This point presents the history, technical aspects and advantages of the High-cycle Regenerative System (HRS) combined with the High Temperature Air Combustion (HiTAC) technology.

History

Since the beginning of the 90's of the last century almost 800 furnaces has been revamped or built as new units using the regenerative burners, mainly in Japan. About half of them, it means about 400 furnaces, utilized the HRS regenerative burners with HiTAC combustion technology invented and developed by Nippon Furnace Kogyo Kaisha Ltd. (NFK).

First industrial application that used HRS burners took place in 1992. Since that time over two thousand HRS burner pairs have been installed in several industry applications. HRS system has been applied for example in the following areas:

• reheating furnaces – 56 applications with installed firing power up to 93,0 MW (38 pairs of HRS-DL Burners),
• ladle furnaces – 6 applications with installed firing power up to 3,2 MW (2 pairs of HRS-DL burners),
• annealing, tempering, carburizing and other heat treatment furnaces – 61 applications with installed firing power up to 15,6 MW (180 pairs of HRS-RT Burners),
• furnaces for ceramics – 8 applications with installed firing power up to 1,7 MW (50 pairs of HRS-U1 Burners).

HRS Burners

The unique features of the HiTAC combustion technology have been successfully applied in the special HRS Burners. The HRS burner concept depends on the High Temperature Air Combustion with high performance regenerative heat exchangers.

Application of the HRS burners using the HiTAC technology provides the following advantages:

• flat heat flux distribution,
• flat temperature distribution,
• low emission of NO_x due to lack of temperature peaks,
• possibility of decreasing fuel consumption,
• lower average temperature in the zone, due to propagation of the combustion process over large volume,
• ability to increase zone capacity, due to making possible an increase in the zone temperature,
• higher refractory lining lifetime due to lack of temperature peaks,
• low noise.

In the HRS burners / HRS combustion systems several techniques are applied in order to achieve the required advantages. The techniques are as follows:

The HRS burner system may operate in two ways: the conventional high velocity combustion mode (F1) and the HiTAC mode (F2). A schematic drawing of the F1 and F2 combustion modes are shown in figure below. During the heat-up the furnace burner works in the F1 mode, however, always as a regenerative burner. When furnace temperature exceeds 800^oC, gas is supplied through F2 nozzles and the burner starts to work in the F2 mode.

Schematic of typical work HRS burner during F1 mode and F2 mode

High-cycle Regenerative System work principle

The idea behind the system is that two burners work alternatively . When burner A works as a burner (the firing mode), burner B sucks the exhaust gas from the combustion chamber (the regenerative mode). The burners change their functions after the switching time. Burner maximum thermal efficiency for such regenerators is achieved during 30-second switching time.

Scheme of the HRS burners pair operation

The HRS burners pair DL- series with regenerative bed called "the honeycomb"

In the regenerative mode the burner draws flue gases through it’s internal regenerative bed and sensible heat is stored in the ceramic Honeycomb whereas cold flue gases are discharged into the flue piping and further to the stack. In firing mode combustion air fed through the burner passes through it’s regenerator where it preheats by recovering stored heat.

Honeycomb superiority

Regenerative heat exchange hardware has a long and outstanding history in reducing industrial processes fuel demand. Excellent thermal properties of ceramic materials, both as a heat storage and heat exchange mediums, give it a clear superiority over common steel heat exchangers. While due to material limitations metallic heat exchangers are limited in thermal capabilities and efficiency, ceramic regenerators allow higher temperatures and heat transfers. Ceramics in many cases demonstrate better resistance against aggressive compounds as well.

Among different types of ceramic regenerative beds the Honeycomb stands out. Ceramic nuggets and balls are both surpassed by the Honeycomb in terms of temperature efficiency, temperature stability, heat exchange surface per volume and mass, lower pressure drop over the bed, lighter weight and thermal inertia.

