Patent Application
20130087085
Recently, there has been an increasing effort to burn fossil fuels such as coal and biomass more effectively to reduce emissions and costs. While the global combined heat and power industry utilizes more coal and biomass than any other fuel, the need for clean coal has been mandated and incorporated in numerous National and International pollution standards in order to prevent health hazards, acid rain, and climatic impact. The present invention describes methods for integration of several solid fuel cleaning technologies with standard processes for preparation of solid fuel and its delivery to the boiler. The present invention provides for better efficiency and reduction of heat loss by ordering the system in steps that sequentially utilize higher temperatures for preparation of the fuel prior to combustion, in order to help presently existing solid fuel power stations operate efficiently while lowering emissions from these plants.
The art of coal preparation has been part of the coal industry and coal fired boilers since the rise of coal as an inexpensive fuel during the Industrial Revolution. Initially based on crushing to desired size, preparation technology became more automated with the use of specific gravity technology first suggested by Archimedes in 300 BC. The 19th century breaker washed coal in a water solution maintaining a specific gravity of between 1.16 and 1.22 to cause the fuel to float while the rock and other non-fuel ash would separate by sinking. More modern techniques include dry separation and the use of fluidized beds to produce similar segregation of fuel and non-fuel based on the specific gravity of each. These separation techniques have been deployed to reduce sulfur in some coals. The prior art of coal preparation has thus been limited to sizing and cleaning through separation or washing.
With the increasingly restrictive pollution control regulation in the past half-century, research has moved to pyrolysis and other thermal or chemical technologies to remove hazardous substances before or during combustion. In many cases these methods have been energy intensive, inefficient, and costly.
The history and detailed time-line of coal pyrolysis are well documented and found in a variety of sources. Details of a pyrolysis process can be found, for example, in “Kinetic Studies of Gas Evolution During Pyrolysis of Subbituminous Coal,” by J. H. Campbell et al., a paper published May 11, 1976 at the Lawrence Livermore Laboratory, Livermore, Calif. Numerous issued U.S. patents describe methods for the reduction of sulfur in coal, for example, U.S. Pat. No. 7,056,359 by Somerville et al. Their process involves grinding coal to a small particle size, then blending the ground coal with hydrated lime and water, followed by drying the blend at 300-400 degrees F. U.S. Pat. No. 5,037,450 by Keener et al. utilizes a unique pyrolysis process for denitrifying and desulfurizing coal. Here the sulfur and nitrogen content of coal is again driven off in gaseous form and sequestered for possible further use.
Furthermore, the recent drive to reduce net greenhouse gas emissions has generated a desire to burn biomass in certain solid fuel plants. The failure to properly prepare biomass for combustion in a boiler designed for coal has led to power output reductions, boiler slagging, additional corrosion, and increased emissions of many hazards.
A system and method is proposed for coal and biomass preparation, cleaning and heating together with and integrated fuel delivery to the boiler that is more flexible and more efficient than prior art. All thermal processes for the cleaning of coal and biomass fuels that are conducted near the mine or source of fuel are prone to higher costs due to energy and heat loss in transportation. Furthermore, the vicinity of combined heat and power boilers is an area that is rich in waste heat. Therefore, by integrating the fuel cleaning processes with the processes necessary to deliver the fuel to the burner tip in the boiler, there are increased efficiencies.
