Achieving the right conditions for optimum combustion is a difficult process that requires accurate monitoring and control of a wide variety of different, yet inter-related, parameters. Matthew Sumner for ABB Measurement & Analytics UK explains how the latest generation of in-situ oxygen and combustibles analysers can help to deliver the data needed for maximum combustion efficiency.
Many industrial processes rely on the combustion of hydrocarbon fuels to generate heat, such as electricity generation, cement manufacturing, iron and steel, pulp and paper and Oil & Gas, to name but a few.
With fuel accounting for a significant percentage of the operating costs of any industrial process, it is desirable to ensure that combustion is controlled as efficiently as possible in order to maximise profitability and minimise the amount of fuel consumed.
A crash course in combustion
Combustion is the result of a sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light. In most combustion processes, the required oxygen is supplied by air.
For the maximum amount of heat to be generated, there needs to be a perfect mix of fuel and air. In theory, this seems simple. In reality, however, matching the correct quantity of fuel to the right amount of oxygen requires constant monitoring in order to compensate for the variety of factors that can affect combustion efficiency.
The chemically correct ratio between the fuel and air necessary to achieve complete combustion of the fuel is known as the stoichiometric ratio. When fuel and oxygen are in perfect balance – the combustion is said to be stoichiometric.
The stoichiometric point is not the point of greatest efficiency, however. To achieve this requires a small excess of air.
The three Ts of combustion
The requirement for additional air can be explained by the three T’s of combustion Temperature, Turbulence and Time.
- The Temperature must be high enough to ignite the fuel
- Turbulence must be vigorous enough to ensure complete mixing of the fuel molecules with oxygen molecules
- Time within the combustion zone must be sufficient to ensure complete combustion
Together with oxygen, these three Ts are inter-related and govern combustion efficiency. In a ‘real world’ combustion plant, it is not possible to achieve or maintain a perfect mix of fuel and oxygen long enough for complete combustion to occur.
To ensure best possible combustion efficiency, additional oxygen is therefore added to help optimise the process.
The result of this excess air being added is a small amount of oxygen appearing in the waste gases, which serves as a control parameter in assessing combustion performance.
Striking the ideal balance
Achieving maximum combustion efficiency requires close control of the amount of excess air being introduced.
The below diagram shows how too much excess air will decrease efficiency. An increase in the amount of excess air will result in a corresponding increase in the volume of gases passing through the system. These gases carry heat released from the fuel through the process and into the environment via the stack. This is wasted heat which has not been usefully transferred to the operator’s process.
Known as flue gas heat loss or stack losses, this wasted heat must be carefully controlled to maximise efficiency.
Conversely, adding too little air will result in incomplete combustion of fuels, leading to unburned combustibles such as carbon monoxide, soot and smoke escaping in the waste gases.
Unfortunately, there is no ‘one size fits all’, generic rule for the amount of excess air that will need to be supplied to a process.
Instead, the amount of excess air required for optimum performance depends largely on the type of fuel being used. The more easily the fuel mixes with air, the lower the amount of excess air is required.
Measuring excess air
Using in-situ zirconia oxygen probes to measure excess oxygen can help to accurately assess the amount of excess air needed to help maintain maximum combustion efficiency.
The probes use an oxygen measuring cell comprised of a stabilized zirconium oxide cell with porous platinum electrodes coated on both its inner and outer surfaces.
When the cell is heated to above 600°C (1,112°F), it becomes permeable to oxygen ions. Vacancies in the crystal lattice permit the mobility of oxygen ions so the cell becomes an oxygen ion conducting solid electrolyte.
The platinum electrodes on each side of the cell provide a catalytic surface for the conversion of oxygen molecules to oxygen ions and oxygen ions to oxygen molecules.
The above diagram shows how oxygen molecules on the high concentration (reference) side of the cell gain electrons to become ions that enter the electrolyte. At the same time, on the other electrode, oxygen ions lose electrons and are released from the surface of the electrode as oxygen molecules. When the concentration of oxygen is different on each side of the cell, oxygen ions migrate from the high concentration side to the low oxygen concentration side.
This ion flow creates an electrical imbalance that results in a voltage potential between the electrodes which is a function of the cell temperature and the ratio of the oxygen partial pressure on each side of the cell.
The potential generated (cell output) conforms to the Nernst equation:
E = RT / 4F x (In P0/P1) ± C
- E = Cell potential
- R = Universal gas constant
- T = Cell temperature in degrees Kelvin
- F = Faraday constant
- P0 = concentration of the reference oxygen
- P1 = concentration of the measured oxygen
- C = the sensor constant (the cell zero offset)
Measuring carbon monoxide
Maintaining the correct level of excess air for combustion can be complicated by various factors. These include variations in the fuel, air, atmospheric pressure and humidity; changes in the heating value of the fuel and the performance of the boiler plant itself.
The target excess air value can be further enhanced by using a combustibles analyser to measure carbon monoxide equivalent (COe) concentrations in the flue gas.
Measuring for carbon monoxide provides an ideal means of compensating for these variations. Colourless and highly toxic, carbon monoxide forms when combustibles are not completely consumed. As it appears before other combustibles such as soot and smoke, it provides an excellent indicator for gauging excess air requirement. By setting the target oxygen value to a given Carbon Monoxide equivalent (COe) concentration, the air-to-fuel ratio can be refined, which in turn can help to minimise NOx levels and further improve efficiency.
ABB’s new AZ40 zirconia oxygen and combustibles analyser features a COe sensor, which consists of an inert coated reference element and a catalyst coated active element. Both elements are resistance temperature detectors, or RTDs.
