Monitoring emission and thermoprocesses
For whichever purpose furnace plants are used, whether for heating, for producing electrical energy, steam or hot water, for the manufacture or surface treatment of particular materials or for the incineration of refuse, in every case familiarity with the compositions of fuels and combustion air, and their relationship to each other, are required for good combustion.
Even the way in which they are brought together is decisive. With mobile measurement technology, all relevant gases can be analysed, thus achieving optimal combustion processes.
In the forefront of every combustion is profitability and the development of flue gas (emission). Added to this in process combustion is the direct contact between combustion or hot combustion gases and the commodity to be treated thermally, so a reciprocal influencing with effects on product quality, combustion and exhaust gas development.
Thus arises in many different regards the necessity for gas analysis with corresponding measurement equipment. The exhaust gas analysis system testo 350 M/XL is a portable and versatile solution for these tasks. Service engineers from PRAXAIR use these instruments for settings on industrial plants running on pure oxygen or enriched air, in order to achieve suitable combustion processes for their customers.
Ideal combustion means reduction of fuel and emission
In addition to the performance level of the furnace predetermined by its construction, the efficiency of combustion significantly determines the energy balance, i.e. the profitability of a combustion plant. The objective is an optimal, complete combustion of all combustible substances in the fuel.
The oxygen needed for this is introduced via the combustion air, and should ideally correspond to the minimum quantity required. In practice however, a minimal quantity of oxygen is not sufficient, among other things because of the incomplete mixture of fuel and oxygen. More oxygen than under stoichiometrical conditions is required. This is called an air surplus.
An unnecessarily high air surplus, however, reduces the combustion temperature and raises the quantity of unused energy lost through the greater quantity of exhaust gas. If too small an air surplus is selected, there arises, in addition to bad utilisation of fuel, a higher level of environmental contamination due to unburnt residues in the exhaust gas.
Information about the correct quantity of air surplus is provided by the exhaust gas components carbon monoxide (CO), carbon dioxide (CO2) and oxygen (O2).
The aim is to achieve a suitable air surplus for an optimal combustion, which in the end leads to savings on fuel. Experience in practice shows that by reducing the oxygen surplus by 1%, the affectivity of a combustion plant can be raised by 1%, and the fuel costs correspondingly lowered. Even adjustments of tenths of a percent bring savings.
If environmental air is used as combustion air, this contains, in addition to oxygen (20.95% if the air is pure and dry, at the earth’s surface), another 78.07% of nitrogen (N2) and, in small quantities, inert gases.
Nitrogen and inert gases, as well as the reaction products from the fuel and the combustion air, reappear with the incombustible components of the fuel in the exhaust.
A balanced air or oxygen surplus fulfils economic aspects and also simultaneously leads to emissions that are low. In order to lower these still further, oxygen-enriched air or pure oxygen are widely used as combustion air.
PRAXAIR provides gases and combustion technology
PRAXAIR is a provider of oxygen and combustion technology. In furnace plants in the glass, ceramics and metals industry, as well as in refuse incineration, the use of oxygen in combination with one of the technologies developed by PRAXAIR helps to reduce pollutant emission and energy consumption. The quality of different products can furthermore be improved by oxygen.
Gas analyses are necessary for monitoring all combustion processes. They facilitate the adjustment of the ratio of fuel to air or oxygen at high productivity and low pollutant emission, and the configuration of the burner and the combustion chamber, based on the composition of the exhaust gases. Robust and easily transported measuring equipment is often required for this.
The right sensors
The portable concept of the exhaust gas analysis instrument testo 350 M/XL consists, in the basic version, of a control unit, an analysis box and an exhaust gas probe. For determining oxygen and pollutant components such as carbon monoxide (CO), sulphur dioxide (SO2) or nitrogen oxides, electrochemical sensors are used.
They are filled, specifically for their task, with an aqueous electrolytic liquid in which there are two or three electrodes, also specifically assembled, between which an electrochemical potential is built up. The sensors are sealed from the outside with gas-permeable membranes.
