Theory

Combustion of coal generates considerable quantities of byproducts, some of which are considered pollutants. The byproducts are mostly water vapor which is what we see coming out of a power plant smokestack, carbon dioxide, and nitrogen that is readily available in the air we breathe, and they do not necessarily pose any direct health hazard. However, the emissions do carry small concentrations of pollutants into the atmosphere, which translate into large quantities of hazardous emissions due to the large amount of coal combusted. The main pollutants that can cause health problems are sulfur oxides, nitrogen oxides, particulate matter (see Combustion Analysis) and such trace elements as arsenic, lead and mercury.

During the combustion process in a coal-fired power plant, nitrogen from the coal and air is converted into nitric oxide (NO) and nitrogen dioxide (NO₂); these nitrogen oxides are commonly known as NO_x. NO_x emissions contribute to the formation of acid rain.

NO_x is primarily formed by two mechanisms: thermal NO_x and fuel-bound NO_x.

Thermal NO_x formation takes place at high flame temperatures. Formation of thermal NO_x increases exponentially with combustion temperature. Fuel-bound NO_x formation is dependent upon the nitrogen content of the fuel.

The best way to minimize NO_x formation is to reduce flame temperature, reduce excess oxygen, and/or to burn low nitrogen-containing fuels.

The DeNO_x Plant Description

The purpose of the DeNO_x plant is for removal of nitrogen oxide from the flue gases. The plant employs a selective catalytic reduction method. The medium used for the reduction is ammonia gas.

The DeNO_x plant includes two selective catalytic reduction (SCR) reactors and an ash silo. Various dampers channel the flue gases either into or bypass the SCR-reactors.

Operation of the DeNO_x Plant

The DeNO_x plant is highly automatic, controlled by Programmable Logic Control sequences. The sequences are S701-Purge, S702/703-Start/stop Reactors, S704/705-Heating of Reactors, S706/707-Ammonia Injection, S708/709-Product Handling and S710/711-Soot Blowing.

Over Fire Air

To reduce NO_x, a quantity of additional air above all burner planes is supplied. This over-fire-air (OFA) reduces NO_x by enabling richer fuel mix in the high-temperature combustion zone at the burners (two-step combustion). Approximately 10 % of the combustion air is added as OFA air. OFA controller failure (MD200 malfunction 0881) is modeled in the simulator:

Over Burner Air

To further reduce NO_x generation, a portion of the secondary air is split off the main duct and directed into a third channel just above the burner. The damper controlling this over-burner-air is abbreviated OBA. OBA controller failure (MD180 malfunction 0780) is modeled in the simulator:

Fuel Quality

In the simulator, the chemical composition of the coal or other fuels can be specified. The sum of the five components C, H, S, O and N should preferably add up to 100%, to avoid confusion, but it is not strictly necessary because the C, H, S, O and N setting is always recalculated to a 100% basis prior to using in other computations. Water and inert matter (ash/slag) should then be added. The water content varies much and has a great impact on the amount of preheating required by primary air. The simulator computes the lower heat value (including water/inert matter) and theoretical combustion air needed and flue gas produced. The air/flue gas values are given in ncm/kg (ncm=normal cubic meter).

In this lab, we will burn both default and lower quality coal for a comparison. Fuel data can be changed using Variable List page 0111 on MD180, for example:

Hints & Tips

Your trend windows should look like the following:

Make sure your trend printouts are labeled properly otherwise, data analysis will be very confusing.

Tabulate your data as shown below:

Percentage deviation can be expressed as

[latex]Percentage\ deviation = \frac{Current\ Value-Reference\ Value}{Reference\ Value}100\%[/latex]

and tabulated as follows: