Patent Application
20130291771
The present invention relates to biomass use in thermal applications. More specifically, the present invention is concerned with a method and a system for delivering heat through gasification of biomass.
Biomass is organic matter that is generally sourced from waste streams in the forestry and agricultural industries. As such, biomass is considerably less expensive, in energy terms, than fossil fuels. Furthermore, its use in a combustion process is considered carbon-neutral. At least for these reasons, biomass is increasingly used in thermal applications in the residential, institutional and industrial sectors, delivering a wide array of heating capacities.
Because of the multiplicity of biomass sources, the characteristics of this bio-fuel vary in chemical composition, moisture, ash content, particle size etc. For instance, biomass sourced from residual wood shavings in a saw mill has very low moisture and ash content, as well as a small particle size. In contrast, biomass sourced from raw animal manure has very high moisture and ash content, as well as a larger particle size. Biomass may also be sourced from corn stalk, rice husk, peanut shell, conditioned corn cob, saw dust, wood shavings, paper mill residues etc.
In its raw form, biomass generally has poor combustion characteristics, due to high moisture, ash content and particle size. For industrial applications, the biomass is usually grinded and dried, and as a result has improved combustion characteristics. However, such “high-quality” biomass is more expensive and is in much shorter supply than “low-quality” biomass.
A number of combustion methods currently exist to burn various forms of biomass, i.e. having various combustion characteristics. Methods using raw biomass have poor results in terms of thermal efficiency and ash release in the combustion chamber.
A dust burning method uses “high-quality” biomass, i.e. typically with moisture content less than 10% and particle size of less than 1 mm. Such biomass has undergone an elaborate and expensive conditioning process, including for example grinding and drying, to drastically reduce its moisture content and size.
Given the very low volumetric density of high-quality biomass, its use also implies high transportation and storage costs. Consequently, the dust burning technology often suffers from uneconomical or insufficient sources of high-quality biomass.
In addition, the combustion of biomass in conventional biomass burners produces residual ashes that are directly admitted to the inside of the boiler, furnace, or other type of heat-demanding equipment installed downstream of the burner. The addition of an emission control and ash removal system is then needed to eliminate or reduce the presence of ashes in the heat-demanding equipment.
There is still a need in the art for a method and a system for delivering heat through gasification of medium-quality biomass.
More specifically, in accordance with the present invention, there is provided a method for providing thermal energy to a heat demanding equipment, comprising partial combustion of biomass in a gasification chamber; and combustion of syngas generated by the partial combustion of the biomass.
There is further provided a system for providing thermal energy to a heat demanding equipment, comprising a gasification chamber provided with a fire-tube; a temperature sensor monitoring the temperature within the gasification chamber; a controlled-speed dosing unit conveying biomass powder or pellets to the gasification chamber; an air blower injecting a sub-stoichiometric quantity of air within the gasification chamber with the biomass powder or pellets; a syngas burner receiving hot syngas generated by gasification of the biomass powder or pellets within the gasification chamber, from the fire-tube of the gasification chamber, for combustion; and a control unit monitoring the temperature and oxygen conditions in the gasification chamber, and adjusting the dosing unit according to at least one of: i) the temperature within the gasification chamber and ii) thermal heat demand of the heat demanding equipment.
Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.
In the appended drawings:
In a nutshell, there is generally provided a system and a method for burning medium-quality biomass, including for example biomass powder, i.e. biomass particles having a particle size of at most 3 mm and a moisture content between about 15% and about 20%, and biomass pellets, i.e. biomass particles having a particle size of about 8 mm with a moisture content of at most about 15%.
The biomass may originate from wood, such as waste or by-product of the forestry industry, or from other sources, such as agricultural and animal wastes, or pulp and paper industries and wastewater sludge.
Biomass pellets may be crushed upstream of the system to obtain a desired particle size.
The present method and system will be described in relation to
The biomass is first injected in the gasification chamber 6 placed upstream of the syngas burner 12, which can be installed onto a heat-demanding equipment (not shown), such as a boiler, a dryer or a furnace for example. The process of gasification in the gasification chamber 6 is a partial combustion, by pyrolysis, which releases an important quantity of synthesis gases, referred to as “syngas”. In the present system and method, gasification is optimized and produces small quantities of residual matter. Ashes are evacuated from the bottom of the gasification chamber, through a screen and ash removal system. Thermal energy is produced by combustion of the syngas.
