Improving Furnace and Convection Equipment
Furnace engineering services are vital to the proper design and reliable operation of flue gas heat recovery in furnaces used
in Chemical Industry. Ammonia, Methanol, Hydrogen, Refining and Specialty Chemicals plants use furnaces and integrated
radiant and convection heat recovery equipment to supply chemical reaction heat and preheating of process and utility
streams used to run the plants. The furnace and convection heat recovery equipment is a large investment, employing
highly complex process design engineering and technologies for flue gas heat transfer to maximize combustion efficiency.
This article describes a base case furnace operation for an ammonia plant, with further improvements resulting from
engineering audits of performance, subsequent coil cleaning, tuning combustion efficiency, upgrading convection coils for
improved performance, and upgrading existing combustion air preheating equipment for improved capacity and efficiency.
Ammonia, Methanol, Hydrogen, Refining and Specialty Chemicals plants use furnaces and integrated radiant and convection
heat recovery equipment to supply chemical reaction heat and preheating of process and utility streams used to run the plants.
The furnace and convection heat recovery equipment is a large investment, employing highly complex process design
engineering and technologies, typically involving several fuel combustion systems further combined with heater coils for flue gas
heat transfer to maximize combustion efficiency.
An example overall heat and material balance for a furnace and convection heat recovery system is shown in Figure 1 (on Page
2) for an Ammonia plant, operating at approximately 1800 Short Tons per day. In the example shown, the Ammonia plant has
been substantially expanded from original capacity of about 1150 STPD to about 1800 STPD for the furnace calculations
illustrated. The plant was expanded in capacity over a period of many years, with both large and small improvement projects.
The original furnace efficiency was about 91.4 % LHV, with flue gas of 300 degrees F. The existing furnace, operating at
expanded plant capacity has deteriorated to 87.5% LHV efficiency with flue gases near 400 degrees F at 2.7 mole %, (dry
basis), exit the combustion air pre-heater before combustion air leakage at the ID Fan. The deterioration in furnace thermal
efficiency has occurred for several reasons, including greater heat and mass transfer loads on the convection coils, insulation of
heat transfer surface due to fouling, mechanical damage or blockage resulting in coil bypassing, or from mal-distribution of
fluids within coils in a tube bank.
All of these causes of performance deterioration can be reduced with resulting improved plant performance with engineering
design and further capital investment. For instance, the original design coil outlet conditions at design capacity were: Mixed
Feed Gas Coil, 950 degrees F (vs 852 Deg F current), High Temperature Steam Superheat, 860 Deg F (vs 836 Deg F
current). Other coil operating differences from original design are impacted from the addition of Superheat burners, firing into
the convection section of the furnace to improve power steam superheat, which results in greater steam turbine capacity, also
enabling higher plant production. (These various coils would operate at cooler outlet temperatures, without the use of the
added Steam Superheat burners for the plant, as shown.)
Combustion, heat transfer and rigorous coil design and evaluation calculations are quite complex and usually not developed
unless a furnace and convection recovery system is having problems or needs investigation for improvement. Such convection
heat recovery equipment commonly lasts about 20+ years when operated properly and well maintained. However, furnace
coils do fail for a variety of reasons, and when this occurs, consideration should be given to upgrade for for performance
improvement, rather than "replacement in-kind."