Walking-Beam Furnace Burners

This article is part of the Advanced Combustion System Makes California Plant Possible article published in Industrial Heating magazine August 2009.

The combustion-system requirements for the walkingbeam furnace were dictated by the Betts Spring specifications, the design philosophy of the furnace builder and the emissions and fuel-efficiency mandates set forth by the California EPA.

To achieve the net input requirement of 2,115,000 BTU/hour, a total of 12 REKUMAT® M200B burners are installed in the rear wall of the furnace. They are mounted 36 inches above the walking beam and fire toward the front of the furnace, against the direction of product travel. The burners’ high-velocity flame causes intense mixing of the products of combustion (POC) within the furnace chamber, resulting in excellent temperature uniformity and improved convective heat transfer (Fig. 3).

The burners are self-recuperative, meaning that the POC exit the furnace through the burners, eliminating the flue opening. An eductor, driven by a combustion-air bypass, creates suction at the burner exhaust port and draws out the POC. The exhaust is collected downstream into a main header and connected to a standard flue. Approximately 90% of the POC are drawn through the burner with the remainder exhausted through the bar-stock entry slot. The rate of draw through the burners is adjusted to maintain a slightly positive furnace pressure and prevent tramp air infiltration.

Before POC exit each burner, they pass over the outside of the burner’s integrated heat exchanger, preheating the combustion air that is entering the burner on the inside of the recuperator (Fig. A).

Fig. A. Self-recuperative principle as implemented in the REKUMAT burner

At zone temperatures of 1750°F, combustion air is preheated to approximately 1100°F and results in combustion efficiency of 73%, a gross input per burner of 241,400 BTU/hour and a net input per burner of 176,200 BTU/hour. A cold-air burner without preheated combustion air would require roughly 352,400 BTU/hour – approximately 32% more. For reference, the total connected load needed to achieve the gross input specification is 2,897,000 BTU/hour.

However, preheated combustion air raises the flame temperature and therefore increases thermal NOx formation. In the past, higher air preheat temperatures meant a trade-off between efficiency and NOx emissions. Now, by using FLOX® (FLamless OXidation) combustion it is possible to significantly reduce peak flame temperatures and thus reduce thermal NOx formation. FLOX is activated at 1550°F (well above the self-ignition temperature of natural gas) and works by redirecting the flow of fuel gas internally and injecting it directly into the furnace. The fuel mixes with combustion air and large amounts of inert POC outside the burner nozzle, reacts in a controlled fashion and lowers the flame temperature, thus lowering NOx. FLOX combustion is nearly invisible and eliminates the typical roaring of a flame (Fig. 4).

Using FLOX it is possible to meet the emission requirements of the California EPA without sacrificing fuel efficiency. NOx emissions were required to be less than 50ppm at 1750°F for the Betts permit. Measurements during production conditions show NOx emissions less than 30ppm when corrected to 3% O₂.

Each burner is equipped with individual solenoid valves for gas and combustion air as well as a CBFF burner control box. Upon receiving a fire command via the Profibus network, the gas and air valves are opened, a spark is generated by the ignition transformer and a flame is created. The UV sensor, mounted on each burner, will initiate a fault lockout in case of unwanted flame loss.

Gas and air-flow volumes are adjusted via the throttling mechanism on the solenoid valves. The proper flow is determined by measuring delta-p settings on the gas and air orifice plates and adjusting flow until the nominal values are obtained.

The burners are pulse fired. The amount of energy introduced to the furnace is determined by the amount of time each burner fires. At high-heat demands (furnace cold), all of the burners fire continuously. As the temperature setpoint is approached, burners begin to turn off for calculated periods of time. The burner off times increase as the furnace temperature approaches the setpoint.

Pulse firing eliminates the need for inaccurate and difficult-to-adjust proportioning devices. The burner is set to fire at its optimum point – typically to achieve excess oxygen content of 2-4%. Regardless of heat demand, the burner will always maintain its optimum setting.

Fig. B. NOx emissions with FLOX® technology