Systems and Methods for Electric and Heat Generation from Biomass

Abstract

A system for producing energy and heat from biomass is disclosed. The system includes a feed system, a gasifier, a thermal fluid oil heater, and a generator based on the organic Rankine cycle (ORC). The system may also include a controller that takes input from a number of sensors and controls, among other things, the rate at which fuel is fed into the system and the speed of fans and pumps that draw the products from one apparatus into the next. In this system, the biomass is fed into the gasifier, the resulting producer gas is flared and used to heat an oil in the thermal fluid oil heater, and the hot oil is used to provide input heat for the ORC generator. Methods for controlling such a system are also disclosed.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to systems and methods for electric and heat generation from biomass.

2. Description of Related Art

Industrial systems for producing power are well known. In the most general terms, typical processes for generating electrical energy involve burning a fossil fuel that heats a working fluid to drive a turbine. The spinning turbine generates electricity. In order to produce power with high efficiency, the temperatures and pressures used in a typical power production cycle are generally very high.

In recent years, there has been a greater focus on the use of renewable energy sources, rather than fossil fuels, in power generation processes. One particular source of biofuel is biomass, a general term that refers to material from living or recently dead organisms that can be used as fuel. Most commonly, biomass refers to plant-based biomass, like wood, forest byproducts, and other cellulosic materials, although there has also been significant interest in algal biomass as well.

Although the use of biomass in power generation is promising, there are significant difficulties in using biomass over fossil fuels, since biomass generally has less energy content per unit mass than fossil fuels.

SUMMARY OF THE INVENTION

One aspect of the invention relates to a system for producing energy from biomass. The system includes a feed system, a gasifier, a thermal fluid oil heater, and a generator based on the organic Rankine cycle (ORC). The system may also include a controller that takes input from a number of sensors and controls, among other things, the rate at which fuel is fed into the system and the speed of fans and pumps that draw the products from one apparatus into the next. In this system, the biomass is fed into the gasifier, the resulting producer gas is flared and used to heat an oil in the thermal fluid oil heater, and the hot oil is used to provide input heat for the ORC generator.

Another aspect of the invention relates to methods for controlling a system like the one described above to produce energy and heat from biomass.

These and other aspects, features, and advantages of the invention will be set forth in the description that follows.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The invention will be described with respect to the following drawing figures, in which like numerals represent like features throughout the invention, and in which:

FIG. 1 is a schematic illustration of a system according to one embodiment of the invention; and

FIG. 2 is a control algorithm for the system of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of a system for producing power, generally indicated at 10, according to an embodiment of the invention. System 10 is particularly adapted to use biomass as a primary fuel, and combines three technologies to produce power: a gasifier 12, a thermal fluid oil heater 14, and a generator that uses the organic Rankine cycle 16.

The process of producing power begins when a user adds biomass to a feed hopper 18. As used here, the term “biomass” refers to any plant-based material that may be used as a fuel. The biomass fuel drops or is fed into a feed screw 20 which is driven by a variable and controllable speed motor (not shown in FIG. 1). As the feed screw 20 turns, the biomass fuel is fed into the gasifier 12 at a defined rate. As will be described below in more detail, the feed rate may be increased or decreased as necessary.

The gasifier 12 of the illustrated embodiment is a cross-draft gasifier, although essentially any type of gasifier may be used in embodiments of the invention. The gasifier 12 includes a flame safety sensor 22. If the flame of the flame safety sensor 22 goes out, that is an indication that system 10 should be shut down.

When the gasification process is complete, the products of gasification, which may be referred to as syngas or producer gas, are sent directly to the thermal fluid oil heater 14. More specifically, a coupler 24, which in this case is a metal flange, is used to direct the products of the gasification process into the thermal fluid oil heater 14.

Thus, as the reader may note, in the illustrated embodiment of system 10, there is no process for cleaning or purifying the products of the gasification process after they leave the gasifier 12. Although such a process may be used in some embodiments of the invention, in the illustrated embodiment, higher molecular weight hydrocarbons and other products of combustion are simply directed into the thermal fluid oil heater 14.

The thermal fluid oil heater 14 is essentially a type of heat exchanger in which high-temperature gases exchange heat with an oil. More specifically, the products from the gasifier 12 (the syngas or producer gas) are flared at a temperature of at least about 2200° F. For that reason, the thermal fluid oil heater 14 would generally include or be immediately associated with a combustion chamber. Additionally, a blower (not shown in FIG. 1) may be included to add additional oxygen for combustion. It should be understood that while FIG. 1 shows a directly coupled gasifier 12, if the gasifier 12 is indirectly coupled to the thermal fluid oil heater 14, the producer gas would flow through insulated pipes from the gasifier 12 to the thermal fluid oil heater 14.

