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Apr-2013

Diagnostic tools and control parameters for refinery heaters

To optimise or manage furnace operation more effectively and safely, engineers and operators need to understand and control the entire process

YAHYA AKTAS¸, HAKAN OGˇUZ, GÜRAY AKSU, DOGˇUS¸UZUN and MUAMMER SEVER
Tüpras¸ Izmir refinery
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Article Summary
The basic principle of good combustion is to fully oxidise the carbon, hydrogen and sulphur in the fuel,  leading to the formation of carbon dioxide, water and SOx. Stoichiometric levels of oxygen should normally be sufficient to complete combustion.1
Improvement of furnace operation is very important to achieve efficient or continuous production, and safety issues have always been the priority for chief engineers in refineries.

Generally, for combustion in refinery heaters, there are two main parameters that affect the thermal efficiency of the entire combustion process:
•  Lowering the value of the oxygen content of the flue gas
•  Lowering the value of the temperature of the flue gas.

The thermal efficiency of furnaces can be improved by increasing heat recovery from the flue gases. Usually, a 20°C reduction in stack flue gas temperature corresponds to a 1% gain in efficiency. In addition, a 1% reduction in the total oxygen level leads to a 0.5% improvement in efficiency.

Reducing CO2 emissions is an absolute necessity and an expensive challenge for refineries to meet environmental targets according to legal requirements. In Izmir refinery, we are trying to decrease the production of greenhouse gases by making some operational changes and investments.

In order to reduce emissions of CO2, CH4 and O3, as well as energy demand, we undertook the following tasks:
•  Heat integration projects, for the significant reduction of emissions and for further energy savings
•  Installation of new preheat exchangers before the heaters, and online cleaning of heaters
•  Optimise the operation of an existing heater.

Detection and diagnostics
Combustion engineering is an intellectually challenging field with plenty of opportunities to enhance fundamental and practical knowledge that will ultimately lead to the development of new products with improved performance.3

Most refinery furnaces generally have a large potential for the reduction of stack gas temperatures. A reduction in temperature leads to an improvement in efficiency. Due to instrument failures in some furnaces, one can observe unrealistic low stack temperatures, indicating cold air intake. Detecting cold air leaks is also very important in the operation of furnaces. Sources of leaks include:
•  Heater header boxes
•  Peep holes
•  Non-activated burners
•  Explosion doors
•  Damper leaks
•  Breakdown of  air registers.

In practice, excess air is necessary to achieve complete combustion. This excess air is expressed as a percentage of the theoretical amount of air.2 Any excess air, above its theoretical value, causes a loss of energy in furnaces.

Stack gas temperatures are usually higher than target temperatures. Excess stack gas temperatures also lead to heat loss, thus decreasing the efficiency of the furnace. One of the most important actions to decrease stack gas temperatures is to constantly keep tube surfaces clean. This can be achieved by smart soot blowing and online cleaning while the furnace is fully operational. When these temperatures are monitored carefully by a distributed control system, any rise in their values can be eliminated by proper soot blowing and cleaning.

In order to increase the energy efficiency of refinery furnaces, there are inevitably some important parameters to consider:
•  Efficient combustion
•  Instrumented protection functions
•  Reducing heat losses
•  Waste heat recovery.

Daily inspection of furnaces is required to make sure that no flame impingement, smoky conditions or bad atomisation occurs.

It is advisable to carry out monthly oxygen measurements to check and compare the percentage of flue gas oxygen, upstream and downstream of the air preheater, to prevent any leakage occurring between combustion air and flue gas. Operation of the air preheater is very sensitive due to the presence of glass tubes. The wash water temperature should not exceed 80°C; otherwise, the tubes may break as a result of thermal shock. To achieve this, the parameters of the temperature indication controller need to be configured for monitoring by the distributed control system. The value of the flue gas temperature should not exceed the target value. Keeping flue gas temperatures below the target value for optimum charge levels also helps to maintain cleaner preheaters, thus minimising shutdowns.

In some cases, furnaces have unrealistically low stack temperatures, probably as a result of broken glass tubes in the air preheater; a furnace’s thermal efficiency will increase if broken tubes are replaced. A portable oxygen analyser will monitor fugitive emissions via induced draught fan sucking patterns.

Visual inspection of the furnace refractory via peep holes revealed the refractory to be in satisfactory condition, with only a few areas of minor damage. Nonetheless, it is necessary to take thermal photos of furnaces as part of regular maintenance procedures, as this helps to maintain clean tube surfaces.

During furnace combustion, when the percentage of hydrogen is higher than the design value in the fuel gas, flame impingement occurs in the convection zone. As a result, tube supports begin to redden. Fuel oil specification is also important to prevent high vanadium and sodium levels exceeding allowable limits.
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