In-situ monitoring system is safe, accurate.

When Minnesota first issued regulations requiring continuous monitoring of emissions in waste-to-energy plants, the city of Red Wing’s WTE plant faced a dilemma. The plant, the first municipally owned WTE facility in the upper Midwest, which opened in 1982, was designed to help reduce the volume of municipal solid waste being landfilled in an environmentally safe manner. It accomplished the first goal, decreasing the amount of landfilled waste by about 85 percent when measured by weight and an even greater percentage by volume.

The second, however, proved to be tougher.

Although concentration levels of most primary pollutants in the plant’s leachate were actually below drinking water standards, sulfide and chloride levels needed to be reduced. And several years ago, the state Pollution Control Agency amended the facility’s permit to require continuous monitoring of carbon monoxide and carbon dioxide emissions.

Despite the fact that the facility is owned by the city, it is run like a private business via an enterprise account. Twelve people operate the facility, which burns waste with minimal processing after it comes off garbage trucks and produces 15,000 pounds of steam per hour that is then sold to a nearby leather tanning company. Profits are currently being set aside to meet anticipated tightening of state and federal air emissions requirements.

A lack of technical expertise led the city to put out an RFP that left recommendations to prospective vendors. Of four proposals received, three followed a conventional extractive approach that requires pulling samples out of the stack, running them through turbines and drying, filtering and measuring them at a remote location.

This process is complicated because the equipment must be maintained at a relatively constant temperature in a climate that varies from 100 [degrees] F in the summer to 40 [degrees] F below zero in the winter. Thus, the line running from the probe to the analyzers requires a heated jacket, and the analyzer itself has to be housed in a climate-controlled shed. In addition to being maintenance-intensive, this adds to the price of monitoring.

The fourth proposal was for an in-situ system that would measure gas concentrations by flashing an infrared beam through the stack.

The beam would reflect off a special mirror, making it possible to put the source and detector in the same enclosure on one side of the stack so the system could be calibrated while the plant is operating. The system, proposed by Air Instruments & Measurements, Baldwin Park, Calif., uses gas filter correlation absorption spectrometry that measures both the sample absorption wavelength and an adjacent reference wavelength. The ratio is independent of light level, depending only on the concentration of the gas present. Multiple gases are measured sequentially with the same unit. This approach eliminates the equipment required to transport the gas samples to a remote locale, thus reducing the price and maintenance costs by about half.

The analyzer was mounted on the side of the stack exiting the 60-foot-high electrostatic precipitator, which is easily accessible via steps and a catwalk.

It is protected from the elements by an air-tight cover and has a built-in cooling system.

The in-situ analyzer provides higher accuracy than extractive analyzers because it avoids sample contact and measures the average of the entire stack composition rather than a single point in the stack. The response of the unit, which can be measured in milliseconds, also is much faster than an extractive unit. The unit continuously monitors gas concentration at three-second intervals and averages over an eight-minute period.

The analyzer is programmed to calibrate itself every four hours. The system includes an internal zero mirror that can be moved into the beam on demand, intercepting the beam and reflecting it back to the detector before it goes through the stack. There is no sample in the beam so it measures zero. This makes it possible to introduce a cell with a known concentration of gas into the beam.

This article was written by Bruce Klair, plant manager, Red Wing, Minn., Solid Waste Boiler Facility.