Notably, the higher molecular weight hydrocarbons, which would form viscous tars at lower temperatures, are, in many cases, combusted before they can condense. The hot products of that combustion are routed into heat exchange coils, where they heat the oil of the thermal fluid oil heater 14. The thermal fluid oil heater 14 is vented to the atmosphere, and a variable speed fan 26 draws the gases through the heat exchange coil and allows them to vent to atmosphere, for which an exhaust pipe or conduit 28 is provided.

Although an exhaust pipe 28 is provided and the products of combusting the syngas may be exhausted to atmosphere, those products are still hot, although at a lower temperature than prior to the thermal fluid oil heater 14. Therefore, in some cases, the gases may be drawn off and sent through a second thermal fluid oil heater 14, or another form of heat exchanger, so that the additional heat can be used for another purpose. Additionally or alternatively, the products may be sent to pollution control equipment, such as a baghouse or an electrostatic filtering arrangement.

An oil pump 30 in communication with a cool oil return pipe 32 returns cooler oil from the organic Rankine cycle (ORC) generator 16 to the thermal fluid oil heater 14 for heating in the thermal fluid oil heater 14. A corresponding hot oil supply pipe 34 supplies hot oil from the thermal fluid oil heater 14 to the ORC generator 16. A additional heat valve 36 is provided, allowing excess heat to be drawn off and used for another purpose.

The ORC generator 16 accepts the hot oil from the thermal fluid oil heater 14 and uses it to heat a working fluid for power generation. The organic Rankine cycle, the power generation cycle used by the ORC generator 16, is a variation on the traditional steam-driven Rankine cycle that uses an organic, higher molecular weight working fluid, such as R134a, instead of water. As such, it operates at lower temperatures and pressures than other cycles, making it particularly suitable both for biomass-driven processes, and for power production on smaller scales closer to population centers.

As those of skill in the art will note, there are multiple places in system 10 where heat may be drawn off and put to other uses. Higher-temperature heat from the thermal fluid oil heater 14 at an additional thermal load valve 29. Relatively lower temperature heat may be drawn off from the ORC generator 16 via the additional heat valve 36 coupled to it. Additionally, the heat in the gaseous exhaust may be recovered by diverting the gas from the exhaust pipe 28.

System 10 is controlled by a controller 38. The controller 38 is in electrical communication with the feed screw 20 and fan 26 to control their speeds. The controller 38 is also in communication with the flame sensor 22 in the gasifier 12, two temperature sensors 40 in the oil circulating pipes 32, 34, and a load sensor 42 in the ORC generator 16. If an additional fan is provided in or in association with the thermal fluid oil heater 14 to provide additional oxygen for combusting the producer gas, that fan would also be capable of variable speed, and the controller 38 would also control it.

FIG. 2 is a schematic illustration of a method, generally indicated at 100, of controlling a system like system 10. Method 100 begins at task 102 and continues with task 104. Task 104 is a decision task based on readings from the load sensor 42. If there is a change in the readings of the load sensor 42 (task 104:YES), method 100 continues with task 106. If there is no change in the readings of the load sensor (task 104:NO), method 100 continues with task 110.

In task 106, the speed of the variable speed fan 26 is increased or decreased as necessary. More specifically, an increase in the speed of the variable speed fan 26 increases the draft through the gasifier 12, which increases the volume of producer gas that is produced. A decrease in the speed of the variable speed fan decreases the production of producer gas. The speed of the variable speed fan 26 may be increased or decreased in proportion to the increase or decrease in heat or electric load, or according to a particular calibration curve.

In some embodiments, a threshold may be used in the decision of task 104. More specifically, instead of determining the whether there has been a change in the heat or electric load on the ORC generator 16, the controller 38 may determine whether or not there has been a change in the heat or electric load on the ORC generator 16 beyond a particular threshold. In that case, method 100 would continue with task 106 only if the load changes more than the threshold. If thresholds are used, the threshold for changing the speed of the fan 26 in response to a drop in load may be different from the threshold for changing the speed of the fan 26 in response to an increase in load.

Once the speed of the fan is changed in task 106, method 100 continues with task 108, in which the controller 38 increases the speed of the feed screw 20. This feeds more fuel into the gasifier 12, so that more producer gas can be produced and used by the thermal fluid oil heater 14. Once task 108 is complete, or after it is determined in task 104 that there has been no change in heat or electric load, method 100 continues with task 110.

