An apparatus and method for operating a gas-fired burner on a liquid fuel. The apparatus integrates a catalytic liquid fuel reformer with a flame burner designed for operation on a gaseous fuel of high Wobbe Index, e.g., natural gas. The method involves reacting a mixture of a liquid fuel and oxidant in a catalytic reformer to obtain a gaseous reformate having a low Wobbe Index; and thereafter combusting the gaseous reformate, optionally augmented with liquid co-fuel and oxidant, in the gas-fired burner under diffusion flame conditions. The invention allows commercial gas-fired appliances, such as stoves, ovens, ranges, grills, griddles, stock pot burners, clothes dryers, hot water heaters, and boilers to be operated on a liquid fuel, which offers advantages in logistics and camp operations.