As the effectiveness of the convection bank drops, stack temperatures increase, leading
to greater stack losses and greater energy consumption as shown in Figure 2, with every 20°C increase in stack temperatures resulting in roughly a 1% increase in stack losses at normal excess figures.
Figure 2. Stack Losses [%] against Stack Temperature (4)
Typically, ethylene plant cracking furnaces are designed with convection section heat
recovery using a BFW preheat economiser as the coldest service. This results in a stack gas
temperature of about 150 °C when the coils are clean, but heavy fouling can result in stack gas temperatures as high as 235°C (3).
In 2005, Foster Wheeler calculated that external fouling can reduce the efficiency of a
fired heater with a duty of 60.5MW by 5.9%, increase the stack temperature by 100⁰C and result in incremental fuel costs of €944,000/p.a. or around €15,600/MW/pa (5).
The run-up in energy prices over the following years has increased the economic impact
of convection bank fouling. 5mm outer dust or scale fouling in both the radiant and convection banks of a small-sized fired heater with 18MW fired duty, results in energy losses of 4.518GJ/h and annual losses of around €614,454/pa or €34,144/MW fired/pa (6). Although simulations are very design-specific, this gives a rough indication of the financial losses that can result from the external convection bank fouling of fired heaters.
While fouling in fired heaters occurs in both the hot and cold sides of fired heaters and
on the hot side, it mostly occurs in the convective section (7). Modern designs such as the use of finned convection tubes mean that it is often possible to attain heat flux in the convection section that is comparable to the radiation section (8), putting greater emphasis on the need to ensure that convention banks are kept free from fouling.
Another issue is that elevated stack temperatures result in the Induced Draft Fan
operating at a higher than intended volumetric flow. While such fans typically are designed with a capacity margin of 20-25%, when convection banks are extremely fouled, the fan may be operating at its limit, restricting throughput by around 1-2% (4).