2007-12-13 - The steady flow of new legislation aimed at reducing industrial pollution is obliging operators of heavy industrial processes to find new ways of cutting their emissions wherever possible. In this article, ABB explains how using the latest oxygen monitoring instruments can help power station operators to begin to improve their operating and environmental performance.
With the continued introduction of increasingly stringent anti-pollution legislation and the UK Government struggling to cut industrial greenhouse gas emissions to its target of 20% below 1990 levels by 2010, the pressure is on heavy emitters of greenhouse gases to find ways to improve their performance.
As one of the three biggest emitters of industrial pollution in the UK, the power sector in particular is expected to make serious reductions in its output of greenhouse gases.
The DTI’s recent Carbon Abatement Technology (CAT) strategy consultation document advocates a number of ways in which carbon reductions can be achieved to meet the Government’s targets.
Admitting that many of these technologies are still several years from realisation and/or commercial viability, the document stresses the importance of improved combustion energy efficiency as a means of cutting carbon emissions in the short to medium term.
Monitoring the levels of oxygen in flue gas emissions to assess the efficiency of the power station boiler combustion process is one way of doing this. By measuring the level of oxygen present in a boiler flue or furnace, it is possible to obtain data that can be used to optimise the air to fuel ratio to ensure maximum heat is extracted from the fuel. With the development and refinement of oxygen monitoring instrument technologies, it is now possible to test flue gas emissions with even greater accuracy.
Achieving efficient combustion
In a perfect world, all operators would be able to achieve stoichiometric combustion, the ideal combustion process whereby all fuel is completely used up.
The potential energy (heating value) of fuel varies according to the type of fuel being used. In burning any fuel, considerable energy is lost. Most of this lost heat is in the stack gases leaving the furnace. The lower the temperature of the exit gases the higher the efficiency will be and the lower the extent of any pollution.
Controlling the amount of air supplied to the combustion process is a fine balancing act. On the one hand, insufficient air supplied to the process will mean incomplete combustion of fuel. This leads to unburned fuel, fouling of heat transfer surfaces and emissions of soot, smoke and carbon monoxide. If, on the other hand, air levels are too high, heat efficiency is reduced because the extra air carries more heat away in the flue gas, resulting in a lower overall boiler fuel-to-steam efficiency. It is worth noting that a 22oC drop in stack temperature can increase boiler efficiency by 1%. This may not sound like much – however, when one considers that fuel accounts for 75% of the cost of producing steam, the extent of any potential savings soon becomes apparent.
The optimum combustion process provides just enough excess air to completely burn the fuel. The starting point is the stoichiometric air to fuel ratio. This is the theoretical ideal ratio at which the air and fuel would both be completely consumed.
The amount of excess air required for optimum combustion efficiency depends largely on the fuel being used.
Oxygen measurement and monitoring
The level of oxygen present in the flue gas is a key indicator of whether the correct amount of oxygen is being used up during combustion.
Various technologies have been used in the past to measure flue gas emissions. The main drawback of these technologies was the need to extract samples prior to analysing them. Techniques such as carbon dioxide and paramagnetic oxygen measurement both required samples to be extracted, cleaned, humidified or dehumidified and then pumped into an external CO2 analyser. All of this incurred considerable time, cost and effort and made it difficult to get a picture of boiler efficiency at any given moment.
The introduction of in-situ Zirconia oxygen sensors in the late 1960s brought a radical change to the way that flue gas oxygen was measured. These systems consist of a Zirconia sensor, thermocouple, filter, support tube, electrical junction box and gas connections for reference and calibration gases. Removing the need for extractive systems, in-situ Zirconia oxygen sensors can be inserted directly into the flue or furnace.
Although Zirconia oxygen sensors are a relatively recent innovation, in fact the understanding of the operation of Zirconia as a means of measuring oxygen concentrations dates back to before 1900. Nernst discovered that Zirconia (ZrO2), the oxide of the element Zirconium, is conductive to oxygen ions, resulting in differences of oxygen concentration when heated to temperatures of over 600°C.
With this method, the basic sensor consists of a thin piece of Zirconia. Applying a coating of porous platinum to the opposite faces provides an electrical connection to the sensor. Air is supplied to one face as a reference gas to provide a constant oxygen concentration. The process gas is then presented to the opposite face of the sensor. The platinum acts as a catalyst in the presence of oxygen gas, where free electrons convert molecular oxygen into oxygen ions. These ions are mobile and can migrate through the solid Zirconia.
When the concentration of oxygen is in equilibrium on both sides of the sensor, migration of the oxygen ions through the Zirconia is zero. Where there is a difference in the concentration, i.e. when the oxygen in the process gas falls, the migration of the oxygen ions will increase in order to re-establish equilibrium. The reaction at the two electrodes will therefore differ, generating a corresponding potential difference. This potential difference provides a millivolt signal that can be converted into a reading of the oxygen concentration in the process gas.
Zirconia Probe Types
Zirconia probes can frequently operate for as long as five years, making them very reliable.
Although all Zirconia probes function in the same way, there are two distinct types in common use, the choice of which depends on the temperature of the application.
High Temperature Probes
These probes are used in applications where the process temperature is between 600 and 1400°C, such as high temperature furnaces and kilns.
Low Temperature Probes
These probes, somewhat more complex in design, are used in applications where the process temperature is below 600°C, such as in steam boilers and incinerators. They incorporate an integral heater, which maintains the sensor at 700°C in order for effective oxygen measurements to be carried out.
Apart from their in-situ monitoring capabilities, Zirconia oxygen sensors require minimal routine maintenance (only an occasional calibration check with a test gas and filter change depending on the type of fuel). They also have a probe life of up to 10 years.
Summary
The UK’s continuing reliance on fossil fuels as a source of fuel, coupled with the increased demand for power and the cost of implementing some pollution prevention measures, makes it unlikely that the problem of pollution from power generation processes is unlikely to resolve itself overnight. By implementing relatively low cost technologies such as Zirconia oxygen sensing equipment, it is possible for operators to begin to take steps towards greater efficiency and to start to improve both their environmental and operating performance.