Autoignition of Hydrogen and Syngas with Air in a Turbulent Flow Reactor

Open Access
Author:
Elies, Daniel Jason
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
September 11, 2012
Committee Members:
  • Robert John Santoro, Thesis Advisor
Keywords:
  • autoignition
  • flow reactor
  • syngas
  • hydrogen
  • premixed
  • ignition delay
Abstract:
A good deal of attention has been given recently to combustion of syngas in gas turbines used for power generation. Syngas is a mixture of hydrogen and carbon monoxide produced from coal gasification, a process where coal is partially oxidized producing a gaseous product with high concentrations of hydrogen and carbon monoxide. Although coal gasification is not a new technology, recent interest has been spurred by concerns about climate change due in large part to increased levels of carbon dioxide in the atmosphere. Coal-fired power plants in 2011 produced approximately 46% of the electricity used in the United States, but contributed 79% of the energy related carbon dioxide emissions. The reason that coal contributes such a large portion of the carbon dioxide emissions is that the carbon to hydrogen ratio of coal is high as compared to other hydrocarbon fuels. As a result, more of the energy released from burning coal comes from the oxidation of carbon rather than hydrogen, which increases the amount of carbon dioxide emitted per kilowatt hour. To reduce these high carbon dioxide emissions, the use of syngas as fuel is part of the "Clean Coal" effort that will use carbon sequestration to remove carbon dioxide from the combustion products and store it underground. One of the challenges of using syngas as a fuel is the variable composition in terms of the amount of hydrogen and carbon monoxide present in the fuel. This variable composition results from the wide variety of coal that can be used to produce syngas. These compositional variations alter the combustion characteristics of syngas. Additionally, present gas turbine technology for power generation utilizes lean-premixed conditions to reduce oxides of nitrogen formation, that is fuel and hot air from the compressor are premixed prior to combustion. If premixed syngas and air was to ignite in the gas turbine premixer, severe damage would occur. Consequently, one of the combustion characteristics of particular importance is the autoignition time. Autoignition is a measure of the time for a mixture of fuel and oxidizer at some elevated temperature to spontaneously ignite. Thus, the present study specifically addresses measurements of the autoignition time for hydrogen and hydrogen/carbon monoxide mixtures under conditions relevant to gas turbines used for power generation. Experiments were conducted using a turbulent flow reactor for the purpose of examining autoignition times for hydrogen and hydrogen/carbon monoxide mixtures with air. Experiments with only hydrogen as fuel were conducted at experimental conditions including ignition delay times of 130 and 210 ms, equivalence ratios of 0.375 and 0.750, and pressures of 10 and 15 atm. Temperatures could be varied between 800 and 900 K using a combination of an electric heater and a hydrogen and oxygen fueled preburner. Experiments were also conducted with a mixture of hydrogen and carbon monoxide, to simulate Syngas, at a pressure of 15 atm, an ignition delay time of 130 ms, and at equivalence ratios 0.375 and 0.75. The temperatures that resulted in autoigntion of hydrogen at the above conditions ranged between 840 to 890 K. The temperatures that resulted in autoignition for the syngas experiments were 8 to 23 K lower than those observed for hydrogen at the same pressure and equivalence ratio conditions. Previous experiments conducted at similar conditions using hydrogen and hydrogen/carbon monoxide mixtures in turbulent flow reactors reported much shorter autoignition times. Furthermore, these earlier studies showed significant disparities, as much as 1 or 2 orders of magnitude, between predictions of autoignition using homogeneous chemical kinetic models and measurements of autoignition time. The results of the current study show agreement with the homogeneous chemical kinetics model within at most a factor of five.