Furnaces
Improved combustion control via advanced CO laser sensor Lieke de Cock, Marco van Kersbergen, Joost Laven and Sjon Brouwer discuss the achievement of improved combustion control in glass furnaces by the application of a CO and O2 laser sensor.
C
ombustion control is of key importance to ensure stable and profitable operation of glass melting furnaces. Most furnaces are fired on >1% O2 excess to be safe and not find any CO in the regenerators. This results in higher NOx values and lower efficiencies, 1% higher oxygen excess ~1% more energy. Understoichiometric firing is not recommended, due to increased refractory corrosion, increased chance of clogged chambers (more evaporation) and higher SOx emissions. The thermic most efficient point can be found just above stoichiometric conditions (~0.5% oxygen excess). Close to stoichiometric conditions, the CO concentrations can vary significantly (+/- 500 ppm) while the oxygen level remains within the accuracy of the oxygen measurements. Small variations in air leakage, temperature or gas composition are not visible in the measured O2 levels, but can easily be detected by the measured CO concentration. Therefore, control is
only possible based on CO concentration measurements when operating at optimal near stoichiometric conditions.
Optimal combustion control There are a series of key factors to achieve optimal combustion control. Measurements of flue species in the flue gas should be taken as close to the combustion process as possible to avoid influences like air leakages (additional O2) and post combustion (decreasing CO). The reaction time of the measurements should be fast and sensitive, especially when operating near stoichiometric combustion. The measurements should also be representative for the real values in the furnace. Most glass producers currently choose to be on the safe side and control on higher O2 excess values to ensure low CO levels. That is why measuring CO is that important. “Combustion control is changed from controlling O2 only to CO control. Target is to
achieve a complete combustion with various settings for the individual burners to achieve an optimized setting for each zone in the furnace. – Sjon Brouwer, Batch and Furnace manager, Ardagh Moerdijk.” Some furnaces are still operating without any measurement of O2 or CO. Concentrations up to 8% of excess oxygen and 4% of CO are observed at these furnaces. Most furnaces are equipped with O2 sensors measuring only at one point (normally the top of the regenerator). In Fig 1 the O2 concentrations in the burner port are shown (range 0-2%). A point measurement does not give a representative value for the O2 values. In the top of the regenerator even larger differences can be seen due to the large flue gas circulations. The laser sensor gives you the average value for the whole width of the burner port which results in a value that is Continued>>
� Fig 1. Oxygen distribution in the burner port of an end-port
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fired furnace (range 0-2%).
33 Glass International March 2018
Furnaces
Flue gases
� Fig 2. Sketch and picture of the sensor setup.
Sensor
Retro reflector*
2 Lasers 2 Detectors
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Also, with aging furnaces, air leakage increases. This can be prevented with repairs, but the furnace will experience varying amounts of leakage air during its lifetime. When leakage air increases, and there is no burner control, the oxygen excess will also increase resulting in a decreased furnace efficiency (less performing regenerators due to additional cold air). If burner control is applied, the amount of combustion air will vary to keep the oxygen excess constant. The primary combustion will therefore become more reducing which might lead to higher CO concentrations in the combustion chamber. Leakage air doesn’t mix well with the flue gases, and therefore CO can move into the regenerator while the measured O2 levels can still be good. The increased leakage air goes unnoticed and the thermic efficiency of the furnace will decrease while in the meanwhile refractory corrosion can take place. Both cases yield to less efficiency, and additional disadvantages like higher NOx levels and higher corrosion of refractory
34 Glass International March 2018
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“There is no post-combustion anymore and we therefore prevent damage to the steam boiler of furnace 1 and the flue gas cooler of furnace 2. – Sjon Brouwer, Batch and Furnace manager, Ardagh Moerdijk.
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more representative for the real O2 concentration. In this way you can go to lower O2 values in a safe way. Operating close to the stoichiometric point without a reliable and representative measurement of O2 is unwise. Adding a CO measurement makes it easier and safer to operate with lower oxygen consumption.
