Oxygen Analyzer in Boiler

Large thermal power plants generate electricity by using the thermal energy of fuel. Most of the power plants are pulverized coal fired boiler that means thermal energy is generated by the combustion of pulverized (crushed) coal.In order to control the combustion process,every power plant has their Burner Management System/FSSS(Furnace Safeguard Supervisory System) system,but Optimum efficiency is a key requirement for any combustion process. As we all know that oxygen is the primary element of any combustion. Thus oxygen analyzer plays a key role for efficient combustion control so that we can get better efficiency.The Oxygen supplied for the combustion usually comes from the atmospheric air, which contains 23% of oxygen and 77% of nitrogen by weight.

Mathematically,the quantity of oxygen required (in gms) per 1 gm of fuel is = 8/3 C + 8 (H-O/8) + S

Where, C= wt. of carbon per gm. of fuel

              H= wt. of hydrogen per gm. of fuel

              O= wt. of Oxygen per gm. of fuel

              S= wt. of Sulphur per gm. of fuel

Thus the quantity of air required (to supply this oxygen) / 1gm of fuel = 4.31 [8/3C + 8(H-O/8) + S] gm.As 1 gm. Oxygen is present in 4.31gms of air.

Concept of Excess Air

This theoretical amount of air for perfect combustion is called as Stoichiometric air,But in practical, more than the theoretical air is required for the complete combustion and that is called Excess air. Because Excess air levels can be affected by many other factors like, Atmospheric pressure, heating value of the fuel, humidity, boiler plant performance, air-fuel mixing and residence time.

If Excess air is too low then flue gas is will contain high percentage of CO instead of CO2, soot and smoke increasing the risk of pollution. Also there will be appreciable amount of un-burnt carbon in ash. This presence of un-burnt fuel is symptomatic of inefficient combustion, as some of the energy in fuel is effectively being wasted.During CO formation 1/3 of the Energy is released in the combustion whereas in the formation of CO2 2/3 of the Energy is released.

If too much of excess air is supplied, then large amount of heat will be dissipated through the chimney as NOx emission will be in higher side. This heat loss in this case is called Dry flue gas loss.It is been calculated by considering the fuel composition that, for pulverized fuel fired boiler 20 -25% excess air is required.

This excess air can be monitored by CO2 and O2 measurement at Air Preheater(APH) inlet,But CO2 measurement for excess air calculation has several limitations, as It is not a direct measure of excess air. As the excess air is reduced, the CO2 % increases until the CO2 reaches maximum. Further if we reduce the excess air,CO2 will decrease. This may be interpreted as if the excess air has increased.So instead of CO2, oxygen (O2) measurement technique is more efficient for calculation and regulation of excess air.

Method for calculation of excess air from oxygen concentration at Air Preheater(APH) inlet:

Excess air % = K [{21/ (21 – % oxygen)} – 1] * 100

The value of K= 0.9 for gas; 0.94 for oil; 0.97 for coal.

Now the question arises how and where to measure the oxygen concentration ?

Oxygen is measured by using the Oxygen Analyzer at the inlet of Air preheater as well as at the outlet of air heater in the flue gas path. Analyzers installed at inlet of air preheater are used for excess air control and analyzers installed at outlet of air heater are used for assessment of air heater leakage.

Oxygen Analyzer

Oxygen analyzer used to measure the concentration of oxygen (O₂) in flue gas.Oxygen analyzers measurement system can be of different types such as :

• Zirconia type measurement system
• Magnetic type measurement system
• Tunable diode laser measurement system
• Electric cell type measurement system

In thermal power plant generally we use Zirconia Oxygen Analyzer.

Principle of Zirconia Oxygen analyzer

For determining the oxygen concentration in gas, Zirconia oxygen analyzer uses the technique of oxygen ion conductivity through zirconia ceramic cell/ Zirconium dioxide (ZrO2).

The figure above shows zirconia cell of oxygen analyzer.With reference gas (Ambient Air) on one side and sample gas (flue gas) on the other, oxygen ions move from higher concentration region to lower concentration region. This movement of ions generates an EMF (Electro Motive Force) which is measured to calculate the oxygen concentration.

Working:

Zirconium dioxide / Zirconia ceramic cell is a solid electrolyte acts as a conducting medium of oxygen ions at higher temperature. Zirconia sensor uses a thin piece of zirconia material coated each side with porous platinum electrode. This platinum electrode is heated up by the help of heater.

Air is supplied to one side as reference gas to provide constant oxygen concentration. The process gas is supplied from the opposite side. The heated platinum electrode converts molecular oxygen to oxygen ion. These ions then migrate through solid zirconia cell from higher concentration to lower concentration. The electrode which is in contact with air having higher concentration of oxygen act as negative electrode. The potential difference generated between two electrodes is called electromotive force.

Equation:

Reaction at Negative electrode: O2 + 4e –> 2O^2-

Reaction at Positive electrode:   2O^2-  –> O2 + 4e

The electromotive force E(mV) between the two electrode is given by Nernst’s equation:

E= -RT/nf ln PX/PA

Where

R – Gas constant

T- absolute temperature

n- 4

f- faraday’s constant

PX- oxygen concentration at positive electrode

PA-  oxygen concentration at negative electrode

**When temperature is controlled at 750⁰C, the value of cell voltage (E),

 E = -50.74 log PX/PA (mV)

Where, PX: Oxygen concentration in the flue gas ,PA: Oxygen concentration in the reference gas (21% O2) i.e. atmospheric air.So, when oxygen concentration is 21% on test side, the voltage generated by cell is, 0 mV. When oxygen concentration is 4%, voltage generated is – 36.54 mV

Oxygen Concentration Vs. Cell Voltage, (cell temperature: 750⁰C)

Calibration of O2 Analyzer

There are three modes of calibration:

1. Manual
2. Semiautomatic and
3. Automatic calibration.

Two different gases with known concentration are used for calibration process.

1.Zero gas of low oxygen concentration

2.Span gas of high oxygen concentration.

For Zero calibration, zero gas is normally used having an oxygen concentration of 0.4% of O2 and for span calibration, gas normally used is atmospheric air/ clean air or we can use test gas of 13% concentration of oxygen.