As the sample gases flow by the sensor, the combustible gases oxidize on the surface of the active element. The heat generated by the oxidation causes a temperature difference between the active and the reference elements.
This temperature difference produces a resistance relationship between the two elements that is directly proportional to the concentration of combustibles in the sample gases.
Where to measure
Together, oxygen and combustion gas analysers can be used to measure efficiency at key points in the power generation process.
Air pre-heaters play a major role in boosting the efficiency of power generation processes.
Using an air pre-heater to recover waste heat from flue gases can typically improve overall efficiency by up to 10 percent. The preheater recovers heat from the boiler flue gas and uses it to preheat the combustion air supply.
Rotating plate air preheaters are commonly installed in many power stations. They comprise of a rotating plate heat transfer element and between two to four sectors which are used to recover heat from the boiler flue gas. The heat transfer section of the preheater slowly rotates, collecting heat from the flue gas stream and releasing it to the boiler air stream.
A common problem with rotating plate air heaters is air leakage caused by deterioration of the heater seals dividing the sectors. Combustion air leaking across to the flue gas stream must be compensated for by an overall increase in the amount of air being moved by the Forced Draft (FD) fan. To compensate for this lost air, the fan has to work harder, increasing its demand for power.
Depending on the heater design and the pressure differential between the flue gas and combustion air streams, leakage rates can vary between eight percent and 30 percent. If allowed to persist, severe air leakage will mean that the fans will not be able to run fast enough for the power plant to run at full capacity, reducing operational efficiency.
Zirconia oxygen probes can play a key role in helping to quantify air leakage. By measuring and comparing excess oxygen levels at the entrance and exit of the air heater flue gas stream, the probes can provide an accurate indication of performance. Higher readings detected by the probe on the exit will indicate seal wear, as combustion air leaks into the flue gas.
By using the data from the probes, a maintenance schedule can be created to check and replace worn seals.
Measuring the flue gas at the economiser outlet will enable operators to assess the effectiveness of the air to fuel mix in the combustion process, which can be fine-tuned if too much air is present.
As the point immediately following the boiler, the economiser provides an ideal location for monitoring the levels of both excess air and carbon monoxide in the boiler flue gases.
By continuously balancing the two measurements against each other, it is possible to achieve and maintain the correct amount of excess air for optimum combustion.
Flue Gas Desulphurisation plant
In some instances, sulphur dioxide from combustion is removed from flue gases by using it either to produce sulphuric acid or gypsum for construction.
In the case of sulphuric acid, adding excess air to the sulphur dioxide and vanadium oxide converts it to sulphur trioxide – SO3. When combined with water, this produces H2SO4, or sulphuric acid.
In the case of gypsum, limestone slurry used in wet scrubbing processes combines with sulphur dioxide to produce calcium sulphite. When subjected to a forced oxidation process involving the addition of water and oxygen, the sulphite is converted into gypsum.
In addition to these processes, oxygen is also required in Flue Gas Desulphurisation processes using seawater. As the seawater absorbs the sulphur dioxide, oxygen is added, creating a mix of sulphate ions and hydrogen, which in turn converts to water and CO2 gas.
In all three cases, using an oxygen probe to accurately measure oxygen concentrations can help to regulate the conversion of sulphur dioxide.
Flue Gas De-nitrification plant
The production and emission of nitrogen oxides, or NOx, is a major issue that is heavily regulated by environmental authorities and legislation worldwide. If uncontrolled, it can result in excessive ozone levels and smog, affecting health, as well as acid rain, caused by NOx reacting with water.
NOx is categorised into three different types, namely Thermal NOx, Fuel NOx and Prompt NOx.
Thermal NOx occurs in high temperature processes above 1,200°C. Its formation is related to the temperature of the combustion flame and the amount of time the nitrogen is present in the flame.
Fuel NOx is created by the reaction of the nitrogen in fuel with excess oxygen in the combustion air.
The role of the Flue Gas De-nitrification plant is to remove residual nitrogen oxides in the flue gas. These plants fall into two main types. Selective Catalytic Reduction (SCR) plants add ammonia to the flue gas before passing it through a multi-bed selective catalytic reduction process, which converts the NOx into water and nitrogen.
Selective Non-Catalytic Reduction (SNCR) plants use urea, which is cheaper and easier to use than ammonia. The urea is added into the furnace to break the NOx down into nitrogen, carbon dioxide and water.
Both Thermal and Fuel NOx levels can be controlled by using oxygen probes to ensure the excess air being added to the combustion process does not exceed the required limit.
The reporting of emissions from power plant flue stacks is a mandatory requirement to help ensure that the release of harmful gases complies with strict global legislation.
Measuring oxygen at the stack provides a benchmark for the normalisation of emissions concentrations of stack gases such as SO2 and NOx. In basic terms, each specific gas will have an oxygen concentration limit for a given type of fuel that will need to be maintained. Deviations in the reported measured oxygen will indicate the need for the plant to correct its pollutant gas concentrations in order to bring them back within the expected range.
With power plant operators facing growing pressure to maximise operational efficiency whilst minimising energy consumption and environmental impact, accurate measurement of the combustion process is vitally important in highlighting what’s happening and where improvements can be made.
Capable of providing accurate, real-time data on operating conditions ranging from flue gas through to steam flow, ABB’s current generation of analytical and instrumentation equipment can form the front line in helping to maximise power plant efficiency whilst meeting the demands of tightening legislation.
For more information about ABB’s zirconia in-situ oxygen and combustibles analyser, visit http://bit.ly/ABBZircOxygen. Alternatively, call 0870 600 6122 or email [email protected] ref. ‘Zirconia Oxygen’.