Patent Application
20050274307
This invention relates to the combustion of coal and in particular to the generation of sorbents to capture mercury (Hg) in flue gas generated during coal combustion.
Emissions from coal combustion may contain volatile metals such as mercury (Hg). There is a long felt need to reduce Hg in gaseous emissions from coal-fired boilers and other industrial coal combustion systems. As mercury volatizes during coal combustion, it enters the flue gas generated by combustion. Some of the volatized mercury can be captured by injected sorbents and removed via a particulate collection system. If not captured, the mercury may pass into the atmosphere with the stack gases from the coil boiler. Mercury is a pollutant. Accordingly, it is desirable to capture a much mercury in flue gas before the stack discharge.
Injection of activated carbon as a sorbent that captures mercury in the flue gas is a known technology for Hg control. See e.g., Pavish et al., “Status review of mercury control options for coal-fired power plants” Fuel Processing Technology 82, pp. 89-165 (2003). Depending on coal type and the specific configuration of the emission control system, e.g., injection ahead of a particulate collector or a compact baghouse added behind an existing electrostatic particulate control device ESP, and coal type, the efficiency of Hg removal by activated carbon injection ranges from 60% to 90%.
The cost of Hg control in coal-fired power plants using activated carbon tends to be expensive. See e.g., Brown et al., “Control of Mercury Emissions from Coal-Fired Power Plants: A Preliminary Cost Assessment and the Next Steps for Accurately Assessing Control Costs”, Fuel Processing Technology 65-66, pp. 311-341 (2000). The typical cost for mercury removal using activated carbon injection generally ranges $20,000 per pound (lb.) of removed mercury to $70,000/lb of Hg. This cost is dominated by the cost of the sorbent. Accordingly there is a long felt need for an economical way to produce activated carbon sorbents. By reducing the cost of sorbents, the cost of removing mercury from flue gas may be substantially reduced.
The invention may be embodied as a method for capturing mercury in a flue gas formed by solid fuel combustion including: combusting coal, wherein mercury released during combustion is entrained in flue gas generated by the combustion; generating a thermally activated carbon-containing sorbent by partially gasifying a solid fuel in a gasifier local to the combustion of solid fuel; injecting the gasified solid fuel into the combustion of coal; injecting the thermally activated sorbent in the flue gas, and collecting the injected sorbent in a waste treatment system.
In addition, another embodiment of the invention is a method for capturing mercury in a flue gas formed by solid fuel combustion comprising: combusting a solid fuel in a furnace or boiler, wherein mercury released during combustion is entrained in flue gas generated by the combustion and flows to a waste treatment system; generating a thermally activated carbon-containing sorbent by partially gasifying a carbon solid fuel in a gasifier local to the furnace or boiler; injecting gasifier fuel from the gasifier into the furnace or boiler; injecting the thermally activated sorbent in a flue gas duct of the waste treatment system; capturing at least some of the entrained mercury with the injected sorbent; collecting the injected sorbent with the mercury in the waste treatment system.
The invention may also be embodied as a system for capturing mercury from flue gas comprising: a furnace or boiler arranged to receive coal and air and further comprising a coal and air injection system, and a combustion zone for combusting the coal and air; a waste treatment system connected to receive flue gas generated in the combustion of the furnace or boiler, wherein said waste treatment system includes a sorbent injector and a sorbent collection device; a sorbent generator further comprising a gasifier having an inlet for a solid carbon fuel, a gasification chamber within which the solid carbon fuel is at least partially combusted to generate sorbent and gasified fuel; a conduit between the gasifier and sorbent injector to convey the sorbent to the injector, and a conduit between the gasifier and the coal and air injection system to convey the gasified fuel to the injection system.
Carbon-based sorbents are effective in removing mercury from flue gas. A system and method have been developed to produce thermally activated mercury sorbent by partially gasifying coal or other carbon containing fuel in a gasifier. The thermally activated sorbent may be injected into mercury containing flue gas upstream of an existing particulate control device (PCD) or downstream of the PCD if there exists a downstream particulate control system dedicated to the sorbent. Thermally activated sorbent is produced from the same coal as fired at the plant or from other carbon containing solid fuel.
The current system and method decrease mercury emissions from the stack of coal-fired boilers by injecting locally generated thermally activated carbon-based sorbent into flue gas and absorbing mercury from the flue gas on the sorbent. Advantages of this method in comparison to traditional activated carbon injection include (without limitation): low capital cost for equipment required to produce thermally activated sorbent; reduced need for a silo to store activated carbon, and relatively low cost of sorbent production.
