Flue gas analysis is vital to ensure that combustion efficiency is maximised and production of greenhouse gases minimised, a point not lost on the manufacturers.
For its part, Rosemount Analytical's X-STREAM oxygen analysers are designed improve the efficiencies of large boilers and furnaces and provide immediate reduction of carbon dioxide (CO2) emissions with minimal investment.
Large industrial boilers and furnaces are primary generators of the greenhouse gas CO2. According to the company, X-STREAM O2 in-situ combustion flue gas analysers are a key tool for optimising combustion and reducing emissions - by providing information to control the fuel-to-air ratio of the burner, much in the same way it's done on today's automobiles.
The analysers have now been improved with AccuMax accuracy specifications, extended temperature capability, and new longer probe lengths of up to 18feet in order to satisfy the need of the industry's biggest boilers and furnaces (Fig.1).
According to Doug Simmers, worldwide product manager for Rosemount Analytical, large furnaces will typically have many burners delivering fuel and air into the combustion zone, which may be the size of a 10-storey office building. Balancing the burners maximises heat rate efficiency, and also minimises the potential damage to the furnace from flame impingement, tube leaks, excessive soot, slagging, and many other potential operational problems.
To achieve balance and avoid these problems, many facilities use permanently mounted O2 probes at different locations inside the flue gas duct and average the readings inside the control system to optimise the total furnace fuel-to-air-ratio. Readings from the O2 probes also provide an indication of the stratification of flue gases inside the furnace.
"Many traditional O2 analysers have had a fixed insertion depth, and engineers have had to guess at the best placement of the probes," Simmers explained. "The new longer probes from Rosemount Analytical not only reach further into these large ducts, but they now have the ability to be adjusted after installation so that the entire width of the flue gas duct can be characterised."
The X-STREAM line of combustion O2 probes offer long life in this difficult service, and are now designed to be repaired without removing the probe from the furnace duct. Additionally, they can provide indication of reducing conditions that may occur inside the furnace during plant upsets.
Another key feature of the new probes is ease of calibration, an activity that can be a challenging problem for plant personnel. The answer often depends on the fuels being burned. However, the new X-STREAM addresses this concern by providing an on-line diagnostic that determines when a calibration should be conducted, eliminating many unneeded calibrations, and the technician and gas resources they consume.
The X-STREAM electronics has an on-line impedance measurement for the sensing cell. This feature can trigger a fully automatic calibration by sequencing soleniods to introduce calibration gases to the sensing cell. The single probe sequencer (SPS) switches CAL gases to a single probe, while a multi-probe sequencer (IMPS) can handle 1-4 probes. Many needless calibrations based on 'time in service' are eliminated. A contact closure notifies the control room when a calibration is taking place. The oxygen output signal can be held at its last value, or released during calibration. The X-STREAM can also initiate calibrations by traditional methods such as contact closure from the user's control room and HART/AMS.
Another advantage is a process temperature range that goes up to 850°C. The X-STREAM oxygen analyser employs a heater and thermocouple to maintain a temperature setpoint at 736°C. Temperature control is maintained within +/-1°C to process temperatures of about 705°C. This is satisfactory for most applications, but excursions to higher temperatures can occur in many processes. In these instances, the heater is turned off and the process temperature is utilised to heat the sensing cell
The X-STREAM is mainly aimed at users of boilers, process heaters, kilns, recovery boilers and reheat furnaces.
Rugged and reliable
The new thermoparamagnetic oxygen transmitter XMO2 from GE provides computer-enhanced, drift-free oxygen measurement in a compact, rugged, online transmitter. It can be used wherever accurate and reliable long-term oxygen measurement is critical to process and product quality. This includes inerting/blanketing liquid storage tanks, reactor feed gases, flare gas, catalyst regeneration, oxygen purity and many other applications.
The e XMO2 is specifically designed to overcome the limitations of conventional oxygen analysers. It does not require analyser shack installation in a general-purpose area, but can be field-mounted at the process measuring point. This minimises sample conditioning requirements and ensures the best sample and the fastest possible response.
Unlike other paramagnetic oxygen sensors, the XMO2 has no moving parts, making it insensitive to mounting position or vibration and providing excellent long-term reliability. The novel, dual-chamber oxygen cell design insures insensitivity to contamination and flow variations and provides a zero and span stability that makes recalibration typically only a biannual requirement.
An on-board microprocessor enables the XMO2 to provide advanced online signal detection and conditioning, real-time error detection and automatic compensation of the oxygen reading for background gas and/or pressure variations. This results in a linearity of better than 0.5 per cent of span, a long-term accuracy of one per cent of span and a stability of 0.8 per cent of span per month. A proprietary fast-response software routine enhances the standard response of the thermoparamagnetic oxygen sensor, providing typical times of less than five seconds for a 63 per cent response.
The user-friendly XMO2 can be programmed in the field or remotely with application-specific data via an RS232 interface and three-level, menu-driven software. When recalibration becomes necessary, it can be accomplished quickly and easily through software, with no pots to adjust.
A complete line of performance-enhancing accessories make the XMO2 even easier to use, says GE. These accessories include custom-designed sampling systems and different styles of digital display packages.
The XMO2 transmitter requires 24-VDC power and provides an isolated 4-20mA output signal for oxygen concentration that is programmable for zero and span. It is available in either weatherproof (NEMA 4X, IP65) or explosion-proof (NEMA 7) enclosures and in ranges from 0-1 per cent to 0-100 per cent, making it suitable for a wide variety of installations and applications.
