With a wide processing capability, endothermic gas is a common industrial process gas. With readily available ingredients, endo is a simple, economical gas to generate.
Endothermic gas (endo) is synthetically created from reacting natural gas or propane and air at elevated temperatures with a nickel catalyst to speed up reaction and reduce the equipment footprint. For new operators, the equations are generally something like:
Generators for endo come in various sizes and configurations. All have construction standards per NFPA 86, specifically 13.5.3. As such, they all fall under the class C standard. This standard applies to the combustion system (fuel gas) and the process system (reaction gas). The fuel-gas requirements are similar to furnace and oven requirements. Interestingly, current NFPA guidelines regarding the safety requirements of 8.5 (purging and trial for ignition) and flame supervision are not applicable. Purging and supervision are always good – for no other reason than not wasting gas.
Reaction gas supply has requirements for safety shutoff valves and controls required for air-fuel gas mixing systems. Manual reset is required for low/high reaction gas pressures, generator low temperature and power failure. Since many systems utilize blowers and pumps, loss of air supply to the mixer also requires a manual reset.
PLC-based control systems on new generators will track these conditions and alert the operators to the shutdown. The ability to track the ladder logic and custom alarm codes allows for quick troubleshooting. Better still are descriptive alarm messages on graphic HMI interface displays (Fig. 1).
Figure 1. Control screen
As noted, a wide range of air/gas ratios for specific processes are used. Measuring the CO or H2O content of the endo will confirm the proper ratio is maintained. Automatic monitoring and control of the air-gas mixture minimizes the monitoring and adjustment of a manually controlled mixer. Automatic systems usually have an additional benefit of electronic records of gas production quality.
Whether or not record retention is required by the heat-treater’s quality policy, these records are an important component in troubleshooting any heat-treat process issues. Manual records can be spotty and are typically infrequent, missing the peaks and valleys in the gas production trend. Endothermic gas synthesis is a production process in its own.
Figure 2. Burn-off flare
As endo demands rise and fall, automatic volumetric control is almost a necessity. Depending on mixing equipment, various methods have differing abilities to adjust the volume of endo-gas generation while maintaining an acceptable gas quality level. Fixed-volume systems burn off excess gas (Fig. 2). While stable in gas quality, this is a very inefficient means of gas generation. Both manual and automatic ratio controls lose the ability to accurately measure air and gas over high and low flows. Both quality and volume stability must be determined for generator sizing and operating specifications for new equipment.
The effects of volume control can be seen in the production costs in Fig. 3. The numbers are based on a 4,000-CFH generator capacity and represent production with automatic turndown.
The costs associated in Figure 3 are based on independently determined national averages. Actual costs depend on local utility costs – trending would remain similar to those shown. Also shown are comparisons of a high-efficiency natural-gas-fired design to an electrically heated design. Finally, the fixed-volume costs skyrocket as the generator burns off excess capacity. In all cases, the most economical operation is at maximum capacity.
Figure 3. Endo gas costs
Heat is required for the production of endo gas. Heating is normally done two different ways: fuel gas or electric. Since reaction gas is required for endo generation anyway, use of the same gas for the fuel gas requires few extra components that are not on the generator already.
Electricity is the zero-emissions alternative. Emissions are handled at the electric generation facility – and priced accordingly. The facility infrastructure can be expensive for an electric generator. A modest-capacity generator can easily require 50-100 kW electric service to the generator. However, electric allows for a virtually unlimited heating turn-down capability.
High-efficiency fuel-gas-fired burners are more cost-effective to operate. The high insulating capabilities of the generator and recuperative burners have actual costs below electric.
All generators use the same amount of reaction gas to produce endo. Typically, endo gas requires 156-402 CFH natural gas (reaction gas) for 1,000 CFH endo gas. Heating the air-gas mixture requires the same energy, regardless of heating method. Energy savings arise from excess gas generation and, to a lesser extent, efficiency of the heating system.
Endo gas is explosive! Simple operation is a safety consideration. Manual control can easily create an unsafe condition with untrained operators. PLC-based control systems on new generators with automatic controls will shut down the generator before a dangerous condition can develop. Simple systems also offer simple training. Simple is good.
Endo gas generation is a hazardous mixture. As such, OSHA may require an SDS for endo (see online exclusive). This is an OSHA requirement for all suppliers of hazardous chemicals and is necessary for workplace HazCom programs. OSHA has several interpretations for use of “generic” SDS in the workplace. They can provide guidance in meeting any of the requirements.
Many local utility companies have rebate programs for energy savings from new, efficient equipment. This is on top of the utility savings from normal operation. Several customers have taken advantage of our energy-efficient designs for a significant rebate. The equipment supplier will be able to provide operational information. The user will have to determine the operation of the equipment (e.g., capacity, hours of operation, max/min flow requirements and fluctuations). If this is offered, do your homework. It is free money.
Endothermic gas is an easily produced industrial gas with a wide range of process capabilities. The materials are readily available and are more economical than other bottled-gas options. Newer equipment offers easy-to-use operator-friendly controls and improved production costs. And, with the increasing availability of rebates from utility companies, replacement of older generators is an option worth careful consideration.