HRS
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 90s, almost 800 furnaces have been revamped or built as new units using regenerative burners, mainly in Japan. About half of them, approximately 400 furnaces, utilized the HRS regenerative burners with HiTAC combustion technology, invented and developed by Nippon Furnace Kogyo Kaisha Ltd. (NFK).
The first industrial application that used HRS burners took place in 1992. Since then, over 2,000 HRS burner pairs have been installed across several industries. Applications include:
- 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 heat treatment furnaces – 61 applications with installed firing power up to 15.6 MW (180 pairs of HRS-RT Burners).
- Ceramics furnaces – 8 applications with installed firing power up to 1.7 MW (50 pairs of HRS-U1 Burners).
HRS Burners
The unique features of HiTAC combustion technology are applied in special HRS Burners. These burners rely on High Temperature Air Combustion with high-performance regenerative heat exchangers.
Advantages of using HRS burners with HiTAC technology include:
- Flat heat flux distribution.
- Uniform temperature distribution.
- Low NOx emissions due to the absence of temperature peaks.
- Reduced fuel consumption.
- Lower average zone temperature due to distributed combustion across a large volume.
- Increased zone capacity by allowing higher zone temperatures.
- Improved refractory lining durability due to uniform temperature profiles.
- Low noise operation.
 Scheme of the HRS-DL type burner |
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The HRS burner system operates in two modes: conventional high-velocity combustion (F1) and HiTAC mode (F2). During the heat-up phase, the furnace burner operates in F1 mode. When the furnace temperature exceeds 800°C, gas is supplied through F2 nozzles, and the burner transitions to F2 mode.
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| Schematic of typical HRS burner operation during F1 and F2 modes | ||
High-cycle Regenerative System Work Principle
The system alternates between two burners. While burner A is in firing mode, burner B operates in regenerative mode, drawing exhaust gases and storing heat in its ceramic regenerative bed. After a set time (typically 30 seconds), the burners switch functions. Maximum thermal efficiency is achieved during this cycle.
 | Scheme of the HRS burners pair operation |
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 recoverySeveral heat recovery solutions are in widespread use, varying in efficiency and usage characteristics. Most common centralized metal recuperators supply preheated air at up to 600oC are by far the least efficient. Central regenerators demonstrate better efficiency and higher preheat temperatures, up to approximately 900 – 950oC. |
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:
nta – air side temperature efficiency
Tao – outlet air temperature
Tai – inlet air temperature
Tgi – inlet flue gas temperature
Actual efficiency attainable varies from over 90% up to 96% with supplied air to furnace contents temperature difference of 50oC.
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 300oC – 450oC and furnace exit exhaust gas temperature is on the range of 900oC – 1100oC.
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.
