Troubleshooting

Elevated (Utility) Landfill Gas Flare

Operation and Trouble Shooting

Introduction

Flares are applied in many industries for saftey, environmental and process reasons. In the case of landfill gas flares their use is for any one or a combination of the three reasons, with saftey and environmetal being the most common.

Landfills produces gas with a nominal charaterization of 50% Methane 50% Carbon Dioxide. Although this is a simplified view of landfill gas composition is is sufficent to describe the major saftey, environmental and benefical properties of landfill gas (LFG)


Operation - Before You Start

Confirm Interlocks and Control Element States


    With the interlocks confimed flare operation is permitted.

   Sequence Complete!

A detailed sequence is found in the table below

Flare Sequence Table

Step

Pilot Ignition

Gas On - Pilot Proved

Sequence Complete
a
Pilot Gas On
No, Check Interlocks
Landfill Gas Blower Starts
(proves on via auxilary switch on contactor)
Failed to start, see "Blower Failure"
Pilot Gas is Turned Off
b
Spark Ignition Initiated
No, See "Spark Ignition Diagnostics"
Inlet / Header Valve Open
(proves open via limit switch)
Failed to open, see "Main Gas Valve"
Flame Monitoring Enabled
(UV Flame Sensor)
Flame Not Established, See, "UV Flame Loss"
c
Pilot Proves on Temperature
(Gas On Set Point Met)
Not Heating, See "Pilot Gas"
Pilot Attains Pilot Off Temperature
Not Heating, See "Pilot Gas"
Main Burner Proves on Temperature
Temperature Not Made, See, "Low Temperature"
d
Temperature Monitoring Enabled
- -
Flare is in Operation

Flare Operation

After the flare is in operation landfill conditions may change. Changes in gas quality and quantity may lead to operation outside of recommended limits for the flare design.
Operation outside recommended limits can result in unstable operation, regulatory violations and / or equipment damage. Typical flare monitoring prevents continued operation outside safe operation but some investigation may be needed to reveal the root cause. Common operational issue are presented as ideas to consider during the investigation

Long Term Consideration

Long term gas quality is affected by the contents of the landfill, moisture distribution, gas extraction rate, gas collection system design and maintenance.

Long term gas production is projected through mathmatical models. This data is then used to determine equipment size and life span. In addtion to setting flare performance requirements the gas model projections are used as measuring stick to indicate if the care and maintenance of the landfill is on track. Long term gas quality is unsually not concern in day to day operation as it is planned into the collection system and flare design

Short Term Considerations

Short term gas quality is impacted by human error, accidental break in the collection system, watered in sumps and gas lines or intentional opening of the collection system for maintenance. The air intrustion dilutes the heating value of the gas.
An important aspect of air intrustion is the possibility of creating an explosive gas mixture.

When significant gas quality and volume are at issue they typically have an immediate and adverse effect on the flare and landfill operation.

Water in the gas collection system can block off wells and increase vacuum on the remaining wells. The reduced total gas production reduces flow to the flare which can lead flame flash back (fire inside the flare stack)

Flash Back Protection

Flame Arrestor - A class D flame arrestor is required to prevent flame travel back to the KOP and landfill gas collecton system (primary protection).

High Temperature Switch - Flame at the flam arrestor will eventually damage the arrestor. To prevent damage to the arrestor and stack the switch stops operation of the gas skid (secondary protection).

Combustion Monitoring

Opening of the collection system increases total volume but dilutes overall methane content by displacing methane with air. This can result in flame loss or blower motor overload.

To insure gas is burning two methods of combustion monitoring are common, temperature and UV (ultraviolet light) monitoring.

Temperature - The most commonly experienced byproduct of combustion is heat. Water in the air is heated by combustion at the flare burner. The elevated temperaure is used to indicate combustion is taking place. Temperature is measured using a thermocouple or "T/C" installed at the burner.

Advantages

Disadvantages

UV Sensor - Another byproduct of combustion is light, specifically light in the ultraviolet range. Significant radiation in quatities that can be reliably detected are present during combustion. The UV sensor is similar to vacuum tube where the UV light impinges on a specially coated grid. A pulsing electron flow from the bulb results . The frequency of the pulses indicate the intensity of the UV light. The signal is applied to electronincs that energize a relay once the frequency of the pulses is suffcient to indicate combustion.

  • Fast, responce time, three seconds or less is common
  • Signal relatively noise resistantant
  • Common wire and materials to connect sensor to relay

      Disadvantages

  • Detection reliablity is affected by water, dirt, debis and deposit from combustion (cleaning may be needed)
  • High wind and flow may move the flame outside of the sensor view.
  • Ambient light can trigger sensor, falsely indicating flame
  • Electronics may indicates failure for transient electrical events during self check

    Blower Protections

    Several items can be monitored to prevent blower damage. The first two are considered mandatory.

    KOP Liquid Level - Liquid carry over result in blower damage.

    Low Gas Flow - Centrifugal blower depend on the process gas to carry heat away from the blower.

    Bearing Temperature - This is an early indication of a process or maintenace issue that will lead to bearing failure.

    Vibration - Excess vibration results in premature failure of the bearing and surrounding mechanical components. Excess vibration also indicates a process problem.

    Operational Problems


    Following is a list of common problems that lead to flame loss. The list is not a comprhensive trouble shooting guide. It is intended to provide some insight to the miriad issues that can lead to flame loss

    Flame Loss - UV

    When flame loss is indicated by the UV flame sensor it can be falsely generated by the following events

    Flame Loss - Temperature
    Low temperature as an indication to flame loss can be caused by some of the following:

      Flame Loss - Equipment Failure