Yankee hoods installed in the past are very often equipped with burners designed to guarantee drying capacity and high reliability, but with no specific target about efficiency and emissions.
In the last few years, energy and environmental issues became important and drove great design changes. Now it is more correct to speak about “combustion systems” and not simply “burners”. A combustion system is made by: the burner itself; combustion chamber; control system (in particular the control of combustion air flow following variable .0).
The design of the whole combustion system must guarantee the compliance with highly restrictive rules nowadays in force in Europe, North America, Australia. Furthermore, in addition to Country regulation, there are local rules that sometimes require special characteristics, mostly in terms of pollution emissions.
Novimpianti is a primary supplier for the paper industry worldwide
Novimpianti is frequently asked to update existing combustion systems, in order to improve thermal efficiency and respect all restrictions to the exhaust. In this case the first step must be an investigation, to understand the current equipment, fix all targets and help mill management to choose the best strategy. Of course there are many parameters to be considered, like:
» Existing burners and combustion chambers characteristics;
» Maximum necessary power (avoiding useless oversizing);
» Available combustion air temperature (some burners can accept more than 300°C);
» Impact in air system layout;
» CO and NOx limits (e.g. 50mgCO /Nm3 @17%O2);
» Burner turndown (usually from 10:1 to 20:1);
» Combustion air-to-gas ratio control;
» In-line or corner burner;
» Process air injection into corner burner combustion chamber.
Combustion chamber – latest developments
Combustion chamber geometry has been modified over time. Combustion chamber “Separated flow type” concept developed by Novimpianti features the flame pipe completely separated from process flow (picture at the top of this article). In this way we can protect the flame from cold process air, up to the complete oxidation of natural gas; furthermore, we completely avoid any risk of endplate overheating, with quick deterioration of burner and combustion chamber. In our last installations, the system is capable to inject a controlled process air flow into the flame pipe, reducing fresh air amount and optimizing CO and NOx emissions.
Air-to-gas ratio control for a significant gas saving
Air-to-gas ratio plays a key role in emissions control and thermal efficiency: high excess of air means lower flame temperature (i.e. high CO values) and limited humidity content in exhaust. With high amount of fresh air the system cannot work at 0.8 – 1 kgw/kgDA, values that can be reached in more stoichiometric conditions. As commonly known, increasing from 0.4 kgw/kgDA to 0.8 kgw/kgDA means reducing exhaust dry air flow to 50%, with immediate gas saving as shown picture 2.
There are a lot of installations, especially in Europe, with fix combustion air. This means combustion air flow isn’t connected to burner output (like often happens in case of in-line burners). In other cases one actuator opens and closes both gas valve and combustion air valve, thanks to a rigid mechanical connection.
Now there are solutions with electronic connection between gas and combustion air dampers, managed by a safety PLC that takes care of the complete burner control system. It means that safety PLC is connected to all burner devices, from pressure switches to gas valves, from purge procedure to flame control. The result is an intrinsically safe and trouble free system.
Corner and in-line burners
Use in-line or corner type can have a big impact in system lay out. In picture 3 a typical corner burner with its “swirl” flame and in picture 4 a standard inline burner with fix combustion air, very common in old installations. Replacing an in-line burner with a corner one can be complicated, sometimes only possible with largescale rebuilding in burner room, i.e. high cost and long machine shut down.