Combustion control and laser analyzers: non-contact measurements


In most industries, combustion processes are used as a source of heat and energy. For this purpose, boilers, heaters and furnaces, burn fuels such as natural gas, biogas or even waste. When deciding on a new system for combustion control we must take into account the following points:

  1. Investment expenditure (CAPEX)
  2. Maintenance and operation expenses (OPEX)
  3. Potential fuel savings
  4. Maximizing heater performance
  5. Minimization of pollutants such as nitrogen oxides (NOx)

 

Combustion efficiency

A look at the theory of combustion shows that the ideal operating point is in a slightly lean regime, that is, with an excess of air. Poor combustion ensures that the fuel burns completely in all conditions. As such, the potential for high concentrations of carbon monoxide (CO) and unburned fuel in the combustion gases is minimized. Otherwise, fuel will be wasted and unsafe combustion conditions could result.

Originally only oxygen (O2) was used as a control measure and the operating point was typically between 5% and 10% excess air, which meant low efficiency and high NOx generation. Currently, additional CO measurements are used to avoid fuel-rich operations, and to provide information on the O2 set point. With this additional measurement, the operating point can be reduced to a range of 3% to 6% excess air.

 

Examples

As an example, we choose a typical ETHYLENE CRACKER , 200 MBTU per heater per hour. By reducing the operating point from 7% excess air to 4%, at a burn rate of 85% to 100%; annual fuel savings are approximately $ 80,000 per heater (assuming $ 2.33 / MBTU). That means that for an Ethylene Cracker with six heating cells, the annual combined fuel savings is almost $ 500,000. At the same time, NOx emission would be reduced by around 33% due to less excess air (figure 1)

Figure 1: Annual fuel savings per heater in k $ (right, blue axis) and reduction in NOx emissions in% (left, yellow blocks) for various operating points relative to a 7% operating point

 

Combustion optimization technologies

Over time, several different technologies have been developed to optimize combustion. Most of them have been based on one-point measurement sensors (probes), which must be in physical contact with the process gas. Zirconium Oxide (ZrO2) probes and electrochemical sensors are currently the most widely used. However, these sensors suffer rapid degradation due to harsh process conditions; catalyst poisoning or inhibition if exposed to reducing gases (eg sulfur). Furthermore, fuel sensors (COe) are not specific for CO, but rather measure the sum of all fuel gases, that is, they also measure hydrogen (H2) and hydrocarbons.

In contrast to these, Modulable Diode Laser Absorption Spectroscopy (TDLAS) performs measurement without contact with the sample, by interaction of laser light and gas molecules. Measurements can be carried out directly in-process (in situ) through the combustion chamber, thus obtaining representative results from the entire chamber, and not just from a point close to the wall.

 

Non-contact measurement

Furthermore, by performing non-contact measurement, the analyzers are not exposed to corrosive gases and high temperatures, and a complex sampling system with high maintenance is generally not required. Also, ZrO2 probes require monthly recalibration due to degradation, unlike TDLAS analyzers which are only validated once a year.

The TDLAS analyzer does not require a sample extraction system and maintenance is much lower, which implies a significant reduction in operating expenses (OPEX) compared to other technologies. Additionally, TDLAS analyzers are highly sensitive and selective, thus achieving very low detection limits without interference from other process gases. This means that unlike COe measurements, TDLAS analyzers measure the true value of CO, which leads to further optimization of the operating point.

 

Combustion analysis solutions

One of NEO Monitors’ solutions for a complete combustion analysis would be two LaserGas ™ III analyzers on site:

  1. Measurement of O2 and process temperature
  2. Measurement of CO, methane (CH4) and water vapor (H2O).

 

 

Each LaserGas ™ III analyzer consists of an emitter and a receiver that are mounted on diametrically opposite sides of the combustion chamber. The installation costs of the transmitter-receiver are somewhat higher than those of the measurement sensors at one point; significantly lower maintenance costs and better combustion optimization compensate for this after a short period of operation.

 

Fuel economy calculations

If we look again at the fuel economy calculation from the previous example and also take into account the difference in CAPEX and OPEX between spot metering type sensors (ZrO2 and CO) and TDLAS analyzers, we get the total TDLAS benefits per heater for the first five years of operation (Figure 2)

 Figure 2: Total TDLAS benefits in k $ per heater during the first five years of operation [/ caption]

 

For an Ethylene Cracker with six heaters, the benefits after five years of operation are more than $ 2.7 million.

Another solution proposed by NEO Monitors that further reduces investment costs (CAPEX) is with its LaserGas ™ iQ2 analyzer.

This analyzer combines the transmitter and receiver units in a single transducer configuration. In this case, a reflector is used to send the beam back to the receiver so that the beam passes through the monitored gas sample twice. There is also a special probe type version, the LaserGas ™ iQ2 Vulcan , specially designed to replace already installed probes from other manufacturers. In this case, only a single flange is required for installation, which reduces investment costs to a minimum, while preserving the other advantages of laser analyzer measurements.

 

 

Other advantages

Other advantages of using LaserGas ™ analyzers for combustion control is that these analyzers can also measure CH4, H2O and process temperature.

  • CH4. During the start-up phase of a combustion process, information on the concentration of CH4 is essential for safety reasons, to prevent explosions.
  • H2O. H2O measurement can be used to detect tube ruptures in boilers, and / or convert wet-base to dry-base measurements, thus ensuring concordance with measurements provided by typical extractive analysis systems (on a dry basis).
  • Temperature. A TDLAS-based process temperature measurement is the best solution for proper compensation of concentration measurements.

IN BRIEF: NON-CONTACT MEASUREMENTS ARE THE FUTURE OF GAS DETECTION.

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