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Advancements in gas-fired process heating systems

Supplier: Hurll Nu-Way
14 October, 2014

Modern burners need to meet energy efficiency requirements.Energy saving not only allows you to do your bit towards saving the planet, it will also save you a significant proportion of your manufacturing costs

The basic process in heating applications, whether they are Ovens, Furnaces or Boilers where whatever enters the process will be heated to the process temperature and will then leave the process carrying energy off to atmosphere. The essence of energy saving is to minimize what enters the process, so reducing what has to be heated, or to pre-heat anything entering the process so reducing the energy needed to heat it. Using the exhaust products leaving the process to carry out the preheat will harvest energy otherwise lost to the system, reducing losses and improving the efficiency of the process.
Energy saving not only allows you to do your bit towards saving the planet, it will also save you a significant proportion of your manufacturing costs. As energy costs have increased over time so the $ savings resulting from improved efficiency have also improved, increasing the Return on Investment, reducing Payback Periods and making energy saving initiatives economically viable.
Product quality must be an over-riding check on the benefit of any energy saving project. It is of no benefit to save 15 per cent of the energy used in a process but produce poor quality products as a result. Scrapped products waste 100 per cent of the energy used in their production! Brick making is a good example of this problem. The Oxygen and Carbon Monoxide levels in a brick kiln will affect the colour of the final product. It is necessary to sacrifice energy efficiency during parts of the firing cycle to create either Red Bricks (excess air) or Blue Bricks (excess fuel).
As fuel costs rose and the need for better energy efficiency grew burners and their control methods evolved to satisfy that need. Improvements were made in three inter-related areas:
  • Burner Heads 
  • Control valves 
  • Energy Recovery
We can examine each of these aspects of the combustion system separately.
Burner Heads
Nozzle Mix burners (where the combustion air and fuel remain separate until they come together in the burner at the nozzle) tended to rely on high excess air levels to cool the burner and to promote good mixing of the air and gas, particularly at low firing rates. This was not an issue when fuel prices were low. If operating temperature or Product requirement demanded low Oxygen levels in the combustion products (i.e. low excess air levels) then a Premix Burner would be used. (Premix systems use a separate mixer where the air and gas are thoroughly mixed together before being delivered to the burner nozzle). If the process did require Premix Burners then their limitations of reduced turndown due to blowbacks inside the nozzle would have had to be accepted.
As fuel prices increased Nozzle Mix burner designs were improved so that they could accommodate lower excess levels, even excess fuel operation, while retaining the Nozzle Mix Burner’s flexibility and wide turndown. Lower combustion air pressures reduced the combustion air fan’s motor size and so reduced its capital cost and running cost. These changes were initially generated in Europe where fuel prices were (and still are) much higher than the USA. High Fire air pressures were reduced from 7 kPa that was common, to 3 kPa or less without loss of turndown.
The flexibility of the Nozzle Mix design allows the discharge port of the burner to be restricted forcing the combustion products to exit the burner at high velocities (up to 152 m/sec). This velocity creates turbulence in the combustion chamber, promoting rapid and uniform convective heat transfer and a saving up to 20 per cent of the energy used by conventionally fired furnaces. But this type of operation creates back-pressures in the refractory tile (quarl) and associated operational issues. The development of the Silicon Carbide tile overcame these issues.
Remember to only operate the burner within its design parameters. There are many burners still available that require high excess air operation to ensure good combustion and internal cooling, and they do an acceptable job on low temperature applications. In addition to possibly causing damage to the burner due to overheating, limiting the excess air could produce Carbon Monoxide (CO). A burner operating well with 10 per cent excess air but producing 500ppm CO will lose 0.64 per cent efficiency due to the CO. This relatively small number equates to a fuel wastage of more than 4200 m3/year on a 3 MWt burner.
Control Valves
Alongside the development of the burner head, came the development of the associated control valves.
Initially burners tended to be controlled by butterfly type air and gas valves mechanically linked together and operated by a single lever. The discharge profile of a butterfly type valve is not linear so the ratio of air flow to gas flow could only be maintained over the turndown range of the valves if both the air valve and the gas valve are the same make/model and size and have a similar pressure drop across them. This is very unlikely to be the case since the burner will need at least 10 times the air flow as the natural gas flow, and there is a substantial difference in the Specific Gravity of air and natural gas. So burners tended to be controlled High/Low with the associated effect on chamber uniformity and product quality.