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� Fig 3. Furnace specific relationship between CO and NOx. materials due to increased CO levels. When CO is measured, the combustion can be controlled on the CO signal with O2 as indicator for leakage air.
Sensor development CelSian started to develop and test the laser sensor many years ago. The proven technology is now being deployed globally on any type of furnace. This sensor is the first of its kind in the glass industry and has become an economically viable solution. The CelSian CO+ sensor is placed on the side of the burner port or flue gas channel to be as close to the combustion process as possible. The CO+ sensor provides average, representative values for the CO and O2 concentration. It is reliable (no drift) and sustainable (non-invasive), because it is not subjected to the harsh flue gases. This leads to low maintenance
and a long sensor lifetime. The typical sensor setup can be seen in Fig 2. “Measurement works stable and is reliable. We now have a stable combustion process due to CO & O2 control. - Sjon Brouwer, Batch and Furnace manager, Ardagh Moerdijk.” Sensor signals can easily be used as an input for the furnace control system. In this way the optimised combustion process can be controlled by varying the ratio between air/oxygen and fuel based on the CO levels. In this way the effect of a changing gas composition can also be controlled. Next to optimising the combustion process (resulting in improved energy efficiency), the CO+ sensor enables the glass plant to operate at the lowest Continued>>
possible NOx concentration of the furnace. There is a furnace specific relationship between CO and NOx when operating conditions of the furnace are stable. Fig 3 shows such a relationship. By controlling the CO values in the flue gas, the NOx levels will be stabilised as well. Increasing CO to the highest allowed levels in the furnace will result in a decrease of the NOx levels to the lowest possible levels for this furnace. Reductions of 20% are observed in industry and this can be enough to avoid implementation of expensive secondary measures to decrease the NOx emissions or, if secondary NOx reduction is needed, to minimize the ammonia consumption of a de-NOx system. Also SOx can be optimised and controlled as this is affected by the CO levels just above the batch blanket. “No CO values above set upper limits anymore, so no premature release of sulphates from batch blanket. – Sjon Brouwer, Batch and Furnace manager, Ardagh Moerdijk.” It is expected that controlling the CO will also lead to a reduction of defects and has a stabilising effect on product quality due to a more stable combustion process.
Installations After a long industrial testing period, commercial sales of the CelSian CO+ sensor were started last year. One of the first customers was Ardagh Glass, who was already involved in the development of the sensor. Ardagh uses CO+ sensors in their production of soda-lime container glass on oxy-fuel furnaces. The results of these applications show that for their typical 280 tons/day oxy-fuel container furnaces, NOx emissions drop by 20%, energy savings are 2% and the oxygen savings are 1100m3/day. Return on investments typically range from six months to two years. “Due to the direct savings of oxygen the application of the sensor is financially even more attractive, cryogenic oxygen is expensive.” – Sjon Brouwer, Batch and furnace manager, Ardagh Moerdijk.” A leading global glass fibre producer has chosen to go exclusively with the CelSian CO+ sensor on any rebuild of its furnaces, while a furnace designer now delivers furnace designs including holes in the flue gas channel/burner port for application of the CelSian CO+ sensor. If you aim for the most efficient combustion and aim to have optimal knowledge and control of your process, you have to measure both CO & O2 and both in a representative way. Only measuring O2 has been the standard for years because there were no other (economically interesting) options. If we want to develop further then the laser sensor is the only solution. �
Lieke de Cock, Project leader, Marco van Kersbergen, Glass Technologist, Joost Laven, Furnace Support Team Leader all at CelSian Glass & Solar. Sjon Brouwer, Batch and furnace manager, Ardagh Moerdijk. CelSian Glass & Solar BV, Eindhoven, The Netherlands tel: +31 40 249 0100 email: lieke.decock@celsian.nl web: www.celsian.nl Ardagh Moerdijk, The Netherlands www.ardaghgroup.com