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Pure oxygen as an alternative

More efficiency and fewer emissions from combustion processes
Pure oxygen as an alternative

The targeted use of pure oxygen as an additional oxidation medium in combustion processes helps to considerably increase the throughput and reduce the related energy consumption. A crucial factor for the successful use of this technique is the individual adaptation of the oxygen added to the combustion conditions and constraints. It is now possible to convert existing installations or equip new plants with general-purpose systems for oxygen addition that have been tried and tested in practice.

Jürgen Flach, Dr. Gerhard Groß

In cooperation with plant operators, the experts at Air Liquide have already upgraded several combustion plants in different lines of industry with an oxygen based system. Typical examples include the mono-combustion of sewage sludges, the combustion of waste waters and hazardous waste as well as the generation of energy from renewable sources like paper industry residues. Apart from increasing throughput and optimising the efficiency of these installations, pure oxygen can also be employed to improve the combustion parameters and the level of emissions.
Traditional combustion processes only actually use around 21% by volume of the oxygen contained in air for combustion. The inert nitrogen is also heated up in the combustion chamber, leading to thermal losses and more exhaust gas. If part of the combustion air is replaced with pure oxygen, the exhaust or process gas volume is reduced, thus relieving the load on the aggregates carrying the exhaust. Depending on the volume of oxygen added, the fuel gas capacity increases by the same amount, while the exhaust gas volume remains constant. Possible increases in the combustion temperatures in any reaction can be avoided by controlling the energy supply. In practice, savings can be achieved by using less primary energy, such as oil or natural gas, or by incinerating waste with a low calorific value. The main factors influencing the carbon monoxide reduction are temperature, residence time and the volume of oxygen added. If the residence time in traditional operation does not suffice for a complete reaction, the targeted use of O2 can accelerate the reactions in the combustion chamber and reduce carbon monoxide peaks.
Depending on the combustion conditions and constraints, there are several ways to transport pure oxygen into the reaction chamber:
  • Special oxy-fuel burners
  • Direct injection into the mixing zone of the flame or into the reaction chamber
  • Enrichment of the oxidation air
The use of oxy-fuel burners usually entails the most substantial modifications to the plant parameters; however, it also helps to achieve the greatest capacity increase in combination with suitable processes such as the Claus process, sulphuric acid recycl-ing or melting and calcination. The optimum method frequently comprises conventional oxidation air enrichment or direct injection of oxygen by means of lances. The enrichment of the oxidation air is, however, restricted for safety reasons due to the higher reactivity of enriched air.
Sewage sludge combustion in practice
In cooperation with a customer, it was possible to raise the throughput of two fluidised bed furnaces for the combustion of sewage sludge and other residues by 40 % thanks to the transverse supersonic injection of oxygen. At the same time, the specific energy consumption was cut by more than 35 %. Using this process, up to 4000 m³/h of pure oxygen are transversely introduced into the two fluidised bed furnaces at supersonic speed. The oxygen is injected by several nozzles built into the refractory lining of the furnace. In addition to intensive mixing with oxygen, transverse injection facilitates cross-mixing of the bed material, which is otherwise much more difficult to achieve.
Another customer has also been using oxygen to increase the performance of its sewage sludge combustion process since the beginning of 2003. Due to the size of the plant, enrichment of the fluidised air is the most suitable method for what is Europe’s largest fluidised bed furnace for sewage sludges. The flexible use of O2 allows both operators to make use of additional capacities for sewage sludge treatment according to requirements.
Increased performance for waste water combustion
Waste waters that cannot be biologically treated due to their high salt content, the presence of toxic substances and/or insufficient biodegradability need to be disposed of in a different way. One standard method is to incinerate the waste water with air in a supporting flame. Through atomisation and vaporisation, the waste water is converted to its gaseous phase as far as possible, then subsequently mixed with air and heated to between 800 and 1200 °C. The organic substances contained in this mixture oxidise completely to form carbon dioxide and water. The residue consists of inorganic substances that cannot be incinerated. Due to the low calorific value of polluted waste water, the energy consumption of this process is extremely high. In addition to utilising production waste with a high calorific value, it thus requires large quantities of natural gas or heating oil as fuel. The capacity of the plant is often limited due to the increased CO in the off-gas.
Since the gaseous phase oxidation of waste water is a particularly energy-intensive process, the specific adaptation of the O2 technology to the respective operating conditions is of utmost importance. In addition to a substantial increase in throughput, this also ensures optimal efficiency by reducing the amount of primary energy required.
Waster water incineration is often based on the use of oxygen by direct injection. The necessary oxygen quantity can be introduced into the combustion chamber as an atomising medium by means of modified waste water atomising lances. This guarantees not only the atomisation and mixture of the waste water with oxygen, even at high volume flow rates, but also the targeted injection of the required quantity of gas. In most cases, enriching the primary air in the main burner does not make sense because an enormous temperature increase occurs. Increased levels of NOx are unavoidable.
Oxygen injection makes the combustion process more stable. In seven combustion plants in the chemical industry that have already been converted, there have been no negative effects on the relevant parameters – especially the CO values – in spite of high waste water volumes, yet capacity has been increased by between 60 % and 100 %. Table 1 compares the typical parameters for operation with ordinary and O2-enriched air in a plant that is used to treat waste water polluted by pesticides. Thanks to oxygen injection, the plant’s capacity is increased while the specific primary energy consumption and emission of pollutants are reduced.
As with any other combustion process, the addition of oxygen during the incineration of waste water does not take place until operation with air, as the oxidation medium, has produced stable combustion conditions. Air, oxygen or a mixture of both can be selected as the atomising medium, depending on the actual load. The injection of oxygen into a rotary kiln and post-combustion chamber also has the desired effect when hazardous waste needs to be disposed of (Table 2). It is thus possible to achieve a higher performance, optimise the combustion result and reduce carbon monoxide peaks of hazardous waste with their fluctuating calorific values and air requirements.
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