The system begins with raw coal or biomass delivered to the coal yard of a boiler or power plant, where it is at ambient temperature. If the particle size of raw fuel is too big to be handled properly in the equipment, part of the fuel preparation requires coal crushing or biomass shredding to manageable size. Many power plants have their fuel delivered in the desired size, but having size management equipment in the system increases the flexibility to reduce costs through the utilization of lower cost off-size raw fuel. In the process of being sized, fuels will have been warmed slightly and the coal fuels are presented to a fluidized bed gravity separator. This begins a process of combining coal and biomass into a homogeneous fuel as in the prior art detailed in U.S. patent application Ser. No. 12/483,620, System and Method for Obtaining Combinations of Coal and Biomass Solid Fuel Pellets of High Calorific Value. This unit should use flue gas with waste heat as an air source for fluidizing the bed. The use of waste heat in this part of the system reduces moisture while upgrading the heat value of the fuel, providing savings in transportation, milling, fuel consumption, ash management, and emissions of CO2, CO, SOx, and NOx. Within the gravity separator the equipment should be managed for separation of non-fuel ash from the biomass and coal fuels. Through careful balancing of the separator parameters, there will be removal of certain heavier kernels of fuel that contain high levels of non-combustible materials, sulfur and mercury. This part of the system processing will further increase temperature and the calorific value of the fuel and generate further savings in materials handling, fuel consumption, ash management, and emissions of CO2, CO, SOx, NOx, and mercury. The fuel will exit the separator at 10-40 degrees C. above ambient temperature.
After separation, the system calls for pulverization of the fuel. Pulverization is a normal on site process for all pulverized coal boilers, but the present invention produces savings because each unit of calorific value in input fuel consists of a reduced quantity of drier, warmer, lower ash fuel that costs less to pulverize. The pulverizer reduces particle size, and increases the temperature through friction. The pulverizer temperature increase of the fuel may be 10-20 degrees C.
The next step in the system that is the present invention is to deliver the hot pulverized fuel to a rotary kiln. Such kiln is not a normal part of prior art power plant boiler operations. In the kiln the fuel is in a rotating chamber between two airlocks that permit the elevation of the temperature of the hot, cleaner, pulverized fuel above the temperature at which the fuel would otherwise combust if there were sufficient oxygen present. At these temperatures, residual moisture is vaporized as well as chlorine, mercury, arsenic, selenium and other heavy metals traces. These vapors of potential hazards are removed and sequestered with small amounts of reagents such as activated carbon or lime, so that there can be no danger of their emission from the power plant combustion cycle. On exiting the rotary kiln, the fuel in the range 200-300 C. just below the ignition temperature of most coals. This fuel product is free of chlorine, importantly eliminating alkalis from the coal and any biomass that has been added. Absence of alkalis prevents reduction of the melting point of ash and helps eliminate slagging of the boiler walls and tubes through ash fusion. In addition, dioxins and furans, which are complex chlorine based hazardous emissions that are formed during combustion in the presence of chlorine, cannot be formed when the chlorine has been removed before combustion. The system supports the addition of a higher percentage of a wide range of biomass fuels as they are combined in a homogenous mixture with coal that presents to the burner tip a single higher-grade fuel.
When the fuel thus prepared and cleaned is then delivered to the boiler, it will have better ignition and burn more cleanly with lower ash, and lower emissions of particulate, CO2, CO, SOx, NOx, mercury, hydrochloric acid, dioxins, furans and heavy metals. This system is efficient in the utilization of heat, and delivers to existing boilers an engineered coal fuel that reduces the costs of operations and maintenance in both combustion and post combustion treatment, for the same level of energy output.
a shows the detail of a biomass shredder in which biomass fuel is chopped into regular sized pieces to aid in materials handling and mixing with coal.
b shows the detail of a coal crusher in which coal fuel is crushed to regular sized pieces to aid in materials handling and mixing with biomass.
c shows the fluidized bed gravity separator that separates ash and other minerals from fuel in order to increase calorific value and reduce ash and emissions.
d shows a pulverizer in which the fuel is reduced in size to a powder that can be blown with air through a nozzle for ignition in the boiler.
e shows the rotary kiln in which the fuel is treated to vaporize moisture and other hazards including chlorine, mercury, arsenic and selenium in order to complete the fuel cleaning prior to combustion.
The present invention describes a system and method for cleaning coal and biomass fuel in a process that is integrated with pulverization fuel prior to being blown into a pulverized coal (PC) boiler. The system places certain equipment in the fuel delivery path both before and after the pulverizer.
Having described my invention,
| Number | Date | Country | |
|---|---|---|---|
| 61545617 | Oct 2011 | US |
