construction & demolition (C&D) waste.

5. The method as described in claim 4 wherein the second feed material is one of:

municipal solid waste (MSW), rubber, refuse derived fuels, wastewater sludge, scrap tires, and combinations thereof.

6. The method as described in claim 1 wherein the monitoring is initiated after the gasifier is at a steady state.

7. A method of syngas production using a syngas production chamber, comprising:

establishing a target production rate at a target energy content;

providing a set of one or more first feed materials, where each of the set of one or more first feed materials has a BTU content value above the target energy content;

providing a set of one of more second feed materials, where each of the set of one or more second feed materials has a BTU content value below the target energy content; and

prior to gasification in the syngas production chamber, mixing first feed material and second feed material to create a mixture;

controlling a feed rate of the mixture into the syngas production chamber such that an output mass flow rate of the syngas from the syngas production chamber is maintained at or near the target production rate;

controlling a feed rate of at least one of the first feed or second materials such that the syngas output from the syngas production chamber is maintained at or near the target energy content.

8. The method as described in claim 7 wherein at least two of the first feed materials are mixed prior to mixing the first feed material and second feed material.

9. The method as described in claim 7 wherein at least two of the second feed materials are mixed prior to mixing the first feed material and second feed material.

10. The method as described in claim 7 wherein at least two of the first feed materials are mixed and two of the second feed materials are mixed prior to mixing the first feed material and second feed material.

11. A computer-implemented method of controlling syngas production where first and second feed materials are mixed and supplied to a gasifier, comprising:

establishing a target production rate and a target energy content for syngas output from a gasifier;

controlling a feed rate of a mixture of the first and second feed materials into the gasifier such that an output mass flow rate of the syngas from the gasifier is maintained at or near the target production rate; and

controlling a feed rate of at least one of the first feed or second materials such that the syngas output from the gasifier is maintained at or near the target energy content.