Q: I am an instrument engineer in an ammonia plant in Malaysia. I would like to seek your comments concerning the safe operation of a tubular reformer. We were recently audited by our corporate HSE division, and were required to install flame monitors at our reformer burners, and to trip the unit on flameout. We were also told to trip on low air/fuel ratio. We would like to know exactly how to implement them in our plant.
Ours is a two-chamber, tubular reformer operating at a temperature in excess of 1,000 °C with 432 staggered, side-fired burners over six levels, all with individual manual isolation valves and manually lit with electric igniters during start-up. I would like to make the installation reliable to protect against unnecessary shutdowns and trips during operation or startup.
Zhafran Abdul Aziz, Petronas, Malaysia / email@example.com
A: Let us first look at the ammonia reforming process and the role of the primary reformer, which is to produce hydrogen that later is used in the synthesis of ammonia. This hydrogen is produced by reforming a hydrocarbon feed like methane (CH4) using high-pressure steam. This reaction (CH4 + H2O = CO + 3H2) takes place at about 450 PSIG and at about 1,500 °F in tubes that are filled with nickel catalyst. These tubes are heated in the radiant section of the reformer furnace (Figure 1). To minimize coking, steam must be supplied in excess of a ratio of 3:1 and the reformer is shut down (S/D) by closing the hydrocarbon feed valve if that steam excess drops to 2.7:1. Figure 1 shows only a few burners while in an actual reformer furnace, there are hundreds.
As shown in Figure 2, the firing of an older induced-draft furnace is often manually controlled because, due to its massive mass and great heat inertia, the process is slow. The furnace has approximately a dozen fuel headers (only three are shown in Figure 2) with about 20 individual burners per header. As was shown in Figure 1, temperature indicators TI-1 and TI-2 are provided at the exit of the reaction tubes. Under manual control, these are monitored and firing is periodically adjusted by manipulating the fuel header control valves (HV-1 to HV-3 in Figure 2, HIC-1-controlled valves in Figure 1). The supply pressure to the header control valves is controlled by PIC-1, which can be set manually or automatically by the firing rate controller (TIC-2 in Figure 1).
Draft vacuum in the firebox is either maintained by a steam turbine-driven induced draft (ID) fan, the speed of which is controlled by PIC-2 in Figure 2, or alternatively by dampers in the stack if the furnace is of the natural draft design (PIC-2 in Figure 1). In either case, the air that supports combustion can be admitted through inlet louvers on the side of the furnace, which are manually set to balance the drafts at various points in the furnace (PI-1 through PI-4).
The main safety concerns are loss of furnace draft and fuel supply interruption. Loss of draft can be caused by ID fan failure, and results in a drop in combustion air flow and formation of hazardous, fuel-rich mixtures. Under manual control (Figure 2), the loss of draft triggers a high-pressure alarm (PAH-2), which should cause the operator to cut fuel flow to the burners (HIC-1,2,3).
Fuel supply interruption causes burner flameout and the resumption of fuel flow can result in fuel-rich air-fuel mixtures. Therefore, purging is required prior to resumption. In manually operated furnaces, fuel interruption is detected by a low-pressure alarm (PAL-1) that triggers the termination of firing by the operator closing FV-1. All control valves (PV-1, HV-1,2,3) should automatically close on instrument air failure.
Now, to answer your questions: If the combustion air is drawn into the furnace through inlet louvers on the side of the furnace, accurate air-fuel ratio measurement is not feasible, because the combustion air flow to the individual burners can't be measured. (Such induced draft furnaces waste much heat energy, because a lot of excess air enters the furnace just for the ride, does not take part in the combustion, and just heats up and leaves through the chimney.) If you have separate combustion air supplies to each burner, you can measure both the fuel and air flows, and low air/fuel ratio (A/F) can automatically close the isolation valve on that burner fuel supply. Naturally, to do that on 432 burners is not cheap. If you can’t measure the individual A/F ratio, you can only have the overall trip I described in connection with PIC-2.
For flameout detection, it's feasible, but expensive, to provide flameout sensors for each burner and once flameout is detected, little else can be done but trip the isolation valve closed, alarm, and request maintenance. Naturally, PLC logic can be applied, so predetermined safety logic will control the safety responses to multiple flameouts on burners in different locations.
Both scanning and individually focused detectors are available. Ultraviolet (UV) detectors measure the flicker of the flame and are not blinded by the hot refractory. Infrared (IR) sensors penetrate smoke or steam, and a single sensor can monitor both the pilot and main flames. Multi-spectrum detectors sense energy in the UV, visible and IR spectra. Flame safeguard costs vary between $1,000 and $3,000, with the higher priced units being explosion-proof and provided with intelligent electronics. Instrument air for purging (and sometimes cooling water) is also required.
Nuisance alarms can be eliminated by the use of smart alarm logic, and diagnostic messages can be generated for operating and maintenance information. All of these capabilities contribute to reducing the frequency of furnace shutdowns and increase its safety.
Béla Lipták / firstname.lastname@example.org
A: Air-fuel ratio trip and flameout detectors are required by Australian and NFPA (U.S.) codes. Most modern ammonia reformers will implement these as part of their safety system.
You will need to install either Honeywell or Siemens flame detectors and wire their flame-out signal to either a SIL-rated BMS trip relay or to your reformer safety system. You will also need to install a second (independent) natural gas flowmeter and an air flowmeter. Wire these two signals to the safety system and set a threshold air-fuel ratio trip parameter. If either of these trips is activated, the main fuel shut-off valve must be closed (double block and vent/drain). Purge the burner chamber to get rid of unburnt fuel, then trip the main ID fan. The required purge volume is dependent on the burner chamber size and fuel volume.
Raj Binney / email@example.com
This column is moderated by Béla Lipták, automation and safety consultant and editor of the Instrument and Automation Engineers’ Handbook (IAEH). If you have an automation-related question for this column, write to firstname.lastname@example.org.