1 May 2005
Savings boil over
By Jane Arnold
Chemical plant saves over $1 million by better fuel management.
In the manufacturing world, there is a strong effort toward improving fuel efficiency and lowering emissions. Natural gas prices have risen from $2 per MMBTU in January of 2000 to $6 per MMBTU, with peaks of up to $10 per MMBTU. The federal and state environmental agencies are also narrowing allowable emissions each year. The federal NOx reportable quantity (RQ) is now only 10 lbs over the permitted limit for each piece of equipment that emits NOx.
Therefore, it is imperative that operating units make every effort to lower natural gas usage and minimize emissions. Fired furnaces and boilers are the leaders in NOx production. Companies are spending millions of dollars each year reducing NOx by installing Selective Catalytic Reducers (SCR) and Non-SCRs, such as ammonia injection. In order to fully reduce NOx, it is imperative to improve the instrumentation and optimize controls prior to hiring consultants or buying technology.
A Waste Oxidizing Boiler (WOB) is a boiler that burns waste fuels created in the manufacturing process. Run time is very critical on a WOB. Any trip could require an immediate shutdown of related process equipment to avoid an RQ on NOx and other Volatile Organic Compounds (VOCs).
Define, correct problems
In the year 2000, Sterling Chemicals had 27 trips on its three WOBs. With new environmental regulations coming in 2002, it was imperative to correct the problems. The new allowable limits on NOx and other VOCs would have forced an immediate shutdown of the reactor train on a WOB trip that seriously cuts into production and increases costs.
Another environmental change on the primary Waste Oxidizing Boiler (A-WOB) was a change in the Boiler and Industrial Furnaces (BIF) limits. A-WOB is a BIF unit because it can burn liquid waste. A-WOB had been running under a Certificate of Compliance (COC), which is interim status for permitting. The state called in the BIF permit and required a Destruction and Removal Efficiency test (DRE). The control temperature needed to increase in order to meet the new requirements for 99.99% destruction. The WOB required a new burner in order to meet the required temperature.
To solve the WOB trips, Sterling Chemicals initiated a program to recalibrate each instrument and check associated wiring or replace the necessary instrumentation to improve field device reliability.
Since the largest number of trips was a direct correlation to control, the company evaluated the WOB controls for improvement.
To give a little background, the control of A-WOB is more complex than a typical utility boiler due to the burning of three different waste fuels, whereas a utility boiler just burns natural gas and produces steam.
In A-WOB, there are three exothermic fuels including natural gas, and one large volume, endothermic fuel that is combusted. Waste fuel A (WF-A) has a high heating value (HHV) of 10,122 BTU/lb.
Waste fuel B (WF-B) has a HHV of 6,509 BTU/lb. Both WF-A and WF-B flow rates are rate dependent and vary significantly based on process conditions. The off-gas is a large volume gas that has a HHV of 315 BTU/lb.
Due to the multiple dynamic variables, the A-WOB ran on local automatic or manual control. This mode required operations to pay close attention to the WOB at all times. Sudden adjustment in fuels sometimes caused the boilers' steam drum level to become unstable, resulting in drum level trips.
In the original control scheme for A-WOB, total air and natural gas ran in local automatic. Operations would watch the reduction and oxidation zone and change the set points for gas and air. The reduction zone temperature control occurred by adjusting the amount of off-gas fed into the reduction zone. Natural gas injected into the primary off-gas to add heat value. The oxidizing zone temperature cooled via hot condensate if the temperature approached the high temperature trip. The steam drum level control was a typical three element level control with steam and level demand fed to the boiler feed water remote set point. With the different variables running on manual or local automatic, it was very difficult for operations to maintain consistent control.
In order to minimize NOx formation and maximize hazardous waste destruction efficiency, the WOB should operate in staged combustion. The first stage is the reduction zone, and it operates fuel rich at about 85% stoichiometric air. The second stage is the oxidizing zone, and it operates at fuel lean.
Managing the combustion process in multiple stages at a lower temperature minimizes the NOx formation compared to single stage oxidation.
