Features Research & Innovation
Preheated combustion air: take full advantage of your heat recovery system

Economics is increasingly the keyword in today’s paper manufacturing industry. Cutting operational costs, while maintaining quality output and maximizing production, is every papermaker’s quest. Unfortunately, this goal will become more difficult to attain as years go by. The main limiting factor is the increasing cost of energy. Some highly renowned scientists even maintain that with current levels of use, the earth will be drained of its natural resources (uranium, petroleum, natural gas, etc.) by the year 2050.Without being alarmist, it is common sense to maximize the amount of heat recovery from the process.

This article covers a relatively new and seldom explored way of minimizing energy cost on Yankee hoods process air systems.

System description

A typical Yankee hood air system consists of two recirculation fans, an exhaust fan, two burners and two combustion blowers. Often, each half of the Yankee hood will have its own recirculation fan, burner and combustion blower, while sharing the same exhaust fan.

Nowadays most gas-fired hoods operates at supply temperature of 800-950F. Typically, exhaust air will be at temperatures of 700F with humidity of 0.45 lb water vapor per lb of dry air. The latent heat contained in the exhaust air makes heat recovery very attractive. Moreover, the single source of heat and relatively small exhaust air volume allows for a compact heat recovery system. This is why most of the gas-fired Yankee hood air systems are equipped with an air-to-air economizer. As air is exhausted out to atmosphere, it goes through a heat exchanger (economizer). The heat recovered is used to pre-heat the make-up air introduced in the system to “replace” the moist exhaust air. Pre-heating the make-up air alleviates the burner load and reduces fuel consumption. The heat recovered will vary depending on the make-up air flow and exhaust temperature, but on average, 2.0 MMBtu/h can normally be expected. A typical arrangement of this set-up is shown in Fig. 1.

Until recently, the heat recovery on Yankee hood exhaust was limited to make-up air pre-heating, sometimes coupled with additional economizers for water heating. However, in Europe, installations have for quite some time spread the use of the heat exchanger by also pre-heating combustion air. In North America, this new arrangement is seen more and more often as a simple and economical way to lower energy costs. The combustion air volume is relatively small and a 25% gain in heat recovered can be attainable by going to pre-heated combustion air system, Fig. 2.

The difference from “conventional system” is that the source of the combustion air will be drawn through the heat exchanger. Typically, ducts will branch out from a make-up air duct to feed the combustion blower. The main consequence is that combustion air density will change during system warm-up. Once the burner is lit, the exhaust air temperature slowly increases, driving the combustion air temperature up gradually, thus lowering the density of the combustion air. Because the combustion blower is a volumetric device (i.e. delivering the same volume [CFM] regardless of air density), the combustion blower will deliver the same CFM and a lower mass flow as it handles warmer air temperature. Therefore lower gas flow will be required to maintain the same air/gas ratio to the burner. This is why a mass compensation control will be required to control combustion air throughout the warm-up cycles.

Figure 3 illustrates a mass compensation system. It is basically a control device that measures the combustion airflow and its temperature, and calculates the resulting air mass. Its output controls the gas flow and continuously maintains the correct ratio between air and gas throughout warm-up heat cycles. Air and gas flow measurement is taken using vena contracta ports across an orifice plate, hooked on to a differential pressure transmitter.

Modern controllers are quite handy to use as they display various useful information on the combustion system, all combined in one small unit. Typical readings include gas flow, airflow, air temperature, air/gas ratio, process demand (from DCS or PLC), alarms, etc. Additional benefit of this control device is that it provides enhanced process monitoring.

Design considerations

There are important factors that must be taken into account when designing this type of heat recovery, either for a new installation or a retrofit system.

First, the fan construction has to be suitable for at least 500F (260C). Glass fibre blowers aren’t suitable for this application. Bearing selection must also be appropriate and a cooling wheel must be added. Given this consideration, it would be advantageous to have the blower located upstream of the air/air economizer, blowing room temperature air through the heat recovery unit and supplying heated air to the burner. However, in most cases it is impractical to operate the fan in blow-through. The main reason is that the air/air economizer mixes fresh air fluids together, including make-up air, which operates at different downstream pressure range than combustion air. If fresh air fluids (make-up and combustion) could be separated in the heat exchanger, then combustion airflow control could be done upstream of the economizer and no mass compensation would be required. Most air-to-air heat exchangers do not lend themselves to fluid separation and to the required level of static pressure resulting from a combustion blower.

Secondly, the fan and intake ductwork has to be properly insulated and cladded so duct skin temperature is within an acceptable range. This has the advantage of providing sound attenuation. Pressure blowers are usually noisy, as they operate at 3500 rpm.

If the source of pilot air is taken off the combustion ductwork, special care should be taken in setting the pilot on the burner. The burner pilot must be set-up so that it will operate smoothly with both cold and hot air. It is a better set-up to use an independent source for pilot air, even for a conventional burner system. A compressed air line provides constant air pressure and, most importantly, a clean air source. With the amount of dust inside a tissue mill, it is not surprising to witness pilot plug-up.

Emissions levels are not normally a concern on Yankee hood burners. Pre-heating combustion air does have the effect of increasing NOx levels, because of the higher flame temperature. This higher value still remains well below maximum allowable level, in the range of 80 ppm.

Finally, duct run must be carefully designed, if both make-up and combustion air travel together in the same duct. Location of make-up connection branch must be such that the combustion blower cannot draw its air from the recirculation duct through the make-up connection. The recirculation air is low in oxygen content (less than 19%), and this lack of oxygen would flame out the burner.

Economic considerations

The additional cost for pre-heating combustion air will depend on several factors such as fan quantity and proximity of economizer to combustion fan. For a system requiring two 10-MMBTUH units, the differential cost of material between conventional and pre-heated combustion air can be summarized as follows (in US dollars):

1) Combustion controller

2 x $4500 = 9000

2) Orifice plates

2 x 300 = 600

3) Diff. press. transmitter

4 x 750 = 1500

4) Temperature transmitter

2 x 750 = 1500

5) Insulated ductwork

2 x 10000 = 20 000

6) Diff. fan material

2 x 7500 = 15 000

Total $47 600

With a nominal natural gas cost of US$3.50/MMBtu, a 2-MMBtu/h heat recovery represents 7$/h savings, or $60 000 per year — or a 10-month payback.

Combustion blower motors must still be selected to handle full load in cold conditions, as they are started up before the system warms up. Although they handle hot air with a lower density, the blowers will have to displace a higher volume in order to provide the same air mass for combustion. Consequently, the operating motor load will be the same as if the air is not pre-heated.

Conclusion

Yankee hoods are significant energy consumers of the tissue-making process. Potential savings in air system energy consumption should not be neglected and heat recovery should therefore be maximized. Pre-heating combustion air has been successfully used in European mills and on recent North American installation. It offers additional benefits in process monitoring and has no tangible effect on the environment. It can be easily retrofitted into existing installations. It should be given more considerations as fuel costs escalates and affect our economy.P&PC

Eric Vanasse is Yankee product manager for Enerquin Air Inc.