Fuel and related compositions – Particulate – Mixed with particles of different size
Reexamination Certificate
1994-09-27
2001-02-27
Medley, Margaret (Department: 1714)
Fuel and related compositions
Particulate
Mixed with particles of different size
C044S505000, C044S590000, C044S595000, C044S606000
Reexamination Certificate
active
06193768
ABSTRACT:
TECHNICAL FIELD
This invention relates to the preparation and burning of particulate waste wood fuel to produce energy. More particularly, the present invention relates to a method for producing energy by combusting particulate wood fuel in the combustion chamber of a particulate fossil fuel suspension furnace.
BACKGROUND OF THE INVENTION
With increasing landfill costs and decreasing landfill availability, the cost of waste disposal has increased dramatically during recent years and is expected to increase further. This is a particularly serious problem for those, such as electric utility companies, who have large quantities of waste wood for disposal. Much of waste wood, however, has potential fuel value. If efficiently combusted, waste wood can provide dual benefits of alleviating landfill problems and producing needed energy. Waste wood is a relatively low cost fuel and, when efficiently combusted, emits relatively low levels of sulfur and ash.
Waste wood has long been burned in furnaces to produce energy. More efficient methods of converting waste wood to energy including comminuting waste wood to a reduced particle size having a maximum dimension from about one to four inches (2.5 to 10 cm) and burning the particulate wood fuel on grates in a boiler, in stoker furnaces, and in fluidized bed reactors. Such methods are effective to produce energy from waste wood but tend to be difficult to control because of variations in particle size, type, and quality of the wood being burned.
There exist a number of older, particulate fossil fuel suspension furnaces for boilers that produce electrical energy via the combustion of particulate fossil fuels such as pulverized coal and atomized oil. Natural gas may also be used in such furnaces in conjunction with the particulate fossil fuel. These suspension furnaces, which include tangentially-fired suspension furnaces and wall-fired suspension furnaces, are still used to some extent by utilities and manufacturing companies such as paper mills, but are relatively inefficient when compared to modern reactors that burn fossil fuels and other types of reactors such as nuclear reactors. Today, particulate fossil fuel suspension furnaces are typically used only in times of peak energy demand to supplement the primary electrical power system. Attempts have been made to more economically produce energy with suspension furnaces by burning wood in such furnaces along with fossil fuels, but those attempts have not been successful in burning wood at a high enough rate to economically produce energy. Because waste wood is inexpensive and available in large quantities, particulate fossil fuel furnaces could produce energy more economically if waste wood could be burned therein at a high enough rate. Thus, burning wood in particulate fossil fuel suspension boilers could increase the usefulness of such boilers and add to their value.
Prior attempts to burn wood in particulate fossil fuel suspension furnaces include co-firing waste wood particles with pulverized coal by mixing the waste wood particles with the coal at the coal pile and introducing the combined material into the furnace through the coal pulverizers which normally pulverize and then deliver pulverized coal to the furnace. This method can be used with combined fuels that contain less than about 10% by weight of wood particles, but such an amount of wood particles is ineffective to more economically produce energy with the furnace than by burning solely fossil fuels. When greater than about 10% by weight of particulate wood was mixed with the coal and fed to the pulverizer, the wood particles inhibited the pulverization of the coal such that the mixed fuel could not be efficiently combusted in the furnace.
Therefore, there is a need for a particulate waste wood fuel and a method for burning such fuel in a particulate fossil fuel suspension furnace to economically produce energy and increase the usefulness of such suspension furnaces and associated boilers.
SUMMARY OF THE INVENTION
The present invention fulfills the above-described need and encompasses a particulate wood fuel, a method for making that fuel, and a method for producing energy by combusting the particulate wood fuel. The particulate wood fuel of the present invention can be co-fired with particulate fossil fuel in a particulate fossil fuel suspension furnace to consistently produce energy in a relatively economical manner and with relatively low levels of sulfur and NO
X
emissions and low levels of ash. With the present invention, particulate wood can be burned along with particulate fossil fuel in a particulate fossil fuel suspension furnace in quantities up to an amount at which the particulate wood contributes about 50% of the total BTUs produced by the furnace. In addition, because the particulate wood fuel of the present invention has desirable flow characteristics and is introduced into the furnace separately from the particulate fossil fuel, such as pulverized coal, the relatively amount of particulate wood fuel combusted in the furnace can be tightly controlled. As will be explained further below, due to the nature of the particulate wood fuel of the present invention, particularly the particle size distribution of the particulate wood fuel, energy can be reliably and relatively economically produced by co-firing the particulate wood fuel and particulate fossil fuel in a particulate fossil fuel suspension furnace.
The particulate wood fuel of the present invention, comprises particles of wood and less than 20% water by weight. The particles of wood have a particle size distribution such that substantially 100% by weight of the wood particles will pass through a sieve having 9.5 mm diameter holes, up to 90% of the wood particles will pass through an eight mesh sieve, up to 55% of the wood particles will pass through a 16 mesh sieve, up to 30% of the wood particles will pass through a 30 mesh sieve, up to 15% of the wood particles will pass through a 50 mesh sieve, and up to 5% of the wood particles will pass through a 100 mesh sieve. Desirably, the particles size distribution of the wood particles is further characterized by having at least 45% by weight of the wood particles pass through a 16 mesh sieve.
The method of making the particulate wood fuel, according to the present invention, comprises shredding waste wood to form wood particles having the above-described particle size distribution. More particularly, the method of making the particulate wood fuel in accordance with the present invention comprises three shredding steps and two screening steps. In a first shredding step, waste wood is shredded in a horizontally forced fed, high speed rotating hammermill shredder to produce wood particles having a maximum dimension of about 10 cm. In a first screening step, the wood particles are screened to separate wood particles having a maximum dimension of less than about 2.5 cm from a first remaining portion of wood particles having a maximum dimension of at least about 2.5 cm. In the second shredding step, the first remaining portion of the wood particles is shredded in a second mill to reduce the first remaining portion of wood particles to wood particles having a maximum dimension of less than about 2.5 cm. In the second screening step, the wood particles having a maximum dimension of less than about 2.5 cm are screened to separate wood particles having a maximum dimension of less than about 6.4 mm from a second remaining portion having a maximum dimension of at least about 6.4 mm. In the third shredding step, the second remaining portion of wood particles is shredded in a third mill to reduce the second remaining portion of wood particles to wood particles having a maximum dimension less than about 6.4 mm. The third mill is desirably a grinder.
Desirably, the method of making the particulate wood fuel in accordance to the present invention includes separating non-wood products from the wood particles with an air separator and magnetically separating other ferrous metal particles from the shredded wood between the fir
Galipeault Claude Joseph
Staab Lawrence Edward
MCX Environmental Energy Corp.
Medley Margaret
Toomer Cephia D.
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