The method comprises feeding biomass into the gasification chamber 6, in which a feeding air provides the source of oxygen for gasification. Sensors connected to a control unit (not shown) are used to regulate the temperature and oxygen conditions in the gasification chamber 6, allowing gasification with an efficiency rate of up to 96%, producing hot syngas, which is injected, together with cooling/combustion air using an air blower 7, to the syngas burner 12, for a clean and efficient combustion of the syngas. Ash matter is evacuated from the gasification chamber 6 through a screen 16 and ash removal system 9.
To start-up the system, the temperature within the gasification chamber 6 is first raised to about 600° C., for example between about 580° C. and about 620° C. This may be achieved by activating an auxiliary fuel burner 4, which operates with a fossil fuel that is fed from a dedicated fuel tank 5.
The gasification chamber 6 has a generally circular or cylindrical shape so as to create a swirl effect for the particles suspension entering it as described hereinbelow.
The walls of the gasification chamber 6 may be insulated using refractory material.
The walls of the gasification chamber 6 may also be cooled as part of a heat recovery process, using a cooling system using air, water or a thermal fluid for heat transfer and recovery. In the case of a cooling system using water or a thermal fluid, the gasification chamber 6 may be constructed using a double walled container, allowing circulation of a cooling fluid inside the double wall. The cooling fluid recovering heat from the walls of the gasification chamber 6 may then be recycled.
A temperature sensor 10 connected to the gasification chamber 6 is used to monitor the temperature changes inside the gasification chamber 6. When the temperature in the gasification chamber 6 falls within a target range between about 600° C. and about 800° C., the temperature sensor 10 causes the control system to activate the feed of biomass to the gasification chamber 6 and to lower the heat load of the auxiliary burner 4. If the temperature within the gasification chamber 6 falls outside of this target range, for example by about 5%, the temperature sensor 10 triggers the control system to effect adjustments in the biomass feed rate and the feeding airflow, as well as the heat load of the auxiliary burner 4, so as to return the temperature within the gasification chamber 6 back to the target temperature range.
The biomass powder is first accumulated in a storage hopper 1, equipped with an airlock 13 at the bottom thereof. When the feed of biomass powder is activated, the airlock 13 is opened, causing the biomass powder to fall onto a dosing conveyor 11 driven with a variable speed motor M, which carries the biomass powder to a pneumatic conveyor 3. Under action of a pneumatic conveyor blower 2, a mixture of biomass powder and feeding air is fed through the pneumatic conveyor 3 to the gasification chamber 6, tangentially to the inner surface of the gasification chamber 6. Thus, following this inner surface, the mixture of biomass powder and feeding air creates a vortex, which insures an homogeneous mixture of air and biomass within the gasification chamber 6, which is found to accelerate the gasification process and facilitates the separation of ash particles from combustible matter within the gasification chamber 6.
The speed of the dosing conveyor 11 is controlled by the temperature sensor 10. For instance, if the temperature inside the gasification chamber 6 rises by 10%, the speed of the dosing conveyor 11 is reduced, causing a reduction of the biomass flow rate entering the pneumatic conveyor 3 and, therefore, of the biomass flow rate entering the gasification chamber 6. Higher temperature, for example 800°, inside the gasification chamber 6 causes the dosing conveyor 11 to stop completely.
The speed of the conveyor 11 may also be controlled by the demand in thermal energy from the heat demanding equipment, since if there is a smaller demand, less biomass has to be fed to the gasification chamber 6. For example, when the demand of steam/hot water in case of a boiler, or of hot air in case of for a dryer, increases, the consumption of the burner 12 increases, so that the dosing conveyor 11 needs to run faster to provide the necessary biomass within the gasification chamber 6.
By using a pneumatic conveyer 3 for feeding of the biomass powder into the gasification chamber 6, some of the air necessary for the gasification process and for spreading the biomass powder inside the gasification chamber, in an homogeneous mixture of air and powder within the gasification chamber 6, is provided.
The dosing conveyer 11 may be a screw conveyor, a single screw model or a multiple screw model depending on the application, with a variable speed motor (M) for modulating the rate of the feed by modulating the speed of rotation. Alternatively, a chain conveyor could also be used.
