Attachment 9 - Natcom Condensation Mgmt Manual.pdf

NATCOM ● 545, Fernand Poitras ●Terrebonne, QC, J6Y 1Y5 , CANADA

OPM-36 – Condensation Management in Air+FGR Mixtures

CONDENSATION MANAGEMENT IN

AIR+FGR MIXTURES

VERSION 1.0

OPERATION AND MAINTENANCE MANUAL

NATCOM ● 545, Fernand-Poitras ●Terrebonne, QC, J6Y 1Y5 , CANADA

TABLE OF CONTENT

1 SCOPE

2 UNDERSTANDING CONDENSATION IN AIR + FGR MIXTURES

3 STRATEGIES FOR MANAGING CONDENSATION IN AIR + FGR MIXTURES

3.1 Air Pre-Heaters

3.2 Hot/warm FGR

3.3 Drains

3.4 Limp Mode

LIST OF FIGURES

Figure 1 : Air + FGR Mixture Condensation Temperature Chart

Figure 2 : Air + FGR Mixture Freezing Temperature Chart

1 SCOPE

The purpose of this manual is to provide guidelines and strategies for managing water condensation in air + FGR mixtures used as combustion air for burners. It also provides a brief overview of the basic physical principles that lead to condensation within these mixtures.

2 UNDERSTANDING CONDENSATION IN AIR + FGR MIXTURES

The combustion of fuel containing hydrogen produces water. For instance, flue gas from typical natural gas combustion will contain 12% mass of water. In comparison, ambient air at 80F and 60% relative humidity contains approximately 1.4% water in mass fraction.

When combustion air and FGR are blended together, the resulting humidity content is a function of the FGR ratio, fuel fired (determines the flue gas water content) and the initial relative humidity of the air. For cases where the combustion air temperature is low, the resulting blend temperature could be below the water dew point of the mixture. Typically, one should verify the mixture dew point for applications with combustion air of 45F or less utilizing FGR. For application with air temperature that go below 32F, freezing could also be an issue.

Water dew point is a property of gas mixture containing water and is easily calculated. It is a predictable characteristic derived from physical properties and of thermodynamics laws. Below are charts that can be used to quickly determine if there will be condensation or risk of freezing of the air/FGR mixture for natural gas firing. The horizontal axis is the

FGR rate and the vertical axis is the air temperature before it is mixed with FGR. The various curves on the chart correspond to the flue gas temperature (200-750F). Flue gas temperature can be found on the boiler performance run. Note that the worst case condition is often at low fire where the flue gas temperature is lowest. Note that these charts were generated for combustion air with 60% relative humidity at the corresponding inlet temperature and 1000 ft elevation.

How to use the Air + FGR Mixture Condensation Temperature Chart

Locate the appropriate % FGR at the bottom of the chart and trace up a vertical line that intersects the FGR temperature curves. Find the FGR curves corresponding to the full load (100% MCR) and minimum fire (or 25% MCR if data is not available for lower loads) economizer outlet flue gas temperature from the boiler run. From these intersection points, draw horizontal lines toward the combustion air inlet temperature. These are the combustion air temperature corresponding to the dew points for the specific air/FGR mixture. Operating temperature must always be above the dew point temperature to avoid water condensation. It is usually the minimum fire condition that determines the minimum combustion air temperature. Mixture freezing temperature graph works the same way as the dew point graph. Combustion air temperatures must be above the temperature found on the graph to avoid risks of freezing.

Example: For 15% FGR with flue gas temperature of 310F at full load and 258 F at Low fire.

Following the 15% FGR line we find a dew point temperature of 32F at the full load condition and a 42.5F for minimum fire. In this case, if the combustion air temperature can be lower than 42 F, a solution to address potential condensation must be selected. Section

3 presents the possible solutions.

Figure 1 : Air + FGR Mixture Condensation Temperature Chart

Figure 2 : Air + FGR Mixture Freezing Temperature Chart

3 STRATEGIES FOR MANAGING CONDENSATION IN AIR + FGR MIXTURES

3.1 Air Pre‐Heaters

Air pre-heaters are the solution for efficient control of combustion air temperature. They prevent condensation and freezing of the moisture in the air/FGR mixture. Set points are usually 10F above the calculated dew point temperature to allow for some control margin and latitude for alarms and trips in case of malfunction.

3.2 Hot/warm FGR

Using flue gas from the boiler outlet will increase the air/FGR mix temperature. Please refer to the dew point curves for increasing FGR temperatures. Using hot FGR means taking energy out of the economizer and putting it into the combustion air. It actually is a direct contact heat exchanger. There are different strategies that can be used to heat the combustion air mixture using hot FGR. The simplest is to tap the FGR line upstream the economizer and use only hot

FGR however this will have an impact on the FD fan which will be required to handle significantly higher air + FGR mixture temperatures. An alternative is to tap the FGR both upstream and downstream the economizer with dampers at each take off and then modulate the hot FGR damper in order to maintain a certain air + FRG mixture temperature. As for an air pre-heater, set points are usually 10F above the calculated dew point temperature to allow for some control margin and latitude for alarms and trips in case of malfunction. This strategy will generally require the addition of an FGR flowmeter in order to control total FGR flow in all conditions.

Note that an existing FGR arrangement should never be modified without first consulting the burner/boiler manufacturer.

3.3 Drains

For cases with occasional condensation issues and no risk of freezing, it is possible to use manual drains installed on the inlet box, fan casing and windbox. These should be regularly drained when the ambient temperature drops below a given level. Note that the inlet box drain shall have two valves because the inlet box is in vacuum. The top valve should be open and the bottom one closed in order to trap the water in the line. The top valve is then closed and the bottom one opened in order to drain the line. If a temperature transmitter is available, the control system can be made to alarm and notify the operator of any risk of condensation.

If the temperature drops below the freezing point, drains can be heat traced and windbox insulated but there would still be a risk of ice formation on the fan wheel leading to serious imbalance. We do not recommend using this solution when there are risks of freezing.

3.4 Limp Mode

To avoid condensation when the combustion air is too cold, it is possible to cut off the FGR flow.

Removing FGR will not only cause an increase in NOx emissions, but will also affect the burner performance because of the reduced velocity through the burner. For these reasons, the firing rate in limp mode is limited to low fire: thus boiler emissions in lb/hr are maintained at an acceptable level. As soon as the combustion air temperature rises above the allowed minimum the boiler resumes its normal operation. A combustion air temperature transmitter is required for this solution.