Task 110 is another decision task, in which the two temperature sensors 40 are read to determine whether the hot and cold oils flowing to and from the ORC generator 16 are at the proper temperatures. If the temperatures are too high or too low (task 110:YES), method 100 continues with task 112, and the speed of the oil pump 30 is changed appropriately. As was explained above with respect to the speed of the fan 26, the speed of the oil pump 30 may be increased or decreased in proportion to the increase or decrease in temperature that is desired. As was also explained above, thresholds may be used so that the speed of the oil pump 30 is only increased or decreased if the oil temperatures have increased or decreased beyond particular thresholds. In other words, some minor variation in oil temperatures may be tolerated without changing the speed of the oil pump 30.

After task 112, or if the controller 38 determines that no change to the oil pump speed is necessary (task 110:NO), method 100 continues with task 114. Task 114 is another decision task in which the controller 38 determines whether or not the flame sensor 22 is still operating. If the flame sensor 22 has gone out, indicating a problem (task 114:YES), method 110 continues with task 116 and the system is shut down. If there is no issue with the flame sensor 22 (task 114:NO), method 100 completes and returns at task 118.

Method 100 may be performed essentially continuously while a system like system 10 operates, or at intervals. The basic tasks of method 100 may, in some cases, be performed in different orders. In addition to the tasks shown in FIG. 2 and described here, method 100 may include other monitoring and control tasks particular to the specific gasifier 12, thermal fluid oil heater 14, and ORC generator 16 that are used in system 10. Methods for controlling system 10 and other such systems may be implemented in the controller in hardware or software. In this context, the term “software” refers to sets of machine-readable instructions on a non-transitory machine-readable medium that are interoperable with a machine, such as the controller 38, to perform the functions that are described.

It should also be understood that while the controller 38 may poll the various sensors in some embodiments, in other embodiments, the sensors may have their own controllers, which automatically signal the main controller 38 if temperatures are too high or low, there is a change in load, or some other condition exists.

The controller 38 itself may be a microprocessor, an application-specific circuit or circuits, or a full, general-purpose computer system. While method 100 ascribes certain automatic functions to the controller 38, the controller 38 may be equipped with a display and input devices, allowing the controller 38 to take input from a user and, either entirely or within defined limits, allow a user to control system 10 or parts of it. If additional components are installed in system 10 to make use of additional heat, they may also be controlled by the controller.

While the invention has been described with respect to certain embodiments, the embodiments are intended to be exemplary, rather than limiting. Modifications and changes made be made within the scope of the invention, which is defined by the appended claims.

Claims

1. A system for producing energy from biomass, comprising: a feed system;a gasifier in communication with the feed system to receive feed, the gasifier converting the feed into producer gas without combusting the feed;a thermal fluid oil heater coupled to the gasifier and receiving the producer gas, the thermal fluid oil heater combusting the producer gas and routing hot products of combustion into a heat exchanger with an oil working fluid to heat the oil working fluid; andan organic Rankine cycle (ORC) generator receiving the hot oil working fluid and using the heat of the oil working fluid to generate electricity.

2. The system of claim 1, wherein the feed system further comprises: a feed hopper; anda variable-speed feed screw.

3. The system of claim 2, further comprising a variable-speed fan in fluid communication with an exhaust of the gasifier and the thermal fluid oil heater to draw producer gas out of the thermal fluid oil heater.

4. The system of claim 3, further comprising: a hot oil working fluid supply pipe or pipes supplying the oil working fluid from the thermal fluid oil heater to the ORC generator;a cool oil working fluid return pipe or pipes returning the oil working fluid from the ORC generator to the thermal fluid oil heater; anda variable-speed oil pump coupled to the hot oil working fluid supply pipe or the cool oil working fluid return pipe.

5. The system of claim 4, further comprising: one or more temperature sensors in one or both of the cool oil working fluid return pipe or pipes or the hot oil working fluid supply pipe or pipes.

6. The system of claim 5, further comprising a flame sensor within or coupled to the gasifier.

7. The system of claim 6, further comprising a load sensor coupled to the ORC generator.

8. The system of claim 7, further comprising a controller in electronic communication with the variable-speed feed screw, the flame sensor, the variable-speed fan, the variable-speed oil pump, and the load sensor, the controller being operable to effect changes in the speed of the feed screw and the fan in order to increase or decrease power production.

9. The system of claim 1, further comprising one or more waste heat ports in communication with the thermal fluid oil heater or the ORC generator, the waste heat ports allowing waste heat to be withdrawn.

10. The system of claim 1, wherein the feed system and the gasifier are adapted to accept biomass as fuel.