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Jul-2011

Controlling CO boilers

UOP Callidus supplies and installs complete systems and various subsystem components for CO boilers and other FCC related ancillary equipment.

Kurt Kraus and Minwoo Kwon
UOP Callidus
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Article Summary
The exhaust emissions from the CO boiler must be controlled reliably and predictably to meet operating company requirements. Principal constituents for control in the exhaust emissions include carbon monoxide (CO), oxides of nitrogen (NOx), sulphur oxides (SOx) and particulate matter. The design of UOP Callidus CO combustors ensures complete conversion of CO to CO2 while minimising NOx emissions with post-combustion treatment of sulphur and particulates.

The UOP Callidus CO combustor
The Fluidised-Bed Catalytic Cracking (FCC) process is the principal means of converting low value heavy oils into more profitable gasoline and lighter products in petrochemical refineries. During the cracking process, carbon coke is deposited internal and external of a catalyst support structure thereby reducing the catalyst activity. To regenerate the catalyst in a continuous process, the FCC regenerator burns the residual carbon from the catalyst. Air is injected in the regenerator converting the coke from the catalyst to CO and CO2. The air also is used in controlling the temperature of the resulting regenerator flue gas. Depending on the regenerator design, a significant portion of the carbon may not be fully reacted to CO2 in a “partial burn” process. The partially burned residual carbon requires further processing, or combusting, before being vented through a stack. The CO combustor with boiler is used to oxidise the CO to CO2 to meet environmental regulations while facilitating the recovery of valuable energy from the flue gas.

Combustion air and fuel control
The combustion air control scheme for a CO boiler is fundamentally different from that of a conventional industrial steam generating boiler. In the industrial boiler, steam supply is the independent control variable. Increased steam demand requires increased air and fuel flow into the boiler to maintain a constant combustion excess air set point while the bulk flue gas temperature is allowed to vary. However in the CO boiler, all of the CO in the flue gas stream delivered to the boiler must be burned thereby requiring a relatively constant combustion chamber temperature. The steam output from the boiler becomes a dependant variable tied to the input requirements necessary to ensure complete combustion of the CO in the flue gas stream.

While the CO content of the flue gas stream from the FCC unit may vary rapidly and considerably over time, the total mass flow of the flue gas stream is relatively constant and slower to change. To respond quickly to the changing CO content of the flue gas stream, make-up, auxiliary or supplemental fuel gas is precisely modulated. By modulating the make-up fuel gas, the heat input from fuel into the boiler is maintained relatively constant.

Meanwhile, the combustion air is modulated to principally control the bulk combustion chamber temperature and secondarily to maintain the proper excess air limits.

The combustion air and fuel control is the key for the control logic for CO combustor control. The main oxidiser (combustor) must be controlled on temperature to ensure complete CO destruction. To achieve complete CO destruction and to meet environmental regulations, a temperature well above the auto-ignition temperature of all constituents is used as the design normal operating temperature set point. As the CO combustor temperature decreases below the set point, the output of temperature controller, TIC, increases, calling for additional combustion air flow. In a “lead-lag” configuration, based upon the additional air flow measured, the fuel gas flow is increased, which ultimately increases the thermal oxidiser temperature. The controller will automatically adjust fuel flow to maintain the set point temperature in the combustor section. Similarly, upon a decrease in heat demand, fuel flow is reduced first, followed by combustion air.

A density analyser, often used to measure fuel gas specific gravity, may be provided to correct for varying fuel gas composition. It is essential to control the ratio of the fuel gas and combustion air to the main burner for NOx control. This analyser is sometimes used with a compensation tool to provide an accurate flow rate. Combustion air is compensated with temperature and pressure to provide the most accurate flow rate to the combustor.

An oxygen analyser is also used to monitor the flue gas O2 level to make sure that there is enough O2 in the combustion chamber. Any excess combustible or hydrocarbon could result in unacceptable after-burning in the CO boiler section. It is very important to keep the O2 level to a preset minimum level. If the oxygen analyser is used to control the air flow rate, it should be used only for override not for trim. If oxygen is added in the temperature control loop as controlling variable, the temperature controller might not be stable as CO flue gas combustible contents will vary.

There are two different combustion air inlets to this CO combustor application, one for primary combustion air and the other for secondary combustion air. Typically, the primary combustion air inlet is dedicated to main burner control. The primary air control loop is tied with the main fuel gas flow meter, controlled by ratio control to maintain and insure the stability of the main burner flame. The secondary air inlet maintains the constant air flow rate to the entire system. As CO flue gas heat content (amount of CO or combustible) continually varies, it is very difficult to control the air with the CO flue gas. The best way to handle this is to set the air flow rate in the Distributed Control System, DCS, by the Hand Indicate Control, HIC. The DCS operator sets the total air flow rate while the primary air flow rate is controlled by fuel gas flow rate. The secondary air flow rate is set for CO flue gas combustion and temperature control purposes. The combustion air for CO regenerator flue gas is introduced through this secondary air flow inlet and into the multipoint injection point with specially designed inducing pipes for proper mixing. Figure 1 shows a simple diagram for a typical CO combustor control scheme. 

CO flue gas injection and refractory selection
Due to presence of SOx and catalyst particles in the flue gas, the refractory lining must be carefully selected for each combustor zone. The most important criteria for the selection will be the erosion factor due to the high velocity and thermal conductivity in the primary combustion zone. Typically, the primary combustion zone is where the velocity and temperature is the highest inside the combustor. Catalyst fines also could cause some refractory failure on the high velocity zone. The FCC CO flue gas injection port is designed to achieve good mixing with the combustion air. The design provides rows of multi-point injection points giving the incoming air and regenerator flue gas a vigorous turbulence creating an intense mixing atmosphere in the combustion chamber. This resulting intensive mixing of regenerator flue gas and air ensures complete oxidisation of combustibles in the flue gas. For this reason, refractory must be selected with high erosion resistant material. Improper selection of the refractory could lead to sulphuric acid corrosion of the steel shell.
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