Operations would watch the reduction zone temperature and the oxidizing zone temperature of the WOB and adjust air and gas manually. Running the WOB in manual is extremely difficult due the varying BTU and flow rate of the various fuels. When the reduction zone is fuel lean, adding more fuel causes the reduction zone temperature to go down, while the oxidizing temperature goes up. If operations were watching the wrong temperature during an adjustment, the WOB would quickly become unstable causing the WOB flame to pulse. The pulsing happens when there is not enough oxygen in the reduction zone and the flame appears to shrink and grow in pulses while burning in the oxidation zone. The instability of the WOB often caused a trip on flameout.
The following outlines all of the control improvements made on A-WOB.
Fuel management is the automatic modulation of air and gas using high/low signal select control and stoichiometry to ensure the proper mix for optimum combustion. The primary control point for A-WOB fuel management is the oxidizing zone temperature. The temperature runs in automatic.
It feeds a remote set point (RSP) to the primary air and the natural gas loops. The calculations occur in a Logic Control Point (LCP) on the DCS system. The LCP allows for a Function Sequence Table (FST) for advanced programming.
The LCP control schematic for fuel management.
You can find the sum of total fuels by using stoichiometry, which is the molecular weight calculation of fuels, to ensure there is enough air for complete combustion. Calculate the total molecular weight, and then multiply it by the stoichiometric ratio to determine how much oxygen you need for complete combustion. The stoic ratio is the percent of O2 present in the reduction zone. One hundred percent stoic means that for every mole of fuel there is one mole of oxygen for complete combustion.
The following calculation shows how to determine the amount of air needed to combust natural gas at 85% stoic:
F1 = F2 / 17.337 * 2.050 * 0.85 / 0.21 * 28.930 (1)
F1 = Primary air to reduction zone in lb/hr
F2 = Natural gas flow to burner in lb/hr
17.337 = Molecular weight of natural gas
2.050 = Moles of O2 required for every mole of natural gas
0.85 = Stoichiometric ratio.
0.21 = Percent of O2 in air
28.930 = Molecular weight of air
A-WOB reduction chamber runs sub-stoic during normal operations. Sub-stoic means the reduction zone will run with less moles of O2 than moles of fuel. A-WOB primary (reduction) zone will run between 60-85% stoic to give true two-staged combustion. Less oxygen in the reduction zone will reduce the amount of NOx out of the stack due to an increased residence time. You can call the concept 'fuel rich' or 'air lean.'
The temperature demand (loop output) feeds to the high and low signal select blocks. The blocks > and < either act as the high or low depending on temperature demand increase or decrease. The high/low signal select control is for fire prevention and comes when you have air leading on temperature demand increase with fuel lagging and fuel leading on temperature demand decrease with air lagging. Normally, natural gas runs on temperature demand, and primary air runs on stoichiometry control. During a temperature upset, when the temperature demand increases, primary air will increase above the stoichiometric set point. When there are changes in waste fuels and temperature demand changes, the natural gas set point may incur limit with the amount of air in the reduction zone. These limits or overrides are set to ensure there is always enough oxygen for proper combustion and to maintain flame quality.
Another change to fuel management on A-WOB was removing natural gas injection to the primary off-gas stream in the reducing zone of the WOB. The old burner was not capable of generating the required BTU to reach the new BIF temperature limit. An attempt at natural gas injection into the off-gas stream occurred as a way to achieve the required temperature, but it was not very successful due to control problems. The new burner installed at the reduction zone inlet, as required by BIF, was a 180 MMBTU burner and allowed enough natural gas to go into the WOB at the burner to achieve the desired temperature.
On the level
The level control in the steam drum on the WOBs at Sterling Chemicals is different than a typical utility boiler. The purpose of a utility boiler is to produce steam. Steam is a by-product of waste destruction in a WOB. Therefore, the WOB pressure just rides the plant 650 lb steam header and does not try to control it.
