Page 16 - Industrial Plant 2020

others as per burner manufacturers proprietary content over the full rangeability of the burner. However,
design; CO breakthrough can occur when operating outside of
• Systems to prevent the NOx release into the the burner capacity range: at overcapacity conditions
atmosphere: de-NOx systems (SCR, NSCR) are or below turndown, mainly due to loss of flame stability,
available on the market for any kind of firing system. incorrect mixing of fuel and air, inaccurate velocity, or
improper staging.
The required NOx guarantee value depends mainly on
local laws and regulation or on the refinery emission In order to reduce the CO content emissions, it is
thresholds. These can be estimated and provided by suggested to adjust the operating conditions by
the burner and heater manufacturer on the basis of the controlling the combustion air or properly setting up the
specific service, burner type, fuel and comburent combustion variables.
properties. These kind of changes in operations can have an
The CO content in the flue gas depends only on impact on the heaters’ performance but solve the
combustion quality with the correct mixing of fuel and emissions challenge.
comburent.
Process Study
CO emissions from a fired heater Wood was requested by one major client to perform
During combustion reaction, carbon and hydrogen technical evaluation to improve the performance of
combine with oxygen to produce carbon dioxide and many existing fired heaters. A thorough and diversified
water. When combustion is complete, no carbon study was carried out to address and propose various
monoxide is formed; if the combustion is incomplete, solutions, both in terms of emissions and efficiency.
part of the carbon is not fully oxidized, producing The analysis covered the refurbishment of many old
primarily carbon monoxide. heaters to improve the efficiency, increase the capacity,
change the service, reduce the coke formation and
The CO emission limit values are stated by API reduce the emissions.
Standard 560 - Fired Heaters for General Refinery
Service: Wood was requested by one major
“For floor-fired heaters, the CO content at the bridge client to perform technical evaluation
wall shall be a maximum of 40 ml/m3 (40 ppm, by to improve the performance of
volume) for gas-fired heaters, or 80 ml/m3 (80 ppm, by many existing fired heaters
volume) for oil-fired heaters, at maximum design firing
conditions.” To reduce the CO emission of heater B-2, an upgrade
of the combustion air control system was proposed.
The CO content in flue gases depends mainly on In addition the thermal rerating of the existing heater
excess air sent to the burners and on the correct was performed with the aim to modify the heater
mixing of fuel and air to produce the required configuration considering the current operating
combustion reaction. conditions, to improve the combustion conditions and
The carbon monoxide concentration is inversely to reduce the CO content in the flue gases as much as
proportional to the excess air level and increase of CO possible. CO emissions were too high with respect to
is linear but slow. At a certain point, called “CO the strict local laws.
breakthrough”, a further drop in excess air will produce The heater B-2 was rerated based on the operating
a sudden increase of CO. data from DCS (Distributed Control System) which was
Burner manufacturers can guarantee very low CO collected during a couple of site visits and the available

B-2 heater DCS data Calculated values

Radiant duty MW N.A. 3.445
Excess air % N.A. 55
Oxygen in flue gases % 8.0 – 10.0 8.0
Flue gas temperature in radiant section °C 421 432
Burner heat fired MW --- 0.260
Burner design capacity MW 0.969 ---
Table 1: Heater B-2
rerating results Burner turndown MW 0.232 ---

IndustrIal Plants - May 2020
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