Thermal flow meter development over the last couple decades has been largely driven to address the needs associated with environmental applications, including acid rain.
Continuous Emissions Monitoring (CEM) for Acid Rain
Thermal flow meters are a relatively new product, having been introduced in the 1970s and 1980s. Like any new product, developments and advancements are usually dictated by the needs of the industry. Early in the product’s life, the need arose for continuous emissions monitoring (CEM) to address the acid rain problem.
Acid rain is an environmental issue that received much attention in the 1990s. It affects large portions of North America by harming lakes, streams, and forests, as well as plant and animal life in these environments.
What is acid rain?
Acid rain refers to rain that has deposited material in it. It can be wet or dry material, but has higher than normal levels of nitric and sulfuric acids. The causes of acid rain stem from both natural and man-made sources. Natural precursors are volcanoes or decaying vegetation while man-made sources include burning fossils fuels. Fossil fuel combustion creates emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx). A significant amount of SO2 (~67%) and NOx (~25%) in the United States comes from power generation, particularly coal.
When SO2 and NOx react with oxygen, water and other chemicals present in the atmosphere, sulfuric acid and nitric acid are formed. These acidic compounds can be carried hundreds of miles from the sources of the precursor emissions by typical winds. When the acidic chemicals in the air are taken to areas where the weather is wet (wet deposition), the result is acidic rain, snow or even fog. If the compounds travel to an area where the weather is dry (dry deposition), the chemicals incorporate into smoke or dust and stick to buildings, cars, the ground or trees. Later the chemicals are washed away with rainfall, and the runoff becomes contaminated with the acids.
Consequences of Acid Rain
Acid rain (whether wet or dry deposition) causes the acidification of our lakes and streams which damage trees and compromises the sensitive balance of our forest soils. It also quickens the decay of building materials, paints, sculptures and statues. Additionally, acid rain derivatives, such as sulfates and nitrates, can be fine particles and when inhaled can cause heart and lung disease.
Thermal Flow Meter
To address the growing concerns of acid rain, in the 1990s, the EPA implemented the Acid Rain Program, which created a need for continuous emissions monitoring (CEM) and required the measurement of SO2 and NOx. By measuring flow rate, as well as the concentration of SO2 and NOx, the acid rain precursor emissions can be measured. Thermal mass meters are ideal for this application and are among the three meter types, aside from averaging Pitot tubes and ultrasonic flow meters, to successfully handle this application.
While the acid rain issue is a major environmental application for thermal mass flow meter success, a greater need was created with the acceptance of global warming, perhaps a topic for a future post.
Perhaps you will find this video helpful on acid rain and coal combustion.
Photo by Origins.gif:NHSavage at en.wikipedia derivative work: Zazou (Origins.gif) [Public domain], from Wikimedia Commons