Valves were developed that allowed their open port area to be trimmed at multiple points between the “Valve Closed” and “Valve Open” positions. This allowed the gas flow to be trimmed to the air flow at those points and created a proportional air/gas ratio control valve that allowed modulating control of the burner. Modulating control allowed closer control to set point, saved energy and improved product quality.
But these valves were expensive, difficult to set in the field, high maintenance, and could not allow for distribution differences on multi-burner furnaces. So an alternative was sought and the close ratio control devices long used with pre-mix burners were considered. Typically gas is presented to the Mixer of a Pre-Mix system at 0 kPa pressure via a Zero Pressure regulator. The correct amount of gas is then drawn into the mixer by the suction created by the combustion air flow. If the burner is firing into a positive pressure the restricting effect of this pressure on the gas flow is overcome by running a loading line from the chamber to the Zero Pressure regulator which then will deliver the gas at 0 kPa + the chamber pressure. This concept was applied to Nozzle Mix burners, a Zero Pressure regulator fitted in the gas line is loaded by the combustion air pressure (i.e. the pressure downstream of the air control valve). As the combustion air pressure increases or decreases as dictated by the control instrument, the gas pressure downstream of the Zero Regulator follows suit resulting in Proportional Ratio Control. The original Zero Regulators were refined over time to become the Ratio Regulators in use today, but their function remains the same.
The Ratio Regulator assumes that the Combustion Air pressure is directly related to the Combustion Air flow. This is not always the case (if a variable chamber pressure exists for example). An effective solution would then be the Differential Pressure Ratio Regulator which was introduced later. The differential air pressure generated across an Orifice Plate or Venturi in the air line is used by the double diaphragm Differential Pressure Ratio Regulator to control the differential pressure across an orifice in the gas line. This will control the air and the gas flows at the required ratio, irrespective of any changes in backpressure.
Also available is a micro ratio control option that is a feature of the latest generation of flame safeguard programmers (suppliers include Honeywell, Fireye and Siemens). Separate air and gas control valves are fitted with independent actuators. At the commissioning stage these valve/actuator assemblies are each set to the correct air/fuel ratio at multiple points throughout the burner’s operating range. This ensures close control of the air/gas ratio throughout the turndown range. Features such as enabling burner start up at rates other than minimum allow the burner turndown range to be extended. Some systems allow the exhaust oxygen level to be monitored and the relative positions of the air and gas valves to be biased to compensate for changes in operating conditions that may affect the ratio (increasing ambient air temperature for example).
These systems control the volume of air and gas supplied to the burner. The burner is actually interested in the mass of oxygen and gas being delivered and the volumes are used as an indicator of those masses. Where the system has variables that disconnect the close relationship between volume and mass (such as combustion air or gas preheat) then a more sophisticated system is required – Mass Flow Control. A Mass Flow Control system is computer based and will meter the flow of air and gas to a burner while simultaneously measuring their pressures and temperatures. The metered flows are corrected for temperature and pressure and the flows compared to the selected ratio. The flows will then be trimmed as necessary and an exhaust O2 monitor used to check the ratio.
Accurate air/gas ratio control is a very effective energy saving device. Reducing the excess air level from 30 per cent to 5 per cent on a furnace exhausting at 10000C will reduce the gas usage by more than 20 per cent.
A modern ratio control system allied to a modern burner allows us to reduce the amount of air that we supply to the burner and that in turn gets heated up then thrown away. However, even with the tightest ratio control, 79 per cent of the combustion air supply is not required – that is the Nitrogen. The only component of the air we actually require for combustion is the Oxygen. If the system could exclude the Nitrogen entirely then the energy saved on our furnace operating at 10000C would be 40 per cent compared to a conventional burner operating with 5 per cent excess air. Of course the effect of the much higher flame temperature on the product, the refractory insulation of the furnace and the burner itself, as well as the much lower volume of combustion products affecting heat transfer, would require a complete redesign and rebuild of the system at significant cost. Oxygen enrichment however – increasing the oxygen level in the combustion air by say 5 per cent - would retain most of the standard system benefits but would still save more than 15 per cent of the conventional system energy usage. It is worth considering when there is a low-cost oxygen supply available.
