Cost of generating Steam
Let's assume that your boiler receives water at 100 ? and 100 psi from a pump. Then the gas heats the water up to 327.81 ?, the saturation temperature of water under 100 psi. The saturated water then transforms into steam at the same temperature under the same pressure.
Assume steady-state boiler operation:
The enthalpy of 100 ? is 68 Btu/lbm;
The enthalpy of 327.81 ?, 100 psi, saturated water is 298.4/Btu/lbm;
The enthalpy of 327.81 ?, 100 psi, saturated steam is 888.8 Btu/lbm.
The boiler needs to take water from liquid to steam so the total increased enthalpy is 888.8-68=820.8 Btu/lbm.
Since no boiler is 100% efficient so let's assume 80% efficiency.
As a result, the gas needs to generate 820.8/0.8=1026 Btu/lbm
Since 1 therm is about 100,000 Btu, the gas requirement is about:
1.026 therm per 100 pounds of water
Notice that the energy required by the pump is neglected.
Hope this helps,