Quick answer: Choose the fuel before choosing the burner. Natural gas and LPG give the cleanest combustion and the widest turndown with the least maintenance; diesel suits sites without a gas supply and where fuel storage is acceptable; methanol is chosen deliberately for low NOx and low sulphur; waste oil is selected because the fuel is already owned, not because it is easy to burn. The fuel choice then sets the nozzle, the air ratio, the materials and the emission profile — all of which are fixed at manufacture and cannot be adjusted on site.
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:
- 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.
- Storage and handling burden. Liquids require tanks, bunding, pumps, filtration and turn-down procedures. Gas requires a supply line, regulation and safety interlocks.
- Combustion cleanliness. Measured by the maintenance interval on the combustion head, and by whether the flue gas carries soot, ash or acidic condensate.
- Emissions. NOx, CO, sulphur oxides and particulates, against the limits applicable in the destination market.
- 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:
| Attribute | Natural gas | LPG (propane/butane) |
|---|---|---|
| Delivery | Piped from network or on-site supply | Delivered and stored on site in tanks or cylinders |
| Density relative to air | Lighter — disperses upward | Heavier — accumulates at low points |
| Safety consequence | Ventilation at high level for gas dispersal | Requires low-level detection and drainage philosophy; pooling is the hazard |
| Storage requirement | None where piped | Tank, bunding, vapour management and periodic inspection |
| Supply security | Depends on network reliability | Depends on delivery logistics and stored quantity |
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:
| Property | Effect on burner design |
|---|---|
| Lower flame temperature | Reduces thermal NOx formation, which is the main reason for selecting it |
| Lower calorific value per unit volume | Higher volumetric flow required for the same heat input — larger nozzle and supply line than diesel |
| Very low viscosity and low lubricity | Pumps and valves must be specified for a non-lubricating fluid; standard diesel components may not be suitable |
| Miscible with water and highly volatile | Storage and handling design must address vapour, sealing and material compatibility |
| Different material compatibility | Some elastomers and metals behave differently than with hydrocarbon fuels; wetted materials must be confirmed |
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
| Criterion | Natural gas / LPG | Diesel | Methanol | Waste oil |
|---|---|---|---|---|
| Combustion cleanliness | Highest | Good with correct atomisation | High, low sulphur | Variable; depends on pre-treatment |
| Turn-down capability | Widest | Moderate; improved with assisted atomisation | Moderate to wide | Narrower; sensitive to fuel consistency |
| Storage burden | None if piped; tank if LPG | Tank, bunding, filtration | Tank with vapour and material considerations | Tank plus filtration, settling and preheating |
| Maintenance interval | Longest | Periodic nozzle replacement | Depends on material compatibility | Shortest; fuel variability drives it |
| NOx profile | Higher flame temperature; needs low-NOx design where regulated | Moderate | Lower, due to reduced flame temperature | Variable |
| Best suited to | Sites with reliable gas supply; processes needing wide modulation | Sites without gas; compact storage requirement | Emission-driven projects; liquid handling with clean combustion | Operators generating a consistent waste oil stream |
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
Can a burner be converted from one fuel to another after installation?
Sometimes, but it is a manufacture-level change rather than a site adjustment. Conversion typically requires a different nozzle or injector, revised air-ratio settings and possibly different combustion head materials. Where conversion is anticipated, it should be specified at the enquiry stage so the burner is built to accommodate both fuels and both operating points can be tested.
Is LPG a drop-in substitute for natural gas?
No. The calorific value per unit volume differs, so gas train sizing and orifice dimensions change. More importantly, LPG is heavier than air and will accumulate in low areas, so ventilation, detection and area classification must be designed for a heavier-than-air gas. Treating an LPG installation as a natural gas installation with a different gas bottle is a safety error.
Why does methanol need different pumps and valves?
Methanol has very low lubricity compared with diesel, so components that rely on the fuel to lubricate moving surfaces can wear prematurely. It is also highly volatile and miscible with water, and it interacts differently with some elastomers. Pumps, seals and valves should be specified for methanol rather than adapted from a diesel installation.
What is the main risk with waste oil burners?
Fuel variability. Because the fuel composition changes between deliveries, the atomisation and combustion settings that worked with one batch may not suit the next. The mitigation is upstream: consistent filtration, water removal and viscosity control through preheating, so the burner receives a more predictable fuel. Without that, maintenance intervals shorten and emissions become inconsistent.
Which fuel gives the lowest NOx emissions?
Broadly, fuels that produce a lower flame temperature generate less thermal NOx, which is why methanol is attractive for emission-driven projects. However, NOx also depends heavily on burner design — staged combustion, flue gas recirculation and precise air control all reduce it. Fuel selection sets the starting point; burner design determines the final figure.
We have both a gas line and diesel storage. Which should be primary?
Natural gas as primary, diesel as backup, is the common arrangement, because gas offers cleaner combustion and wider modulation for normal operation. The diesel train then serves supply interruptions. In this configuration both operating points must be specified and tested, and the changeover procedure — including purge and safety interlocks — must be documented for the operators.
Not sure which fuel configuration suits your site? Tell us what is available — gas line, LPG storage, diesel, methanol or a waste oil stream — together with your required heat input and emission limits, and we will recommend the burner configuration and the handling equipment it requires.