Depending on the gas component to be measured, the construction details and function of the sensors differ. In oxygen sensors (two-electrode-sensors), flue gas, or oxygen molecules contained in it, reach the cathode through the gaspermeable membrane. Because of the material composition of the cathode, a chemical reaction takes place, in which OH ions are formed.
These travel through the electrolytic liquid to the anode, creating an electric current proportional to the concentration of O2. The decrease in voltage of a resistance arranged in an electrical circuit serves as a measurement signal. The negative temperature coefficient of the integrated resistance serves to compensate temperature influences. It ensures the temperature-stable performance of the sensor.
In three-electrode-sensors, as used for CO, SO2 or NOx, the gas molecules reach a working electrode through the gas-permeable membrane, where H+ ions are formed in a chemical reaction. These travel in the electrical field to the counterelectrode, where an electrical current is created in the outer circuit, in a further chemical reaction using O2 from the fresh air which is also introduced.
A special feature is the precise CO2-infraredsensor module
With standard fuels and known maximum CO2 value, it is usual to calculate the CO2 value via the O2 measurement. With fuels that have changing compositions, however, this calculation no longer corresponds to the actual concentrations. Even if additional CO2 is emitted from the process commodity in process plants, direct CO2 measurement is necessary. In combustion with oxygen as the combustion air, CO2 measurement serves to compensate CO2, and with that also serves the accuracy of the O2 measurement. The CO2 measurement module is based on infrared technology.
The non-dispersive infra-red sensor (NDIR) is Testo ’s own development, and has a fully usable measuring range from 0 to 50% Vol% CO2. The sensor works on the so-called Single-Channel Double-Beam method. Two infra-red receivers with two different filters (test filter and reference filter) are built into a miniature curette.
The sensor can thus be tested by the user himself and adjusted with the absorption filter included in delivery. In this simple way, drift and beamer aging are eliminated. Testo 350 M/XL measuring instruments already in use can easily be refitted at any time.
Sophisticated combustion technology
“The exhaust gas analysis instrument testo 350 XL is used by us mainly for spot checks in servicing or for regular routine analyses that we carry out for certain customers. We use the XL version because with it, NO and NO2 concentrations can also be recorded”, says Gerald Adler, Sales Support Engineer with PRAXAIR in Biebesheim, Germany, and adds, “NOx emissions are of particular significance from the point of view of environmental protection, which is made clear by the very tight limit values set by the German Federal Emission Protection Law (BimSchG).
These demands can often only be fulfilled with sophisticated combustion technology. A proven method here is staged combustion by staged oxygen input.”
Additional information on Nitrogen oxides (NO and NO2, sum formula NOx)
In combustion processes, the nitrogen out of the fuel, and at very high temperatures out of the combustion air, combines to a certain extent with combustion air oxygen, creating first nitrogen oxide (NO) (fuel NO and thermal NO) which, in the presence of oxygen, turns into hazardous nitrogen dioxide (NO2) in one step, already in the exhaust pipe and later in the atmosphere. Both oxides are poisonous.
NO2 especially is a dangerous pulmonary poison and, in combination with sunlight, contributes to the formation of ozone. Staged combustion is an effective primary measure, as it reduces the formation of both fuels NO and thermal NO. Directly at the burner, the air supply (primary air), thus the air surplus, is reduced. This causes an increase in the formation of CO.
In contrast, the NOx content remains low, and is further lessened by the formation of stable N2 molecules. Above or below the burner, combustion air is again introduced through a second lance, the so-called burn-out air. This causes a further combustion, vastly reducing the CO, and since the nitrogen already exists in the form of stable N2 molecules, hardly any NO is formed. With sophisticated settings, the concentration of NOx can be considerably reduced.
Gerald Adler on this: “The principle of staged combustion, which has been used in furnaces with several burners for some time, also works in furnaces with one burner. By systematic underprovision and dosing of the oxygen, different temperature zones with the same effect are created in the flame. To implement such combustion technology, the testo 350 XL is an appropriate tool.”