The flue gas flows through the boiler and into the ducts 24 of the waste treatment system where the flue gas cools. The waste treatment system 16 includes a sorbent injection system 26, a particulate control device (PCD) 28 with an ash discharged 30, and a stack 32 for flue gas discharge. The sorbent injection system may inject sorbent into the duct 24 upstream of the PCD. In addition (or alternatively) the sorbent may be injected downstream of the PCD if a dedicated sorbent particulate collection device 34 is included in the waste treatment system 16.
The sorbent flows from a sorbent discharge chute 36 from a sorbent generator 38. In the generator, coal or other carbon containing solid fuel 40 is partially gasified in a gasifier 42 that produces thermally activated carbon sorbent. The gasifier may discharge the sorbent along with the gases into the duct 24 through chute 36. Alternatively, the thermally activated solid sorbent generated in the gasifier is separated from the other gasification products in a cyclone separator 44. A mixture of sorbent and gaseous fuel products enter the inlet of the cyclone separator 44. The solid particles of sorbent are discharged from the cyclone into the sorbent chute 36. The gasifier and cyclone may be on site with the waste treatment system 16. The gaseous products from the gasifier flow through a conduit 46 to the coal injectors 18 and flow into combustion zone 22 in the boiler.
In one example, the gasifier 42 may be formed from stainless steel and its inner walls are refractory lined. Heat required for solid fuel gasification is supplied by the combustion of natural gas and air. The horizontally aligned heating chamber 41 may have an internal diameter of 8 inches (in.). Coal 40 is injected into the gasification chamber 50, which may have internal diameter of 12 in. Nitrogen or air may be used as a transport media for the solid fuel.
The solid fuel 40 is injected at an upper end of the gasification chamber 50 through an water jacketed injector 58. A transport gas 51 is injected through the fuel injector 53 to carry the solid fuel particles into the gasification chamber 50. The heat added to the gasification chamber causes the solid fuel particles to partially gasify, e.g., by partial combustion, and to generate reactive sorbent particles. The walls of the gasification chamber 50 and the auxiliary heat chamber 41 are refractory lined 62 to accommodate the heat within the heating chamber.
Heat required for partial gasification of the solid fuel, e.g., coal, is provided by a heat source 60 and/or by partially combusting the solid fuel in the gasifier. For example, natural gas and air 60 are mixed in the heat chamber 41 to generate heat that is provided to the gasification chamber 50. Cooling ports 64 in the heat chamber allow water 66 to cool the walls of the heat chamber and solid fuel injector 58. The cooling of the heating chamber 41 allows the temperature to be controlled and avoid excessive combustion of the solid fuel in the gasification chamber 50. The temperature in the gasification chamber is preferably in a range of 1000 degree to 2000 degrees Fahrenheit.
Conditions in the gasification chamber 50 are optimized to enhance the generation thermally activated sorbent having relatively high reactivity. For example, the sorbent may be produced to have a relatively large surface area and high carbon content. Process parameters in the gasifier include fuel residence time in the gasification chamber 50, the stoichiometric ratio (SR) of carbon containing material to air, and the temperature in the chamber 50. By controlling these process parameters, the generation of reactive sorbent can be enhanced. Optimum process conditions in the gasifier are also affected by the type of carbon containing fuel 40 and its reactivity.
Tests were conducted to determine the effect of gasifier parameters on the reactivity of the thermally activated carbon-containing sorbent. Sorbent reactivity may be viewed as the carbon content in the sorbent.
The temperature profile in the gasification chamber 50 was measured using several thermocouples 56 located along the chamber wall and in the heating chamber 41. Ports 68 located near in the gasification chamber allowed for gas and solid samples to be taken and analyzed. Solid samples were analyzed to determine loss-on-ignition (LOI), which provides a measure of the carbon present.
It is desirable to have thermally activated sorbent with higher carbon content. Thus, short residence times and lower SR favor high carbon content in the sorbent. On the other hand, the extent of coal gasification at very short residence times results in relatively small surface area of the sorbent. Sorbent particles having large surface areas are effective at capturing mercury. Thus, conditions in the gasifier have to be optimized to achieve high reactivity of the sorbent.
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While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
