CO and Hcvoc Controls

CO is normally created when carbon-containing fuels are burned in insufficient air or where the normal combustion process is halted before completion. CO production mechanisms include the presence of cold surfaces in the flame volume where the flame is quenched below the kinetic reaction temperature, direct leakage and quenching of partially reacted gases, and poor fuel-to-air mixing and subsequent quenching. CO emissions also depend on fuel type and can vary substantially, even in otherwise properly operating systems. In boilers, for example, observed CO emissions levels can be 10 ppm or below, or can exceed 300 ppm, and are often associated with severe NOx control.

Control technologies for stationary sources include: improved combustion control, capture systems that reuse CO-rich exhaust gases as fuel, and afterburners or oxidation catalysts to destroy CO in exhaust streams. Fuel switching to cleaner fuels, notably natural gas, is also a commonly used option for stationary sources.

While CO can be reduced with control modifications, such modifications are often at odds with those required for NOx control. Methods that treat exhaust gases directly (e.g., SCR and SNCR) do not exhibit this effect. In areas where CO is strictly controlled, NOx reduction methods using combustion modification techniques may be limited by CO control considerations or require oxidation catalysts to reduce VOC emissions.

VOC emissions result from incomplete combustion and vaporization. There is considerable overlap between emissions classified as HC or VOC, since VOC are compounds containing combinations of carbon and hydrogen. However, they may also contain oxygen, sulfur, nitrogen, and halogens like fluorine and chlorine. Methane, ethane, and a few other compounds are excluded from definitions of VOC, giving rise to the term non-methane HC (NMHC).

Industrial VOC emissions are dominated by three types of sources: 1) incomplete combustion of fossil fuels; 2) solvent emissions resulting from coating and printing; and 3) organic emissions resulting from the handling and manufacture of petroleum products, chemicals, and chemically derived products. While all combustion sources may emit some VOC emissions as a result of incomplete combustion, internal combustion engines typically have the highest VOC levels and are, therefore, a primary focus of VOC control efforts. For the purpose of the discussion on emissions resulting from fossil fuel emissions, VOCs are addressed as HCs. To some extent, NOx control methods can have deleterious side effects on HC emissions levels, and thus require oxidation catalysts.

As noted previously, oxidation catalysts are typically made of platinum (Pt), palladium (Pd), rhodium (Rho), or Pt/Pd/Rho catalyst formulae. CO emissions are effectively controlled from 450 to 1,250°F (230 to 670°C). HC control is dependent on the HC species. Typically, exhaust gas temperatures from 750 to 1,250°F (400 to 670°C) are sufficient for effective control.

Reciprocating Engine Emissions Controls

A number of emissions reduction technologies can be applied to new or existing engine designs, including lean-burn operation and add-on catalyst, with and without supplemental air. Figure 17-46 shows nox, CO, and NMHC (VOC) production for natural gas-fired reciprocating engines as a function of air-to-fuel ratio. Engines used for small utility applications are calibrated at very rich air-to-fuel ratios (typically ~12:1). Shifting to a leaner calibration can produce very large reductions in engine-out HC and CO emissions. Leaner operation, however, also tends to increase engine-out nox emissions so that the net change in combined HC and nox emissions may not be significant.

Exhaust treatment catalysts have been used on passenger cars since the mid-1970s to reduce emissions. They can apply in many instances to stationary engines as well. Oxidation catalysts are used to reduce HC and CO. This requires a net oxidizing exhaust environment, which is achieved through the use of lean engine calibrations or supplemental air injected into the exhaust manifolds.

The air-to-fuel ratio set point for operating in the catalyst window for post-combustion controls in stationary stoichiometric engines is very narrow. Air-to-fuel ratio, which depends on intake air density, temperature, fuel pressure, fuel flow, fuel heat value, and carburetor adjustment, must be precisely controlled. Figure 17-47 illustrates a simplified control system.

There are several ways to achieve the desired air-to-

Stoich Lean

Rich Lean Combustion

Stoich Lean

Rich Lean Combustion

Air/Fuel Ratio 14 16 18 20 22 24 26 28 30 lambda .88 1.0 1.13 1.25 1.38 1.5 1.63 1.75 1.88

Fig. 17-46 Summary Illustration of NOX, CO, and NMHC Emissions for Natural Gas-Fired Reciprocating Engines. Source: Waukesha Engine Div.

fuel set point for maximum catalyst performance. Typically, an oxygen sensor is placed in the exhaust stream. The sensor compares ambient oxygen on one side of the sensor with the exhaust gas sample on the other side. Some controllers also place a thermocouple next to the oxygen sensor for control limiting and temperature compensation due to oxygen. Through micro-circuit or microprocessor action, an output signal is generated to interface with a number of end devices. On charged reciprocating engines, it is common to control the waste gas, air bypass loop, or fuel regulator. With naturally aspirated engines, biasing the fuel regulator or some form of flow restriction is used. When an air-to-fuel ratio controller and catalyst are applied to a stoichiometric engine, standard emissions reduction efficiencies of 90% nox, 85% CO, and 50% HC are typically achieved.

Particulate Matter (PM) Control Technologies

As with other pollutants, fuel choice is an important determinant of particulate (PM) emissions. While uncontrolled particulate emissions for natural gas are negligible, particulates must be controlled for most other fuels, especially heavy oil and coal. Combustion control can improve particulate levels by reducing the level of unburned carbon. However, after-treatment, add-on controls are common for coal, oil, and many industrial material handling operations. The post-combustion control of particulate emissions from combustion sources and control of particulates from point sources in general can be accomplished by using one or more of the following devices: • Electrostatic precipitators (ESP), which have no adverse effect on combustion system performance, remove particles by charging them so that they are attracted to a collection surface.

Fabric filters (or baghouses), which consist of a number of filtering elements (bags) along with a bag cleaning system contained in a main shell structure incorporating dust hoppers.

Wet scrubbers, which include venture and flooded disc scrubbers, tray or tower units, turbulent contact absorbers, and high-pressure spray impingement scrubbers, are applicable for PM control, as well as for SO2 control on coal-fired plants. A disadvantage is the stringent disposal requirements for the resulting wet sludge.

Cyclone or multi-clone collectors are referred to as mechanical collectors because they do not rely on electrical, liquid, or barrier principles for removal of PM from a gas stream. The collection efficiency depends strongly on the effective aerodynamic particle diameter. These are often used as precollectors upstream of other controls because they are relatively ineffective for collection of the finer PM-10 and are not effective over wide load ranges. Side-stream separators combine a multi-cyclone and a small pulse-jet baghouse to more efficiently collect small diameter particles that are difficult to capture by a mechanical collector alone.

In internal combustion engine applications using liquid fuel, zeolite SCR NOx abatement catalysts have achieved up to 40% soot/particulate reduction.

As the soot, most of which is carbon, passes through the catalyst, it is converted to CO2.

Filterable particulate emissions can be controlled to various levels by all of these devices. Cyclones, ESPs, and fabric filters have little effect on measured condensable particulate matter (CPM) because they are generally operated at temperatures above the point at which most CPM remains vaporized and could pass through the control device. Wet scrubbers, however, reduce the gas stream temperature so they can remove some of the CPM. Side-stream separators are typically used on small stoker boilers as a lower cost (less effective) Fig. 17-47 Simplified Example of Air-to-Fuel Ratio Control. Source: Caterpillar Engine Div. control method than ESP or baghouse.

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