Turn Waste to Energy Without Wasting Energy

1 Feb, 2017  |  Written by  |  under Combustion, Conductivity Analysis

by Sara Wiederoder, Product Manager, Rosemount Combustion Products, Emerson

Hello. My name is Sara Wiederoder and I’m your Analytic Expert for today. I’d like to share an interesting and innovative waste-to-energy application in which a number of Emerson products are used including the Rosemount OCX8800. The company in this application is Sustainable Waste Power Systems (SWPS) and they’re building a garbage in/power out (GIPO) system, or more specifically, a two-stage, wet, thermal conversion of wet carbon-based waste into synthesis fuel gas (SynFuel). In the system, a devolitization stage reduces the wet feedstock into a bio-char and light volatiles. A large pressure drop between the stages causes fluidization and pulverization of the flow. Gasification completes the fuel synthesis through the water-gas shift and cooling of hot oil and process systems provides thermal power to the customer.

One of the key challenges of the system is to provide stable burner and air control. In general, it can be said that the concentration of excess oxygen is one of the best indicators of how efficient a combustion process is. Industry quickly discovered that if you do not have excess oxygen in your flue gas (indicating there is too much fuel), there is a good chance your boiler will explode, so excess air/oxygen is required. Adding too much air/oxygen will cool down the combustion process, which is undesirable since combustion is being used to produce thermal energy, and the more you cool it, the less heat you can get out of it. Typically, combustion processes are controlled between 2-5% excess oxygen. The actual value varies upon the type of fuel. Gaseous fuels combust very efficiently and quickly, so there is less excess oxygen required for optimal efficiency. However, solid fuels don’t combust as well, so adding extra oxygen ensures the solid particles are fully combusted. Having a continuous measurement of oxygen, and inputting this data into a control system that automatically controls air flow to the combustion burner, ensures the combustion process is operating at optimal efficiency.

The Rosemount OCX8800 Combustion Flue Gas Transmitter provides a continuous, accurate measurement of not only the oxygen, but also the combustibles remaining in flue gases from a combustion process. The renowned zirconium oxide sensor is the basis for the oxygen measurement. This, combined with the combustibles sensor, detects oxygen and combustibles concentrations in flue gases with temperatures up to 2,600°F (1,427°C).

For this application, complete and efficient combustion within a very tight set point was crucial. So in addition to oxygen, carbon monoxide (CO) measurements give greater detail into the current condition of a combustion process. CO is an indicator of un-combusted fuel. When processes are short on oxygen it’s usual to see a lot of CO, but it’s normal to have small concentrations of CO right near the optimal combustion set point. Right around the point of greatest efficiency, it’s typical to see trace amounts of CO around 200 ppm. Using both CO and oxygen measurements gives the user greater control over their combustion process.

The solution in the SWPS application included:

  • A burner safety control panel that utilizes a pass-through 4-20mA signal once safety sequences are complete
  • Configuration of PID for fuel valve with furnace temperature as process variable feedback
  • Configuration of PID for air valve with O2 sensor as process variable feedback
  • Using the easy “manual track” feature in the built-in PID loops, which made the changeover to automatic control seamless
  • Feed forward adjustment to air valve output as factor times change in fuel valve output
  • A burner system that is very stable without the need for air or fuel mass-flow instrumentation

Ultimately, the commercial scalability of the GIPO was proven including:

  • High Total Carbon Conversion Efficiency > 90%
  • High Quality SynFuel Gas – 80% Fuel Gas Fraction
    • 16-30% CO2 (maximum)
    • 10-20% Methane
    • 30-45% Hydrogen
    • Balance Carbon Monoxide
    • No Tar Production

While your application may not resemble this unique GIPO system, talk to our analytic experts about achieving efficient combustion in your demanding applications.

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