Decarbonisation Technology May 2022 Issue

rate by mass or volume. If the plant measures flow rate by volume, 3X more hydrogen (by volume) is needed to achieve the same heat release as methane at the same pressure. If the plant measures flow rate by mass, then the hydrogen mass flow rate will be 3X less than the mass flow of methane for the same heat release (at the same pressure) because of the differences in heating value. Piping and burner nozzles sizes should be evaluated to ensure they can handle the required change in fuel flow and to avoid material embrittlement and unexpected pressure changes. In addition, hydrogen requires 20% less combustion air than methane to achieve the same heat release. While burner adjustments can be fixed when permanently switching to a new fuel, these differences in heating value and combustion air requirements have implications if the hydrogen content of the fuel varies over time and/or if the burner switches from natural gas to a high hydrogen fuel, which may be the case in using natural gas during light-off and switching to hydrogen fuels during normal operation. For this reason, it is vital that operators monitor the combustion process and ensure the burner always has adequate combustion air, and flue gas analysis is one approach to safe monitoring of these burner-related parameters. Critical threefold role of flue gas analysis and its measurements for safe combustion In any combustion process, it is necessary to monitor the inlet flow rates of fuel and air to the burner. While these flow rates provide baseline parameters to set a flame, they do not provide feedback to reveal or alert any potential concerns with the combustion reaction, such as incomplete combustion from imperfect mixing in the burner or safety risks such as fuel leaks or loss of flame. Flue gas analysis offers one approach to monitoring the process and providing feedback using measurements made in the flue gas. It is especially important to consider when firing high hydrogen fuels. That said, flue gas analysis plays three critical roles in the combustion process:  The operational set point for the air-to-fuel ratio

 The role of fuel efficiency and optimisation  The detection of unsafe conditions. Each role will be highlighted along with its corresponding measurement. Together, the measurements ensure combustion safety, proper operation, and fuel efficiency. Role of excess oxygen setpoint As the first, most critical role, flue gas analysis provides the operational air-fuel ratio setpoint at the burner using the ‘excess oxygen’ measurement. To sustain a flame, a sufficient amount of air must be diverted to the burner to consume all the fuel. This balance is represented by the air-fuel ratio. In practice, the burner must have enough ‘excess oxygen’ in the flue gas to ensure safe combustion control. Unlike the ‘total oxygen’ concentration in the flue gas, the ‘excess oxygen’ measurement is unique in that it correlates directly to the air-fuel ratio. This excess oxygen level refers to the amount of oxygen present after all the combustible content in the stream is consumed; hence, it monitors the ‘excess’ of the remaining oxygen. This measurement is also referred to in industry as the ‘residual oxygen’ or ‘net oxygen’ reading. It is also important to note that a 0% excess oxygen reading means there is no safety margin of air to the burner, and this presents an unsafe condition if held for too long. When it comes to setting the air-fuel ratio, the excess oxygen measurement is an operational setpoint to ensure that the burner operates with sufficient ‘excess’ oxygen at all times. In most cases, operators set the excess oxygen setpoint anywhere between 1% and 5%, depending on the fuel type and the variability of the fuel composition over time. If the reading is too low, more combustion When it comes to setting the air-fuel ratio, the excess oxygen measurement is an operational setpoint to ensure that the burner operates with sufficient ‘excess’ oxygen at all times