Sulfur removal after combustion

It is more efficient, and usually essential if emission controls are to be met, to remove the SO2 formed from the gaseous products of combustion. 

The objective of these treatments is to reduce as far as is practicable the release of sulfur (as SO2) into the atmosphere. Considerable progress in this direction has been made through the development of chemical methods of sulfur removal from the gaseous products of coal combustion (see Section 8). 

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The methods for controlling sulfur oxides from coal, which, in turn, impact the combustion system design can be classified into three types (1) sulfur removal after combustion, (2) sulfur removal during combustion, and (3) sulfur removal before combustion. 

The first approach (i.e., sulfur removal after combustion) is the one that is currently receiving the most attention in many industrialized countries including the United States because it does not represent a significant departure from existing coal-fired power plant technology. Ordinarily, a wet scrubber can be used to remove sulfur oxides from the flue gas. However, currently available sulfur oxide control technology has proven to be expensive, is subject to operational difficulties, and produces a liquid or dry waste product that must be disposed. It is, however, the approach that is now generally accepted by the utility industry. 

The CO2 required for the cathode reaction is expected to be supplied by recycling the anode gas exhaust (after combustion of the residual H2) to the cathode. Therefore, any sulfur in the anode effluent will be present at the cathode inlet unless provisions are made for sulfur removal. In the absence of sulfur removal, sulfur enters the cathode inlet as SO2, which reacts quantitatively... 

Much of the sulfur contained in a crude oil remains with the tar, asphalt or coke after refining. As crude prices rise, even this residuum becomes valuable as a feedstock for cracking to make lighter products. This involves deep desulfurization. Whether the coke is burned as a fuel or used for anodes, desulfurization will take place before or after combustion or processing. Sulfur removal from coke plants is a currently feasible process. 

There are strong incentives to develop processes for removing sulfur from coal before combustion (precombustion cleaning), during combustion, or after combustion (postcombustion cleaning)... 

The mineral matter content of coal varies considerably and may even be as high as 35% w/w of the coal. The composition of the mineral matter in the coal (or the composition of the mineral ash after combustion) is of importance for, as examples, the performance of design of postcombustion cleanup equipment, such as electrostatic precipitators and FGD units (Kelly and Spottiswood, 1982, 1989 Hjalmarsson, 1992). The alkali metals (sodium, potassium, and lithium) affect (decrease) the resistivity of the ash and can influence sulfur removal. 

In the entrained-bed combustor, the feed coal must be introduced to the combustor as small. The particles (generally <200 mesh) are carried by the gas into the furnace and travel in a suspended state through a hot zone where they are consumed. After combustion, approximately 20% of the ash falls to the bottom of the furnace and is removed there. Combustion gases, which contain about 80% of the ash, pass out of the furnace and are treated to remove the remaining particulates (usually in an electrostatic precipitator) and sulfur compounds (usually in a stack gas scrubber). This type of combustion system (PCC) is in common use for large-scale utility boilers. Higher combustion rates can be attained in pulverized fuel combustors than in fixed-bed combustors. 

Post-combustion capture involves separating the carbon dioxide from other exhaust gases after combustion of the fossil fuel. Post-combustion capture systems are similar to those that already remove pollutants such as particulates, sulfur oxides, and nitrogen oxides from many power plants. 

WEW Process (24,25). Vereingte Elektrizitatswerke Westfalen is a German electric utility that has developed a unique dry feed, entrained-flow gasification process. The VEW concept relies on partial gasification with preheated air to produce low-Btu-coal-derived gas and char. After waste heat recovery, steam generation, and sulfur removal, the coal gas is fired in a combustion turbine. The char is fired in a pulverized coal boiler with the hot flue gas from the combustion turbine serving as the preheated combustion oxidant. This design requires additional cleanup of the pulverized boiler flue gas to meet emission requirements. 

A number of processes are being used to remove sulfur and sulfur oxides from fuel before combustion and from stack gas after combustion. Most of these efforts concentrate on coal, since it is the major source of sulfur oxides pollution. Physical separation techniques can be used to remove discrete particles of pyritic sulfur from coal. Chemical methods can also be employed for removal of sulfur from coal. 

In AFBC units, heat is removed from the flue gas by a convection-pass tube bank. The particulates leaving the boiler with the flue gas consist of unreacted and spent sorbent, unburned carbon, and ash. Multiclones after the convection pass remove much of the particulate matter and recvcle it to the combustor, increasing the in-furnace residence time an improving combustion efficiency and sulfur retention performance. Bubbling PFBC units do not have convection-pass tube banks and do not recycle solids to the boiler. 

In the infrared detection system, the sample is weighed into a special ceramic boat which is then placed into a combustion furnace at 1371°C (2500°F) in an oxygen atmosphere. Most of the sulfur present is converted to sulfur dioxide, which is then measured with an infrared detector after moisture and dust are removed by traps. The calibration factor is determined using standards approximating the material to be analyzed. 

There are several approaches available to a utility to construct a boiler that will meet New Source Performance Standards. These approaches can be classified according to the position in the combustion system at which pollutant control technology is applied. Precombustion control involves removal of sulfur, nitrogen, and ash compounds from the fuel before it is burned. For coal combustion this approach involves the application of coal-cleaning technology. Combustion control relies on modifications to the combustion process itself or the addition of material to the combustion process to reduce pollutant formation or capture the pollutants formed in the combustion chamber. Examples of combustion control include staged combustion, boiler limestone injection, and fluidized-bed combustion with limestone addition. Post-combustion control involves removal of pollutants after they have been formed but before they are released into the atmosphere. Traditionally, flue gas desulfurization has meant the application of postcombustion control either alone or in conjunction with another... 

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