Natural Gas, LPG, Diesel, Methanol or Waste Oil: Choosing a Burner Fuel

Why fuel is the first decision, not the last

Buyers often approach a burner enquiry by picking a model and then asking whether it can run on the fuel available on site. This reverses the correct order, and the consequences are not cosmetic.

The fuel determines four things that are fixed when the burner is built:

• The nozzle or injector geometry, tuned to the viscosity and calorific value of the fuel. A diesel nozzle on gas will not meter correctly, and vice versa.
• The air-to-fuel ratio range the control system must hold. Gaseous fuels meter by volume and mix readily; liquid fuels must first be atomised, which adds a variable — droplet size — that gas does not have.
• The materials in the combustion head and downstream path. Sulphur content, ash and corrosive compounds differ by orders of magnitude between natural gas and waste oil.
• The achievable emission profile, particularly NOx. This is largely set by flame temperature and fuel chemistry, so it is designed in rather than tuned out.

A burner that is "compatible" with several fuels is usually optimal for one. Where multi-fuel capability matters for supply security or price hedging, both operating points should be specified, quoted and — most importantly — tested before delivery. Assuming equivalent performance on the secondary fuel is a common and expensive error.

Five fuels compared on the parameters that matter

The fuels encountered most often in industrial heating are natural gas, LPG, diesel, methanol and waste oil. They are not ranked on a single scale; each is the right answer under different site conditions.

The parameters that actually decide the choice

Five parameters separate them in practice:

1. Supply availability and cost stability. A piped gas supply removes storage entirely. It also ties the operation to a network with seasonal and geopolitical price variation.
2. Storage and handling burden. Liquids require tanks, bunding, pumps, filtration and turn-down procedures. Gas requires a supply line, regulation and safety interlocks.
3. Combustion cleanliness. Measured by the maintenance interval on the combustion head, and by whether the flue gas carries soot, ash or acidic condensate.
4. Emissions. NOx, CO, sulphur oxides and particulates, against the limits applicable in the destination market.
5. Turn-down and control quality. How far output can be reduced while remaining stable, and how quickly the burner responds to demand changes.

Each fuel trades these against each other differently, which is why the correct approach is to fix the site constraints first and then select.

Natural gas and LPG: the default choice

Gaseous fuels are the default where they are available, for three engineering reasons.

First, they mix with combustion air at the molecular level before ignition, so combustion completes with modest excess air. Clean combustion means less soot on the head and a longer maintenance interval.

Second, they meter accurately by volume, which makes precise ratio control straightforward and gives a wide, stable turndown range. Modulating gas burners commonly hold stable output well down into their range, which suits processes that spend most of their time at holding duty.

Third, they contain essentially no sulphur or ash, so there is no acidic condensate to manage and no particulate emission.

Where natural gas and LPG differ

LPG is often treated as interchangeable with natural gas. It is not, and the difference is physical rather than commercial:

The density difference is the one that most often catches out an installation planned as a gas job. LPG leaking in a pit or trench will not disperse upward; it accumulates. Detection, ventilation and electrical classification must be designed for the heavier gas, not adapted from a natural gas layout.

Diesel: when there is no gas line

Diesel is selected where no gas supply exists, where the site is remote, or where the process needs the fuel's higher energy density in a compact storage footprint. It burns well when atomised properly, and the technology is mature.

The engineering price is atomisation. Diesel must be converted into a fine mist before it can burn efficiently, which requires either pressure atomisation through a fine nozzle or air-assisted atomisation. Two variables then enter that gaseous fuels do not have:

• Droplet size distribution. Coarse droplets burn slowly and incompletely, producing soot and carbon monoxide.
• Fuel temperature and viscosity. Viscosity changes with temperature, so the nozzle that performs correctly with warm fuel may produce a poor spray pattern when the tank is cold.

Practical consequences follow. Diesel installations need filtration to protect the fine passages in the nozzle, a preheat strategy where the site is cold, and a maintenance regime that treats the nozzle as a wearable item with a defined replacement interval rather than a component to be cleaned only when the flame degrades.

Turn-down and staging on liquid fuels

Liquid-fuel burners generally do not modulate as widely as gas burners, because stable atomisation becomes difficult at low flow. Two strategies compensate:

Mechanical or air-assisted modulation maintains acceptable atomisation across a range by changing the atomising air independently of fuel flow. This extends the stable range but adds a control loop that must be commissioned properly.

Staging uses a burner with two or more discrete firing stages rather than continuous modulation. Output steps between them, which is acceptable for processes with thermal inertia — a large oven or a boiler — but produces temperature oscillation in processes with low mass and fast response.

Methanol: selected for emissions, not cost

Methanol is not usually chosen on price. It is chosen where low nitrogen oxide and low sulphur emissions are the primary requirement, or where the fuel must be stored and handled as a liquid while still burning cleanly.

Its combustion properties differ from conventional liquid fuels in ways that affect burner design:

The low lubricity point is the one most often overlooked. A pump selected for diesel and reused for methanol can fail prematurely because the fuel does not lubricate its internal surfaces as diesel does. This is a component selection issue, not an installation adjustment.

Waste oil: burning what you already have

Waste oil is selected because the operator already possesses the fuel — used lubricating oil, recovered oil from service operations, or process residues. The economic case is that the fuel is otherwise a disposal cost. The engineering case is harder than it looks.

Waste oil is not a specification. Its composition varies with every delivery, which means viscosity, calorific value, water content, particulate loading and the presence of contaminants all move. A burner designed around a stable fuel must be reconfigured for that variability:

• Filtration and pre-treatment. Particulate and water must be removed before the nozzle, or atomisation degrades and the nozzle blocks. Filtration stages and a settling or separation step are normal parts of the installation, not optional extras.
• Preheating. Viscosity control is essential to atomisation; waste oil is typically heated to bring it to a consistent spraying viscosity.
• Material selection. Contaminants can include compounds that corrode standard combustion head materials, so the wetted path is specified accordingly.
• Emission management. Higher ash and contaminant loading produces more particulate emission, and the flue path must handle what it carries.

Where the fuel is genuinely free, waste oil can be an excellent economic answer. Where the fuel stream is inconsistent, the maintenance burden can exceed the saving unless the pre-treatment and viscosity control are designed properly from the start.

Fuel selection comparison

Read this table as a set of trade-offs rather than a ranking. The correct fuel is the one whose constraints the site can actually meet — including the storage, handling and maintenance obligations that come with it.

Frequently asked questions