In this case study we are going to study why corrosion occurs in boilers that use diesel for their operation.
Operation of a boiler motor
As we know, a boiler is an equipment whose aim is to transmit heat to a heat transfer fluid, generally using water, and using the energy power of fuels for this.
The oxidizer used is air, which is made up of:
Combustion is a chemical reaction. Fuel associates with oxygen creating new compounds and releasing energy in the form of heat.
THERE ARE 2 BASIC TYPES OF BOILERS
The hot fumes flow through the interior of small tubes surrounded by the water that will be used as heat transfer fluid. They are those that are usually used to produce heating and sanitary hot water.
The water is what goes through the tubes and the hot gases through the outside.
Operation of a boilers
- The heat energy transmitted by the flame to the walls of the fireplace in the combustion process.
- The hot fumes that are generated in the oxidation of the fuel.
The heat exchange with water is carried out by conduction through the metal surface of the tubes. Hot water forms a closed circuit that takes its energy from the fuel and transfers it to the medium to be heated. At the end of the process, a fuel has been burned and hot water is available to use as a heat transfer fluid.
The circuits are usually calculated for a thermal jump between 10ºC. and 20 ° C. It is very common to adopt temperatures of 80ºC. in impulsion, and 60ºC. in return.
The routes that the water and smoke take are reversed: The smoke reaches its maximum temperature at the exit of the fireplace, and its minimum temperature upon arrival at the smoke box, where it connects with the
The water returns to the boiler through the place where the fumes are at lower temperatures, and that is where the heat exchange begins, increasing its temperature more and more on its way to the impulse. It is at this point of entry where the water and fumes have a lower temperature, where corrosion begins.
In Spain the “ROYAL DECREE 1700/2003, of December 15, which sets the specifications for gasoline, diesel, fuel oil and liquefied petroleum gases, and the use of biofuels”, developed taking into account the European Directives and published in the BOE of December 24, 2003, it sets the sulfur content.
The diesel C commonly used in heating has a part of sulfur in its composition. Given the polluting problems caused by sulfur, the laws establish the maximum allowable quantities.
Regarding diesel C, which is used for heating, the maximum sulfur content of heating oil (class C) will be governed by the following:
a) As of January 1, 2008, the sulfur content shall not exceed 0.10 percent by mass.
b) Notwithstanding the provisions of Annex III and the preceding paragraph, the Ministry of Economy, following a report from the Ministry of the Environment, may authorize the use of said diesel with a sulfur content between 0.10 percent by mass and 0.20 percent by mass, upon reasoned request from the interested parties, and as long as the air quality standards regarding SO2 are respected and the emissions produced by such use do not contribute to exceeding the critical loads.
Said authorization must be made public and communicated to the European Commission 12 months in advance and will not be valid after January 1, 2013. Sufficient information will be provided to the European Commission so that it can check whether the criteria mentioned above are met.
Currently the maximum proportion of sulfur allowed is 0.10% of the mass, although up to January 2013 in some cases up to 0.20% could have been authorized.
The process starts with a uniform mixture of fuel and oxidizer.
Heat is applied to that mixture to start the chemical reaction.
Once combustion has started, it is the same heat generated that is responsible for maintaining the temperature necessary for the ignition of the fuel.
The basic composition of fuels is:
- Carbon (C)
- Hidrogen (H)
- Sulfur (S), is also a combustible element, although it tries to reduce itself by being contaminated.
There are different types of combustion:
It is the ideal, the theoretical, the exact air ratio, the perfect chemical reaction.
It is the combustion that is carried out with the strictly necessary air, without missing or over.
The reactions that occur are:
C + O2 = CO2 + heat
H + O2 = H2O + heat
S + O2 = SO2 + heat
combustion with lack of air.
There is not enough oxidizer, and therefore oxidation of all fuel elements cannot be achieved. Unburned are produced, and fumes are evacuated into the atmosphere with fuel that can continue to provide energy.
