Gas vs Diesel Steam Boiler Fuel Consumption: Buyer’s Guide

Start with the same steam duty

Record steam output in kg/h, outlet pressure and steam condition, plus feedwater temperature at a defined inlet. Here, one tonne means 1,000 kg. State whether pressure is gauge or absolute; steam-property calculations require an unambiguous pressure. Dry saturated steam, wet steam and superheated steam have different specific enthalpies.

A “2-ton boiler” label alone is insufficient. Ask whether the capacity describes actual evaporation under specified conditions or an equivalent evaporation rating. Spirax Sarco’s explanation of boiler ratings shows why feedwater temperature and steam conditions matter.

The JINHONG WNS gas and oil steam boiler page lists reference consumption using specified fuel heating values. Use those figures for initial selection; request the calculation conditions before using them in an operating budget.

Check fuel units before using the formula

For diesel or a fuel described locally as DO, obtain the actual supplier specification. The abbreviation alone does not establish composition, density or heating value. A light-oil reference figure does not confirm compatibility with every locally supplied diesel grade.

• Oil: use a mass heating value in kJ/kg. Convert consumption with litres/h = kg/h ÷ density in kg/L. Density and purchased volume must refer to compatible temperatures; one kilogram is not one litre.
• Gas: use a volume heating value in kJ/Nm³ with flow on the same reference basis. Specify reference temperature, absolute pressure and dry or wet basis. An actual meter m³ and a normalized m³ are not automatically interchangeable.
• Both fuels: pair an LHV-based efficiency with lower heating value, or an HHV-based efficiency with higher heating value. Mixing the two bases changes the answer incorrectly.

NIST’s natural-gas heating-value report explains reference-condition effects. Spirax Sarco explains gross and net calorific values; our boiler-efficiency comparison guide develops the efficiency definitions.

Calculate heat demand and count blowdown once

For a simplified steady-state boundary with no leakage or stored-energy change, feedwater flow equals exported steam plus blowdown. Let S be steam flow and B blowdown flow, both in kg/h. Specific enthalpies h_s, h_fw and h_bd describe steam, entering feedwater and leaving blowdown, in kJ/kg.

Feedwater flow = S + B

Q_water = S × (h_s − h_fw) + B × (h_bd − h_fw)

The result is kJ/h. Define η_w specifically as the fraction of fuel energy absorbed by the steam and blowdown streams together. Then:

Fuel input = Q_water ÷ η_w
Diesel kg/h = fuel input ÷ diesel LHV
Gas Nm³/h = fuel input ÷ gas LHV

This defined η_w is not a combustion-analyzer reading. If a quotation instead supplies fuel-to-exported-steam efficiency, η_s, use fuel input = S × (h_s − h_fw) ÷ η_s. Blowdown losses are already reflected in that efficiency; adding them again double-counts them.

DOE’s blowdown guidance explains its energy cost and water-quality purpose. Confirm whether a blowdown percentage uses steam flow or feedwater flow as its denominator. Our blowdown and heat-recovery guide covers the wider assessment.

Worked example: 2,000 kg/h of steam

All values below are assigned illustration inputs. The two LHVs match the published WNS reference basis; they are not fuel analyses for your site. The rounded enthalpies are assumptions for this arithmetic, not certified steam-table values. Final engineering must obtain properties at the confirmed pressure, temperature and steam quality.

The gas reference conditions above are assigned for this example; they are not a statement of the WNS table’s unspecified gas reference conditions. Blowdown is 2.5% of steam output, not 2.5% of feedwater. Feedwater is 2,050 kg/h. No external blowdown heat recovery is credited.

1. Steam heat: 2,000 × (2,780 − 377) = 4,806,000 kJ/h.
2. Blowdown heat: 50 × (780 − 377) = 20,150 kJ/h.
3. Fuel energy: (4,806,000 + 20,150) ÷ 0.88 = approximately 5,484,261 kJ/h.
4. Diesel: 5,484,261 ÷ 42,700 = approximately 128.44 kg/h, or 152.90 L/h after division by 0.84.
5. Natural gas: 5,484,261 ÷ 35,588 = approximately 154.10 Nm³/h.

For example, (kJ/h) ÷ (kJ/kg) gives kg/h: the units provide a useful error check. Equal assumed efficiencies do not establish equal real-world performance between oil and gas firing.

Convert the result to fuel cost per ton of steam

Divide hourly fuel cost by actual steam output in tonnes/h:

Fuel cost per tonne = fuel quantity per hour × matching unit price ÷ (S ÷ 1,000)

Assume purely hypothetical prices of US$0.90/L for diesel and US$0.40/Nm³ for gas, on the same volume bases used above. These are arithmetic inputs, not current market prices.

The price assumptions determine this comparison; they do not prove that gas is always cheaper. Obtain delivered tariffs and identify fixed charges separately. These figures exclude electricity, water, chemicals, labour, maintenance, upstream feedwater preparation and distribution losses. Steam delivered to a process needs a separately defined cost boundary.

Recalculate for actual load and operating hours

A plant averaging 1 tonne/h cannot assume half the rated hourly fuel use. DOE describes how fixed losses and cycling affect efficiency. Request performance at expected low, normal and peak loads, separately for each proposed fuel.

As a sensitivity calculation only, halve steam and blowdown in the example and assume η_w falls from 0.88 to 0.80. Gas becomes approximately 84.76 Nm³/h; diesel becomes 70.64 kg/h, or 84.10 L/h. Hourly consumption falls, but fuel per tonne rises by 10%, because 0.88 ÷ 0.80 = 1.10. The efficiency change is assumed, not a predicted boiler curve.

For annual budgeting, calculate each load period and add separately estimated start-up and standby fuel. Alternatively, use representative metered fuel and exported steam over the same period. Do not add start-up fuel again if those meter totals already include it. Review turndown and short-cycling considerations when the production schedule varies substantially.