The HRS burners enclose a regenerative bed called “the honeycomb”, made of ceramics resistant to high temperature of flue gas. The high performance heat exchanger allows combustion air of ambient temperature to preheat up to the temperature close to the sucked flue gas temperature during the regenerative mode of the burner.

Comparison of ceramic honeycomb and ball A typical honeycomb used as a regenerative medium has 100 cells per square inch. This great number of cells per square inch ensures the following features of the regenerative heat exchanger in comparison to the conventional ball type regenerator:

• high specific surface area equal to 1307 m²/m³, about 7 times bigger than in the case of the ball type (a ball diameter – 20 mm),
• high equivalent heat transfer rate per volume equal to 165 kW/m³*K, about 5 times bigger than in the case of the ball type,
• low unit weight - about 3 times less that in the case of the ball type,
• low unit volume - about 5 times less that in the case of the ball type. This factor causes that burners are compact and easy to install, especially during furnace revamping.
• short optimum switching time equal to 30s - the time where the highest regenerative efficiency is obtained. It is about 2 to 4 times lower compared to the ball type. Short switching time results in small fluctuation of the preheated air temperature.
• low pressure drop, about 3 to 4 times lower than in the case of the ball type,
• no problems with plugging due to the construction of the honeycombs (lack of flow dead zone).

Advanced heat recovery

Distributed recuperators and regenerators (like is applied in HRS burner) both share the advantage of being less demanding towards auxiliary burner piping, as lower temperature flue is released from burners and at the same time obtain the best efficiency.

Honeycomb’s bed temperature efficiency can be described by:

Actual efficiency attainable varies from over 90% up to 96% with supplied air to furnace contents temperature difference of 50^oC.

Excellent fuel efficiency, fuel savings, downsized furnace, increased furnace capacity

The HRS technology was created for fuel savings and successfully meets this goal. High efficiency of the honeycombs results in fuel saving after the revamp of furnaces. The highest level of fuel saving takes place when the furnace before revamping is equipped with a poor recovery system or even does not have one (the savings can reach even about 50%).

The fuel savings for the typical furnaces are on the range of 20% - 35% because of standard preheated air temperature is on the range of 300^oC - 450^oC and furnace exit exhaust gas temperature is on the range of 900^oC - 1100^oC.

Due to high heat release overall better characteristics compared to conventional burners, HRS systems allow downsizing furnaces or improve furnace capacity. Typically both furnace capacity increase as well as unitary fuel consumption reduction can be achieved. Fuel saving as a function of furnace exhaust gas temperature

Ease of deployment and maintenance

Burner setup is easy and adjustment to existing conditions is uncomplicated. The HRS burners do not require complicated maintenance schemes. Burner maintenance is limited to periodic replacement of parts of the Honeycomb bed. Typically after two years of operation top four layers of the Honeycomb bed have to be replaced, however, it depends on process conditions. Replacement after two years of operation are recommended to get reliability of the system, often depending on the combustion and furnace conditions the honeycomb bed can work longer without replacement.

Ease of control

Through simple construction and automatic control system operating the HRS system is easy and integration with an existing system is fast and smooth. Firing power rate can be managed online with both Flow Control and Time Interval Control methods. Burners can be switched on and off during system’s work. All burners on a furnace heating system can operate with a custom switching pattern to regulate power distribution between furnace zones or between burners in a zone. The burners working range covers full 0 – 100% firing power range, individual units as well as the whole system can be operated in stand-by mode at hot conditions without fuel supply.

Summary

A continuous increase in ecological consciousness and high pressure to reduce the energy cost in many areas within the companies result in the high interest of industry in applying the newest combustion technology. Thus, every type of new combustion technology has to guarantee reduced energy consumption of the process, low pollutant emissions, a furnace capacity increase, the combustion process and product quality improvement and, at the same time, reliability and dependability in industrial conditions. The presented unique features of the High-cycle Regenerative System (HRS) make this technology the best solution to reach the all goals which are listed above.