What are boiler flue gases?
Before we get to describing the benefits of monitoring boiler flue gas, it is worthwhile to give a brief overview of what boiler flue gas is, how it is produced and how it affects performance.
Let’s start by describing what they are.
Boiler flue gases are nothing more than the product of combustion that takes place in the firebox of the boiler or generator. The health of the system can be deduced from an analysis of their composition and temperature. However, before moving on to combustion technicalities, it is necessary to understand the general structure of the boiler.
All boilers consist of a burner, which is responsible for combustion, and a boiler body, where the heat exchange between flame, flue gases and water takes place.
There are different types of conduction, depending on the burner used and the applications: as a result, fumes with different characteristics are produced.
Like all fumes produced by the combustion of hydrocarbons, boiler fumes are pollutants and must therefore be monitored. But the analysis of flue gases is not only useful to understand their level of toxicity: through this analysis, it is also possible to derive the performance trend at which the boiler is working.
Index:
- Combustion in the boiler body
- Boiler efficiency and flue gases
a. How the concentration of carbon dioxide and carbon monoxide affect efficiency;
b. Flue gas temperature and its impact on efficiency. - How to carry out flue gas monitoring
- Conclusions
1. Combustion and flue gas composition
Having set out the basics and refreshed some concepts on how boilers work, let’s look at some combustion technicalities and how they affect the performance of a thermal system.
Let’s first define what combustion is.
Combustion is nothing more than the oxidation of a fuel (usually methane in boilers) by a comburent (oxygen).
The fuels that are used in most cases belong to the hydrocarbon family (methane, diesel, LPG, etc.).
Their combustion always produces a certain percentage of carbon dioxide (CO2), water vapour (H2O), oxygen (O2), nitrogen (N2) and, in small percentages but very heavy on the environment, carbon monoxide (CO) and nitrogen oxides (NOx).
It is easy to see, even without being an expert in chemistry, that the gases present in higher percentages are not really polluting gases. In fact, nitrogen, water vapour and oxygen are already present in the atmosphere and are not toxic.
Instead, the three polluting fractions are:
1. Carbon dioxide
2. Carbon monoxide
3. Nitrogen oxides
As we have heard a thousand times before, carbon dioxide is not a toxic gas in itself, but it is responsible for the greenhouse effect. Eliminating it is practically impossible as long as hydrocarbons are burned in cars, boilers and generators, because carbon dioxide will always be a product of their combustion.
The only way to take it out of the equation would be to find mechanisms to remove the produced dioxide from the atmosphere, but that’s a whole different subject.
Let’s get back to our combustion.
Carbon monoxide (CO) and nitrogen oxides (NOx) are toxic gases for both humans and the environment.
In boiler combustion, carbon monoxide occurs when combustion is not complete.
Incomplete combustion can occur during start-up and shut-down of the system but should not occur during normal operation.
Carbon monoxide concentration should therefore tend towards 0 as steady state conditions are reached.
A good way to reduce the total CO emissions of a thermal system is to have a boiler with a wide flame modulation range, which switches off and on as little as possible.
During the normal boiler operation, whether it produces hot water or steam, nitrogen oxides (NOx) are always emitted into the atmosphere.
These gases are highly toxic and harmful to the environment, which is why there are strict rules to limit their emissions. In Italy, for example, the limit is 100 mg/kWh for power above 1 MW burned. Other more advanced countries in terms of environmental protection, such as the USA and in particular the state of California, have even lower limits: 18 mg/kWh.
2. Boiler efficiency and flue gas
As we have begun to see, the composition of the boiler flue gas gives us clear indications of its state of health: if, for example, we have excessively high CO concentrations, it means that combustion is incomplete.
Monitoring flue gases is therefore a fundamental practice in order to keep system performance under control.
Let’s look specifically at how the composition of the flue gases and their temperature affect performance.
a. How the concentration of carbon dioxide and carbon monoxide affect efficiency
Let’s first take the case of carbon monoxide.
Excessively high levels of carbon monoxide (CO) in the flue gases indicate incomplete combustion, with a risk of explosion and fouling of internal surfaces, which can therefore affect heat exchange.
Even more than carbon monoxide, dioxide has a major influence on yields.
Too low a level of carbon dioxide, resulting in high oxygen levels in the flue gas, indicates too much air in combustion.
Excess air means a higher than necessary volume of hot flue gases being dispersed into the environment, resulting in lower boiler efficiency.
It goes without saying then that lower efficiencies mean higher operating costs. A few percentage points may not seem like much but on an industrial system this ensures significant savings over the year.
a. Flue gas temperature and the impact on efficiency
Together with the concentration of carbon monoxide, flue gas temperature gives us an important indication of the state of combustion and therefore the efficiency of the boiler.
In fact, if we look at a case where we burn methane at 3% oxygen, for every 22 degrees of increase in boiler flue gas temperature we lose approximately 1% in efficiency.
If, for example, under these conditions, we have an efficiency of 92%, with flue gas at a temperature of 197°C at a temperature of 219°C, we have a gross efficiency of 91%.
2. How to carry out flue gas monitoring
Boiler flue gas monitoring can involve both its composition and temperature.
Depending on the type of system, there are different timings with respect to the cyclical nature of the monitoring.
Generally for an industrial type system, flue gases should be checked at least once a year by an expert technician.
The check must verify both that the parameters fall within the regulations in force in that area but, mainly, that they reflect the manufacturer’s conditions of use.
As regards their temperature, on the other hand, it is always advisable to keep it constantly monitored.
The flue gas temperature can be used to calculate the efficiency of the boiler at that precise moment.
By keeping it constantly monitored, the performance of the boiler or system can thus be outlined over long periods of time.
By obtaining the performance trend, for example for a season, it would be possible to study how to increase the average value by changing certain modes of use.
Conclusions
We have therefore learned that boiler flue gases play a fundamental role in its proper functioning: from their composition to their temperature.
Both of these parameters allow us to establish the health of combustion and monitor the efficiency of the entire system.
Knowing that high efficiencies correspond to lower operating costs, the degree of importance of boiler flue gas monitoring goes without saying.