Energy Recovery
The ideal use for recovered energy is on the application that produces the energy in the first place, because then the production of the energy and the need for the energy will always coincide. Using recovered energy from a furnace to heat water for example may result in the hot water being available when not required (necessitating storage) or being required when the furnace is not running (necessitating a back-up heater). Using recovered energy for space heating may result in the recovered energy not being used through the summer so extending the payback period for the project. Beneficial though those uses may be, using the recovered energy from a furnace to preheat the load entering that furnace or the combustion air to those burners means that when the energy is recovered it is used and when the energy is needed it is available.
The hot exhaust products from a gas fired furnace will carry a lot of energy away to atmosphere if not addressed, and those losses will increase at higher furnace temperatures and higher excess air levels. Our reference furnace exhausting at 10000C for example will lose 55 per cent of its energy input when operating with 5 per cent excess air but 65 per cent when operating with 30 per cent excess air.
If the furnace exhaust were to be 5000C then the stack losses would reduce to 30 per cent and 35 per cent respectively.
So recovering energy from the exhaust gasses is a very effective energy saving device and burner systems have evolved to allow this.
A widespread method of recovering the energy is to use the exhaust to preheat the incoming combustion air supply to the burners. The exhaust gasses themselves are oxygen depleted so cannot be supplied directly to the burners. The system requires a mechanism to transfer the heat energy from the exiting combustion gasses to the incoming fresh air. The device may be a Heat Exchanger (aka Recuperator) or a Regenerator.
The hot and the cold air streams are separated in a Heat Exchanger and the energy is exchanged across (usually) stainless steel plates. Various devices (e.g. fins) may be fitted to increase the heat exchange surface area. The amount of energy transferred is limited by the heat exchange area and the temperature difference between the hot and cold streams. The maximum exhaust gas temperature entering the Heat Exchanger is limited by the materials of manufacture. In practical terms the inlet temperature is usually kept below 6500C (so our reference 10000C furnace would require a cooling by the addition of cold air prior to the Heat Exchanger) while the hot air temperature out is limited (by heat exchanger size) to approximately 4300C. Nevertheless this arrangement would still save 16 per cent+ of the energy input to the reference 10000C furnace compared to operation without a Heat Exchanger.
The energy recovered is often increased by also passing the exhaust gasses (taken before or after the Heat Exchanger) through a chamber to preheat the next load for the furnace. This is also an effective device for saving energy as long as the load then enters the furnace hot. Preheating a load and then allowing it to cool while the furnace is readied will not save energy!
It is important that the final temperature of the exhaust gasses does not approach dew point as this will cause problems in the baghouse.
The final energy recovery method to be considered is the Regenerative Burner. This adapts the regenerator concept used on glass melting furnaces and also employed on Regenerative Oxidizers, to a size that can be applied at burner level. A Regenerator differs from a Heat Exchanger because both the exiting hot exhaust gasses and the incoming cold combustion air pass through the same chamber but at different times. Basically each burner on a furnace is replaced by two. Each of those burners is fitted with a Regenerator which is a refractory lined container, holding a refractory heat store (often fused alumina balls approximately 2 cm diameter). One burner of this pair will fire and its products of combustion will exhaust through its partner. As the exiting gasses pass through the Regenerator the heat store is heated, retaining some of the energy otherwise being lost to atmosphere. (Exhaust gasses entering the regenerator at 20000F will leave the regenerator below 2600C). The burners will then switch and the exhausting burner fires and the firing burner exhausts. The cold combustion air is preheated by the hot Heat Store and energy is returned to the furnace. This cycle will repeat every 0.5 to 3 minutes (varying with the burner manufacturer’s heat store capacity).
Energy recovery is very effective. The energy saved on our reference 10000C furnace would exceed 40 per cent compared to the cold air operation. The burners are very robust, refractory lined to withstand the high exhaust and preheat temperatures. The problem with Regenerative systems is their initial capital cost, which is high due to the number of burners, change-over systems and controls required. Regenerative burners would only normally be applied to furnaces operating continuously above 8000C, typically steel heating and aluminum furnaces where payback periods are usually less than a year.
So you can now appreciate that burner systems have evolved over time in order to maximize the effective use of the energy available from Gas, by improving heat transfer and reducing losses. Burners are part of a system. They must be matched with effective controls, and good instrumentation. Better insulation and sealing of furnaces and ovens have also played their part in reducing the energy cost of products.