Reading Flame Colour: What It Tells You About Biogas Combustion

When biogas burns, the colour of the flame is one of the quickest visual indicators of how well combustion is proceeding and, indirectly, of the quality of the gas being burned. Operators of palm oil mill flare stacks and burners often learn to read flame colour as a first-line diagnostic long before instruments confirm what their eyes already suggest. Understanding what the colours mean helps catch problems early.

Why Flame Colour Changes

Flame colour is governed largely by combustion temperature and the completeness of the reaction. A hotter, cleaner burn with the right air-to-fuel ratio produces a flame at the blue end of the spectrum, while a cooler or incomplete burn shifts toward orange, yellow, and red. With biogas, the methane content and the presence of impurities both influence how hot and how cleanly the flame burns.

Reading the Colours

• Blue flame: the hottest and cleanest, indicating complete combustion and good methane content with sufficient air
• Yellow to orange flame: cooler and often a sign of incomplete combustion, insufficient air, or lower gas quality
• Red or dark flame: the coolest, typically indicating poor combustion, low methane, or excess particulates

The Link to Biogas Quality

Biogas from palm oil mill effluent is primarily methane and carbon dioxide, with methane being the combustible fraction. When methane content is high, the flame tends to burn hotter and bluer. When the gas is diluted with more carbon dioxide or moisture, or when hydrogen sulphide and other impurities are present, the flame cools and shifts colour. A persistent yellow or smoky flare is often a prompt to check gas composition and upstream treatment.

Why It Matters for Compliance and Efficiency

An incomplete burn is not just an efficiency problem. Unburned methane released to atmosphere carries a far higher global warming potential than the carbon dioxide produced by complete combustion, so a poorly performing flare can undermine the very emissions goal it exists to serve. Reading flame colour, alongside proper instrumentation, helps operators keep combustion efficient and compliant.

Flame Colour Is a Guide, Not a Gauge

Flame colour is a valuable at-a-glance indicator, but it should support rather than replace proper gas analysis and temperature monitoring. Ambient light, viewing angle, and stack design all affect how a flame appears. Used together with instrumentation, though, it remains one of the fastest ways for an operator to sense that something in the combustion process has changed.

How THAGO Can Help

THAGO designs and commissions biogas burners and flare stacks engineered for efficient, stable combustion across a range of gas qualities. Our team can advise on burner and flare selection to suit your mill's gas composition.