Combustión con exceso de aire
It is the actual combustion and the one that is achieved in an installation that has a good maintenance. Heat for combustion is applied to a fuel-oxidizer mixture. No matter how well the mixing process between fuel and oxidizer has been carried out, in practice it is not possible to achieve it 100%.
If only the theoretical necessary air were applied, unburned would be produced. The amount of additional air that must be supplied will depend on the fuel and the perfection with which it is mixed with the oxidizer.
When the fuel is gas, the mixture is more homogeneous and the excess air that must be added is a small percentage. In liquid fuels such as diesel, it is divided into very fine droplets while the fuel and air are stirred to obtain a good mixture.
The percentage of excess air to be applied is higher than in the case of gas.
Actual combustion fumes
In addition to the combustion products, there is excess oxygen as it has been necessary to introduce more air.
This oxygen, under these conditions and high temperatures, makes associations with other elements of combustion such as nitrogen NOx, (óxidos de nitrógeno) and with part of the SO2 btained in combustion, forming SO3, which when reacting with water gives rise to nitric and (NO3H) y sulfuric (SO4H2), acids, which cause acid rains.
In a pure substance, there is a relationship between its pressure and its temperature, which delimits the gaseous and liquid states, and is represented graphically by a curve.
In the case of water:
At a pressure of 1 atmosphere it has its boiling point at 100ºC. Above 100ºC. everything is steam. Between 0ºC., And 100ºC. At the pressure of 1 atmosphere, the water is in the vapor state and in the liquid state.
The air admits water vapor, although not all the steam that it is intended to provide. Let us suppose that in an environment of 26ºC. With 60% Relative Humidity (R.H.), a glass with beer whose surface is at 5ºC is placed on the table.
The water vapor contained in the air is stable, that is to say that under these conditions it does not have any tendency to become liquid.
The absolute amount of water is 12.64 grams per kg. of dry air. The air at 26ºC. and at a pressure of 1 atmosphere, it admits up to 21.35 grams of water vapor per kg. of dry air, a value that coincides with its saturation curve and corresponds to 100% R.H.
From there, any contribution that is intended to be made in the form of steam, would not be admissible and would condense. When taking a glass whose outer surface is at 5ºC to an environment under these conditions, exterior condensations are observed that cover the outside of the glass with water.
Water at a pressure of 1 atmosphere and 5ºC, admits a maximum amount of water of 5.40 grams per kg. of dry air. The difference between the water in the environment and the maximum allowable condenses.
In This case:
12,64 – 5,4 = 7,24 grams of water per kg. of dry air, which remain in the form of liquid water surrounding the glass. As the temperature increases, maintaining the same pressure, the air admits a greater quantity of water vapor.
If the temperature rises to 35ºC, for example, at a pressure of 1 atmosphere the amount of water vapor is 36.58. grams per kg. of dry air.
Si aumenta la presión, el aire admite una mayor cantidad de vapor de agua por Kg. de aire seco, y al contrario si disminuye.
The amount of water vapor in a high temperature smoke mass is relatively high.
In a conventional boiler it is very important to know the temperature at which condensation occurs, because it must be avoided so that it does not react with SO3, generated in combustion.
The percentage of water in the fumes can be important because they are at temperatures above 100ºC, and at pressures below atmospheric. It will depend on the type of fuel.
In the case of natural gas in which H is more abundant in its composition, water vapor occupies a higher percentage of the volume of smoke, when compared to diesel.
Dalton’s Law establishes that the total pressure exerted by a mixture of gases is equal to the pressure that each of them would exert if it occupied the entire volume.
In the case of combustion in which there is CO2, SO2, N, O2, SO3, NOx, H2O, Among others, the total pressure is the sum of the pressures that correspond to each of these components, if each one occupies the total volume.
In the Technical Guide “Procedures for periodic inspection of energy efficiency for boilers”, published by the IDAE, in section 2.6.3, an example is given of at what temperature the fumes would condense at atmospheric pressure if the water occupies 15% .
