COe Control – The New Kid on the Block

Published:  07 August, 2014

Oxygen Trim (O2 trim) is widely acknowledged as an essential element of burner control that allows boiler operators to reduce both energy costs and associated harmful emissions. Over the past two decades O2 trim has evolved from basic systems that adjusted mechanical linkage characterising cams using Bowden cables through to today’s microprocessor controlled electronic linkageless burner management systems that employ highly accurate and repeatable servo motors to position air dampers and fuel drives. Mick Barstow, sales manager UK & Republic of Ireland, reports.

The concept of O2 trim was the result of the development of in-situ zirconia based flue gas measuring technology. Efficient and safe combustion requires a precise mixture of fuel and air. Too much air results in energy being wasted up the chimney; too little air results in incomplete combustion. Incomplete combustion is particularly undesirable and results in the formation of Carbon Monoxide (CO) & Hydrogen in the form of H2.

To combat the chances of incomplete combustion burners are always commissioned with an element of ‘excess air’. Combustion is complex and there are many variables such as air temperature, humidity, barometric pressure and fuel quality that affect the whole process. Excess air ensures that even if the combustion variables change detrimentally then the combustion process remains safe.

O2 sensors allow combustion systems to become ‘closed loop’. This means that any changes in the combustion variables or controlling elements is detected and can be corrected accordingly.

So, how does O2 trim work? The answer is fairly simple in that it adds or reduces either fuel or air to compensate for changes in the combustion variables. For each point on the combustion profile there is an O2 setpoint. If the O2 reading for any point increases, then air is reduced, or fuel added, to bring the process variable back to the setpoint. If the O2 decreases the opposite happens. Most systems work by adding or subtracting air as this has less effect on the power output. Decreasing air will reduce costs whilst adding air will increase costs. However in the latter case systems will ensure the combustion process remains safe and CO/H2 is not produced.

In an ideal world, combustion, without excess air, would result in the best efficiency possible; this is called stoichiometric combustion and is a theoretical state where exactly the right amount of oxygen molecules combine with fuel molecules to complete the combustion reaction.

CH4 + 2O2 = CO2 + 2 H2O + Heat

COe control allows combustion systems to get closer to stoichiometric conditions while remaining safe. COe sensors use a modified version of zirconia O2 sensors that enable them to detect the products of incomplete combustion. These include CO, H2 and CH4. This is why the term COe is used instead of CO; COe is effectively a CO equivalent.

COe control produces savings over that of O2 trim by enabling the burner to operate a fuel/air mixture on the edge of stoichiometric conditions. One important aspect that allows this level of control is the rapid response of the COe sensor to the detection of the products of incomplete combustion. COe control is a self-adapting algorithm that ‘learns’ each point on the programmed combustion curve by reducing air to the point where COe is detected and then ‘backing-off’’ to a safe setpoint. After the learning process has been completed, if at any time incombustibles are detected, the system simply readapts by ‘backing off’’ to the next safe position. Each ’learned’ point has a lifetime of eight hours after which it is ‘learned’ again. This ensures that if external conditions have improved then the COe control will readapt to compensate for this and increase efficiency.

Quantifying cost savings when employing O2 trim and COe is always difficult as the starting point, i.e. the base profile set by the commissioning engineer, is somewhat subjective. However, as a rule of thumb, COe control can generate an additional saving of approximately 50% over conventional O2 trim systems.

COe control has been widely adopted in Europe but its benefits have not been recognised in the UK.

LAMTEC has extensive experience of COe control. It introduced the first systems back in 2004 and since then, over 1500 systems have been installed. COe control is available as an option on all its combustion control systems from the new BT300 through to the well-established Etamatic and FMS/VMS.

The KS1D is the latest version of LAMTEC’s combination zirconia based probe that detects both O2 and COe. By using the KS1D with the latest LT-3F failsafe transmitter COe control can be implemented using a single probe located in the flue.

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