To ensure that the biomass powder entering the gasification chamber 6 has a particle size of less than 3 mm, a 7 MESH screen (not shown) may be positioned at the entrance of the pneumatic conveyor 3. The pneumatic conveyor 3 provides simultaneously the biomass particles and air needed for the gasification process. The feeding airflow is controlled to be sub-stoichiometric and to keep the biomass particles in suspension in the gasification chamber 6.
The high temperature, the sub-stoichiometric quantity of air and the suspension of biomass particles inside the gasification chamber 6 allow the gasification, i.e. the partial combustion of the biomass and, therefore, the generation of syngas. This gasification also generates heat in the gasification chamber 6.
In this case, the pellets are directly handled from the hopper 1 to the gasification chamber 6 by the dozing conveyor 11. As people in the art will appreciate, a pneumatic conveyer for pellets would require pipes of a large dimension and the pellets would break inside the chamber of the pneumatic conveyer upon impact.
A thermocouple or a limit switch may be used to control feeding of the screw conveyor 11 in case the temperature inside the screw conveyor 11 increases too much, for example in case of backfiring, i.e. presence of a flame that would circulate from the gasification chamber 6 to the hopper 1 by propagating through the dosing conveyor 11, Another way to prevent backfiring is to install the dosing conveyor 11 with an upward inclination, so that the discharge of the dosing conveyor 11, at the gasification chamber 6, is higher than its inlet, at the hopper 1. Since a flame has always the tendency to go upwards, in such an arrangement propagation of a flame through the dosing conveyor 11 back to the hopper 1 is hindered.
The sub-stoichiometric feeding air provided by the blower 2′ is directly injected from below the screen 16 into the gasification chamber 6, in an upward and swirling trajectory, and keeps the pellets in suspension during the gasification phase. Moreover, the control unit takes into account the higher residence time of pellets, by lowering their feed rate into the gasification chamber 6 correspondingly.
During the gasification process, ashes fall to the bottom of the gasification chamber 6. The bottom of the gasification chamber 6 is equipped with a screen 16, under which an ash removal system 9 is installed to collect and evacuate the ashes away. The ashes may be periodically or continuously removed, depending on the heating capacity of the system, using a mechanical automated system, such as, for example, a simple screw conveyor, a chain conveyor or a reciprocating container, i.e. an automatic drawer that opens upon command. The ashes may be also removed manually, provided a temporary stoppage of the blowers when the de-ashing doors are open. The removal of ashes from the gasification chamber 6 allows the system to maintain high combustion efficiency.
The syngas generated by combustion of the biomass rises to the top of the gasification chamber 6 and exits through a syngas duct 14, towards a syngas burner 12. A large portion of the feeding air inside the gasification chamber 6 is consumed by the gasification process. A blower 7 connected to the syngas duct 14 supplies additional air to the syngas burner 12. The air flow provided by the air blower 7 allows controlling the temperature in the syngas burner 12. Being equal to or above the stoichiometric level, this air flow also ensures a complete combustion of the syngas.
At the end of the syngas duct 14, the mixture of syngas and cooling/combustion air crosses a flame starter 8, which creates a spark to ignite the combustion process. Once the mixture is ignited, a flame is created inside the syngas burner 12, in which the syngas is completely consumed.
Thus, according to an embodiment of the present method, a supply of biomass powder or biomass pellets is stored inside a storage hopper, with an airlock valve or trap, which, when activated, allows the biomass to fall on a dosing unit. With a speed controlled manually or automatically, the dosing unit transports the biomass to the entrance of a pneumatic conveyor in case of powder, or to the gasification chamber in case of pellets. The biomass powder is pushed, through the circuit of the pneumatic conveyor, to the inside of the gasification chamber with the help of an air blower (
The present method and system allow achieving good combustion performances, in terms of maximizing the combustion's thermal efficiency and minimizing ashes release in the combustion chamber, while using biomass that does not require an elaborate and expensive conditioning process.
The present method and system allow and optimized combustion of medium-quality biomass in a gasification-combustion burner in order to produce heat in a manner that minimizes the release of ash formed in the combustion chamber
The scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.
This application claims benefit of U.S. provisional application Ser. No. 61/640,849, filed on May 1, 2012. All documents above are incorporated herein in their entirety by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61640849 | May 2012 | US |