1 Atmósfera 103.125 pascales (Pa)
15% de 101.325 15.199 Pa
It is the pressure exerted by the water vapor in the fumes. With this pressure, and using the Pressure-Temperature diagram, the water vapor condenses at a temperature of 54.3ºC.
In diesel the dew point without excess air is around 50ºC.
Chemical reactions that produce corrosion:
Among the combustion products there is a proportion of sulfur dioxide (SO2).
As a consequence of excess air, and therefore oxygen, the following reaction occurs:
SO2 + O2 = SO3 (sulfur trioxide)
If water condenses on the metal elements of the boiler, the following reaction occurs:
SO3 + H2O = SO4H2 (sulfuric acid)
That it is a very corrosive acid and ends up destroying the surfaces on which it is generated, and that they are none other than those where condensation occurs.
A boiler that provides heating to a building, if it is expected to work between 80ºC. and 60ºC., It would only have to do so, a short period of time, because the designs are made for the most unfavorable conditions without taking into account other calorific inputs, and with extreme outdoor conditions, with a very conservative correction percentile .
If the boiler keeps working between these values, the energy losses are higher than working at a lower temperature. For this reason, in some cases, those responsible for Maintenance guided by energy saving criteria, may be tempted to lower the temperature of the boilers, especially if they only provide heating services.
Another case is those heaters that are put into service intermittently. They are held during the day and disconnected at night, such as office buildings, universities, housing communities, etc.
When put into operation the water is cold, and if the start-up sequences are not correct, returns to low temperatures can occur.
En la actualidad existen calderas de baja temperatura en las que se evita el contacto con las superficies de intercambio mas frías, y que permiten obtener un mejor rendimiento energético al trabajar a menor temperatura.
However, with a conventional boiler, it is necessary to maintain return conditions higher than the dew point to avoid corrosion, even if this means greater energy losses in the primary circuit.
It is necessary to control the return water temperature.
For this, a thermostat set at a temperature slightly higher than that which condensation can appear is installed. Traditionally, a so-called anti-condensation pump has been placed at the boiler outlet, which is controlled by this thermostat, and which joins the impulsion with the boiler return.
When the return temperature is lower than that set, the thermostat gives the order to start this pump and a mixture is produced between the flow provided by the anti-condensation pump and the return that causes the boiler inlet temperature to rise.
When the return temperature is higher than the set temperature, the thermostat will give the order to stop the anti-condensation pump. It may happen that if this pump is selected too small, it is not enough to increase the return temperature.
It is important to properly program the start-up processes, and not send water to the installation, until the boiler has reached its nominal conditions.
Let us suppose that the boiler works between 80ºC. and 60ºC., and it has been set that the water does not return in any case below 55ºC. If at the time of start-up, the water in the circuit is at 25ºC, and the anti-condensation pump has been calculated for 30% of the flow, an initial return temperature of:
T = (0,3*80 + 0,7*25) = 41,5ºC. valor menor de 55ºC., y que puede provocar condensaciones.
In some treaties it is recommended that this pump is not less than 40% of the return flow, although it will have to be designed taking into account the characteristics of the installation.
Another method used lately would be to place a proportional regulation 3-way valve in the boiler return circuit. In this way, the temperature is always controlled by diverting the necessary flow towards the return, which can be from values close to 100% at start-up to 0% in normal operation.
Among the elements that make up diesel C, there is a small part of sulfur.
This sulfur in a complete combustion comes out in the form of sulfur dioxide (SO2). A part of the SO2, due to the excess air necessary for combustion, combines with oxygen giving rise to sulfur trioxide. SO3. The fumes resulting from combustion contain water vapor.
If the return temperature of the water to the boiler is lower than a certain value, condensation of water vapor occurs on the metal surface. The mixture of water (H2O), and sulfur trioxide (SO3), gives rise to sulfuric acid which is highly corrosive and destroys the boiler zone affected by condensations.
There are techniques applicable to conventional boilers to raise the return temperature, such as the anti-condensation pump and the three-way valve.