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:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 12/927,406, filed Nov. 15, 2010, the contents of which are incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to systems and methods for electric and heat generation from biomass. 
         [0004]    2. Description of Related Art 
         [0005]    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. 
         [0006]    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. 
         [0007]    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 
       [0008]    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. 
         [0009]    Another aspect of the invention relates to methods for controlling a system like the one described above to produce energy and heat from biomass. 
         [0010]    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 
         [0011]    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: 
           [0012]      FIG. 1  is a schematic illustration of a system according to one embodiment of the invention; and 
           [0013]      FIG. 2  is a control algorithm for the system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      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 . 
         [0015]    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. 
         [0016]    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. 
         [0017]    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 . 
         [0018]    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 . 
         [0019]    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 . 
         [0020]    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. 
         [0021]    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. 
         [0022]    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. 
         [0023]    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. 
         [0024]    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 . 
         [0025]    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. 
         [0026]      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 . 
         [0027]    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. 
         [0028]    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. 
         [0029]    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 . 
         [0030]    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 . 
         [0031]    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 . 
         [0032]    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. 
         [0033]    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. 
         [0034]    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. 
         [0035]    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.