2nd Meeting of the Oxy-Fuel Network

Publication Overview

The aim of this Network for Oxy-Fuel Combustion is to provide an international forum for organisations with interest in the development of Oxy-Fuel Combustion Technology to discuss various issues relevant to the development of the technology.

Publication Summary

The future directions for the development of the oxy-combustion technology would have three different legs namely:

• The short term development of the oxy-combustion technology would look at the enabling technologies that would be suitable for plant retrofit or new built; and would be totally based on the current conventional power plant equipped with new highly efficient and air-tight boiler.  This type of boiler could also allow operation of the boiler based on air firing mode. (i.e. Projects developed by Vattenfall, CS Energy/IHI and TOTAL)
• The intermediate term development of the oxy-combustion technology would look at enabling technologies that would build the next generation power plant purposely for oxy-combustion.  It could be perceived that this type of boiler would still be similar to the current conventional boiler but never planned for air firing. (i.e. CANMET development in HYDROXY Burner)
• The longer term development of the oxy-combustion technology would look onto new technologies that would be totally different from the current conventional boilers (i.e.  Chemical Looping Combustion, Praxair’s OTM membrane, CES technology).

For the short term development of oxy-combustion process, the following points could be highlighted:

• Validated simulation of the oxy-combustion process will be the key to allow plant retrofit with confidence. It is important to establish how to extrapolate what has been learned (i.e. sub-models developed) for an air fired case to oxy-fired case.
• There is still a need to develop simulation tools that would allow modelling of heat transfer, ignition, devolatililisation and char burnout kinetics, and ash partition and deposition.
• Particularly, there are still a lot of uncertainties in the understanding of impact of ash deposition to the heat transfer model.  This also includes the gap of knowledge in the gas to surface thermal resistivity.
• It could be identified that there is a critical gap of information on the ash deposition and its impact to the fireside of the boiler (i.e. corrosion).
• There is limited information on particle deposition and slag formation under oxy-combustion firing mode. 
• Critically, it is important to note that further data should be gathered to establish the relationship between flue gas recycling, impact of CO₂ rich environment, recycling of SOx and NOx to the deposition and slag formation (which are also dependent on coal types).
• There is already a good understanding in NOx and SO₂ formation (including sulphation of the ash).  However, there is still a need to obtain more data for SO₃, Hg and trace metal emissions.
• It is well accepted and established that recycling of flue gas containing NOx species could reduce NOx emissions (on mass emissions basis).
• The reduction in NOx was primarily attributed to the reburn mechanism.
• It could be established that there is about 30% reduction in sulphur emission due to higher sulphation rate of the ash promoted by the ash’s Ca content.
• There is still some uncertainty on the SO₂ concentration that can be tolerated in the recycle loop, and whether desulfurization before recycle is necessary for high sulfur coals.
• Likewise, there is some indication that SO₃ formation could be potentially higher during oxy-combustion.  But further data should be obtained to validate such an indication. It should be noted that understanding of SO₃ formation is important to the operation and estimating the performance of downstream units (i.e. ESP and Fabric filter operation – primarily its implication to the dew point temperature).
• The issue of air ingress is very critical to the viability of oxy-combustion process.
• The limit on non-condensables is driven by the limit on the physical separation process and the cost of separating these impurities from CO₂ during compression. (This would definitely impose the severe constraints on furnace inleakage or air-ingress for plant retrofit).
For the intermediate term development of oxy-combustion processes, the very focus of the research would be in the aspect of reduction (or elimination) of externally recycled flue gas. This could be achieved by use of steam as the attemperation medium or application of combustion techniques applied in the glass furnaces (i.e. high velocity jet burner / flameless combustion technique).
• The critical barrier to such developments are identified which are:
• Furnace / boiler materials selection.
• Understanding the aerodynamics of directed O₂ injection point.
• Simulation tools necessary to aid boiler / furnace developments are essential.
• Heat transfer that would allow flame temperature cooling (including the prediction of aerodynamics and temperature profile differentiated by high jet injection of oxygen and fuel).
The longer term development would probably involve the development of techniques and processes that could reduce the energy penalty from oxygen production. There are two main common development issues in any types of oxy-combustion technologies.
• To address the energy penalty related to the oxygen production
• To address the requirements for the CO₂ quality
The longer term development addressing the energy penalty due to oxygen production would include:
• Development in chemical looping combustion
• Development and integration of alternative oxygen production system
  • OTM (development undertaken by Praxair)
  • ITM (development undertaken by Air Products)
  • CARS (development undertaken by BOC / Linde)

Efforts in the development of the oxygen production could be subdivided into two directions:

• Near term development would look onto the improvement of current cryogenic distillation process:
• • What could be the optimum oxygen purity in terms of minimising the CAPEX and the OPEX?
  • What are the potential for integration with the boiler island and the CO₂ processing unit?
  • What could be the permissible maximum capacity of current ASU cryogenic technology? (i.e. Could we have a single train ASU that could produce greater than 5000 TPD O₂?)
• Medium to long term development would look into the development of various breakthrough technologies: (It should be noted that this leg of development would be a merger between the long term development of oxy-combustion boilers / furnaces and oxygen production). Breakthrough technologies include:
• Development in membrane technologies (i.e. ITM, OTM, CARS)
• Chemical looping combustion process (no oxygen production required)

It is important to discuss the issue of CO₂ purity. It should be noted that concentration limits for on-site storage are uncertain, both technical and regulatory.

• Would the CO₂ rich products be considered as a waste? or as a resource?
• What are the requirements of the CO₂ storage site with regard to the major impurities (i.e. O₂, N₂ and Ar)
• What are the different technical issues involving the removal of minor impurities (i.e. SOx, NOx, Hg and other trace metal)?
The compression and condensation process proposed by Air Products is considered one of the most elegant solutions for CO2 processing presented during the workshop. It is important to note that possible reaction between NOx and SOx during the compression stage of the CO2 rich product is considered an important learning. However, the reaction mechanisms and capture efficiency of these minor impurities should be further verified.”