The natural gas supply in the United States is changing. As the volume of liquefied/natural gas (LNG) imported to the United States increases over the next 10 years or so, the characteristics of the natural gas supply will change even more.

     Recognizing the need to understand and manage this situation, a coalition of gas transmission and distribution companies, gas utilities, GAMA, and other interests was formed: the Natural Gas Council. One of the products of the National Gas Council was the development of a set of interim guidelines for gas interchangeability. These interim guidelines were submitted as a recommendation to the Federal Energy Regulatory Commission in early March.

     In essence, the interim guidelines state that for any specific service territory, the quality of the new gas supply, as measured by the Wobbe number, should not vary by more than plus or minus 4 percent from the gas that has been supplied historically in that service territory. Furthermore, except as noted below, the Wobbe number of the new gas supply should not exceed 1,400 and its heating value should not exceed 1,100 Btu/ft3. The guidelines include an exception from the maximum Wobbe number and heating value limits for those service territories that have supplied natural gas that exceeds one or more of these limits in the past and have experience showing that there were no problems with the end use equipment receiving that gas.

      The guidelines also limit the amount of higher hydrocarbons (identified as Butane +) and inerts that may be in the gas. These limits have a practical effect of keeping the Wobbe number above 1,200.

     On the one hand, these guidelines represent the consensus agreement of the most knowledgeable group in the industry today. On the other hand, they are identified as interim because this is an evolving situation with changing factors. At the same time as these interim guidelines are being implemented, the industry hopes to learn more about the performance of end-use equipment using the anticipated new natural gas supplies. In GAMA's case, that equipment is a specifically residential and commercial appliance.

BREAKING IT DOWN  

These are some of the things that are known about the use of different natural gases:
      If the new gas has a higher Wobbe number, which usually translates to a higher heating value, the gas appliance will burn hotter. On furnaces, boilers, room heaters, and commercial heaters, this will increase the temperature of the heat exchanger and increase the thermal stressing of that component. The increased level of heat at the burner and in the combustion chamber will raise the temperature of components that are in direct contact with the burnerflame or installed within the

appliance. If those components are heated beyond their temperature ratings, component failure and appliance operational problems will occur.

     Even if the elevated component temperature does not exceed its temperature rating, it still may be beyond the designed or normal operational temperature range of the component in that case, premature failure of the component may result

     If the new gas has a lower Wobbe number, which usually translates to a lower heating value, the gas appliances will burn cooler. This will generally result in decreased performance of gas appliances. Space heating appliances of all types will have to operate longer to provide the heat required for the space, water heaters will have slower recovery times, and cooking appliances will either cook longer or cook unsatisfactorily.

     This last example is of particular interest. Cooking appliances, particularly the equipment used in restaurants and other food service establishments, are - in a real sense - production equipment. Underfired commercial food service equipment will either cook food slower (i.e., provide less product per unit of time) or undercook the food (i.e., provide an unsatisfactory or poor-quality product ).

     A significant change in the Wobbe number in either direction presents potential problems of increasing carbon monoxide production. It can be caused by incomplete combustion that results from major changes in the air/fuel mixture occurring at the burner, or it can be caused by flame impingement on surfaces within the combustion chamber that alters the combustion process. On vented appliances, this is not, of itself, an immediate problem. But it is indicative of improper operation of the appliance. If such a condition develops, it should not be allowed to continue as normal operation. 

     The interim interchangeability guidelines include the critical assumption that appliances in the field were put on rate when they were installed. In other words, the new gas can range from -4 percent to +4 percent of the historical gas, if the appliance is set to operate in the very middle of that range. But, if the appliance was installed such that it is operating at some point off the center of the range, then a new gas supply that fits the :plus or minus 4-percent criteria may nonetheless put that particular appliance beyond its designed range of flexibility to safely and properly burn natural gas.

      As new appliances are installed or serviced, it is vitally important that installers and service technicians make sure that those appliances are operating at their nameplate hourly input rating.