A classic 3-element level control manages the boiler level. A v-cone replaced the flow orifice on steam, and they added temperature compensation to provide mass steam flow measurement and improved Boiler Feed Water (BFW) response. Pressure was not in the mass compensation because the pressure affect on a v-cone is insignificant.
Ft = Temperature Correction Factor; multiply by flow for mass flow
T = Flowing temperature of gas or liquid in deg F
t = Conversion to absolute temperature (0'F = 459.69' Rankin)
Tc = Meter calibration temperature in deg F
The purpose of boiler feed water control is to maintain steam drum level at set point. Steam flow is a feed forward set point to boiler feed water adjusted by the drum level controller. The amount of BFW is dependent on the steam flow out of the boiler with the level controller working to correct any difference between the two transmitters.
The Logic Control Point contains the following equation to calculate the Remote Set Point (RSP) to the BFW flow:
RSP BFW = (Level demand output (IVP) – 50) / 100 * BFW Scale + STM FLOW (3)
RSP BFW = Remote Set Point to Boiler Feed water in lb/hr
Excess oxygen control
The primary measurement for excess O2 is the combustion O2 analyzer. When the analyzer loop runs in automatic, the O2 controller establishes a remote set point to the secondary air HIC valve. The secondary air is in a 48" line. Therefore, the analyzer loop has a clamp on the output from 15% to 35%, which minimizes air flow to the oxidation zone. Without the clamp, the HIC could rob the reduction zone of primary air and cause an unstable environment in the WOB. When the HIC is in manual, it can be stroked full range, 0-100% and the analyzer loop tracks the HIC for bumpless transfer.
The oxidation zone is the second of a two-staged combustion. More oxygen in the oxidation zone completes combustion and minimizes the amount of Carbon Monoxide (CO) out of the stack, which the government also tightly regulates. Less oxygen minimizes the amount of NOx emitted in the stack. Therefore, it is imperative to maintain tight control on oxygen to keep down CO and NOx.
Running the WOB on fuel management and closing all the loops allowed operations to run at a much lower temperature control set point and lower excess O2. The changes resulted in lower natural gas, lower NOx, and increased on stream time. The reduction of trips on A-WOB from 17 trips to two results in a savings of over $420,000 for lost production time.
Lower temperature and oxygen set points combined with improved controls significantly reduced the amount of natural gas needed to keep A-WOB at control temperature. There is a reduction of 2,000 lbs per hour reduction of natural gas to A-WOB while not burning waste fuels. At $6 MMBTU, this reduction is a savings of over $400,000 per year. Additional savings in natural gas occurs while burning waste fuels because fuel management will automatically back out natural gas if it does not need it to maintain temperature.
Lower temperature and oxygen set points also significantly reduced the amount of NOx emitted from A-WOB stack. NOx reduction was over 80 lbs per hour on A-WOB. This improvement allowed for a delay in purchasing an SCR or NSCR for A-WOB.
Similar results occurred on B-WOB and C-WOB after the implementation of improved control. The total savings is in excess of $1 million dollars a year in natural gas and lost production. NOx also went down by over 40% for the three WOBs. And just as important, the company achieved this savings without any significant capital expenditures.
Behind the byline
Jane Arnold is a senior process control analyst at Sterling Chemicals in Texas City, Texas.
Oxygen content key
It is crucial to maintain enough oxygen to fully combust fuels. The following control scheme maintains a constant supply of air regardless of the amount of fuels present. As the primary and secondary air valves move, the discharge pressure on the blower changes. A pressure loop ties to total air with a speed safety override.
The Low Signal Select (<) shown above selects the lowest value between total air and speed. Normally, it is in flow control unless the speed is />= overspeed setpoint (SP). The speed override will hold the output until the flow SP drops and flow control resumes. Overspeed protection is necessary to keep the blower from tripping on high speed. Minimum air flow occurs at the speed controller. Operations can not move the speed controller below a minimum of 3200 revolutions per minute. Therefore, the control scheme can not inadvertently trip the blower on low speed.
Total air control