Feedstock Variability: Why Your Gas Quality Moves and What It Does to the Engine

 

A biogas engine is specified against a gas analysis. The analysis is a snapshot. The gas is not.

Methane content drifts. Hydrogen sulphide spikes. Siloxanes appear when the feedstock changes. Moisture rises in summer. None of that is a fault, it is the normal behaviour of a biological process fed by a variable input, and an installation designed as though the gas is constant will spend its life derating, tripping or wearing out components early.

Understanding what moves, and what each thing does downstream, is the difference between a biogas engine that runs and one that becomes a maintenance problem.

What actually varies, and why

Methane content. The main determinant of calorific value. It moves with feedstock composition, digester temperature, retention time, organic loading rate and the health of the microbial population. A digester recovering from an upset produces different gas to one in steady state.

Carbon dioxide. The balance of the gas, and it moves inversely with methane. Higher carbon dioxide means lower energy content per cubic metre, which means the engine needs more volume for the same output.

Hydrogen sulphide. Driven by sulphur in the feedstock. Certain feedstocks are inherently high in it, and a change in what is being fed shows up here first. This is the single most damaging component for engine internals if it is not managed.

Moisture. Gas leaves a digester saturated. What matters is what happens to it on the way to the engine, and that changes with ambient temperature, pipe routing and whether drying is working properly.

Siloxanes. Present where the feedstock includes material containing silicone compounds, which is why landfill gas and some wastewater applications see them and agricultural digesters generally do not.

Oxygen and nitrogen. Usually indicate air ingress somewhere in the system, which is both a gas quality issue and a safety one.

Flow rate. Gas production follows feeding, and feeding is rarely perfectly constant. Batch feeding, seasonal feedstock availability and process upsets all move the flow.

The underlying point: each of these traces back to something upstream. Gas quality is a symptom of process condition, which is why monitoring it is useful for more than just protecting the engine.

What each one does to the engine

Falling methane content reduces the energy delivered per unit volume. The control system compensates within its range, and beyond that range the engine derates. Sustained operation on lower quality gas than the specification assumed means the output you planned for is not the output you get.

Hydrogen sulphide is corrosive, and in combustion it forms compounds that end up in the lubricating oil. The practical consequences are accelerated oil degradation, corrosion of engine internals and exhaust components, and shortened oil change intervals. Untreated, it is the fastest route to unplanned engine work.

Moisture combined with hydrogen sulphide is worse than either alone, because the combination is acidic. Condensate in gas lines also causes flow problems and, in cold conditions, blockages.

Siloxanes form hard silicon dioxide deposits during combustion. Those deposits accumulate on valves, pistons, cylinder heads, spark plugs and any downstream heat exchanger. The deposits are abrasive and difficult to remove, and they are the most expensive contaminant to ignore.

Ammonia can contribute to emissions issues and to corrosion.

Particulates cause wear and foul filters.

Varying flow forces part load operation, which affects efficiency, and frequent load swings increase thermal cycling on components.

The treatment train, and what each stage is for

Gas conditioning is not one device. It is a sequence, and the sequence should be designed against the actual gas analysis rather than a generic specification.

Desulphurisation. Several approaches exist, from biological methods inside or adjacent to the digester through to chemical and adsorbent based systems. Which suits depends on the concentration, how variable it is, and the required outlet specification. High and variable inlet concentrations are harder than high and steady ones.

Drying. Cooling the gas to condense out moisture, then reheating so the gas arriving at the engine is not at its dew point. The reheat step matters and is sometimes omitted, which produces condensate exactly where you do not want it.

Siloxane removal. Typically adsorbent based, and only required where the feedstock warrants it. Where it is required, it is not optional.

Particulate filtration.

Condensate management. Drainage points at low spots, correctly trapped, and actually maintained. A large number of gas supply problems are just condensate sitting in a pipe.

Ask the design question: was this train specified against a gas analysis from this site, across a period long enough to capture variation, or against a typical composition? The two produce different equipment. See biogas treatment solutions.

Designing for the range, not the average

This is the core engineering decision.

Specify against the range. An engine sized on average gas quality will underperform whenever quality dips below average, which is roughly half the time. The specification needs to state the expected range, not a single figure.

Confirm the tolerance. Every engine has a range of gas quality it can accept while maintaining output, and a wider range it can accept while derating. Know both numbers and know where your gas sits relative to them.

Consider modular capacity. Multiple smaller units allow capacity to follow gas availability, so units run at good load rather than one large unit running poorly loaded. It also means maintenance on one unit does not stop generation entirely. See biogas installation options.

Provide buffer storage. Gas storage smooths short term production variation and decouples the digester from the engine, which improves the operating profile of both.

Plan for flaring. A flare handles the excess when production exceeds what can be used, and it is a safety and compliance requirement regardless.

Treat the treatment as capacity too. Conditioning equipment sized for average conditions becomes the bottleneck during a peak.

Monitoring, and what it is worth

Continuous methane and carbon dioxide tells you the energy content arriving and gives early warning of process change.

Hydrogen sulphide, before and after treatment. The before reading tracks the feedstock and process. The after reading tells you whether the treatment is still working, which is the number that protects the engine.

Flow rate, to reconcile production against consumption.

Oil analysis on a schedule. This is the most underused diagnostic available. Trends in oil condition reveal what the gas is doing to the engine before it shows up as a failure, and it lets oil change intervals be set on evidence rather than on a fixed calendar.

Exhaust and component inspection during scheduled maintenance, for deposit build up.

Trend everything. A single reading tells you the current state. A trend tells you what is happening, and it is the trend that lets you intervene before something breaks. See service and maintenance.

The operational habits that matter

Change feedstock deliberately. A new input stream changes gas composition, sometimes substantially. Sample the gas after a feedstock change rather than assuming.

Maintain the treatment train on condition. Adsorbent media has a finite capacity. Once exhausted, it passes contaminants straight through while looking exactly the same from outside.

Drain condensate traps. Simple, routine, frequently neglected.

Investigate oxygen readings rather than accepting them. Air in the system is a safety matter.

Feed consistently where you can. A stable process produces stable gas, and stable gas is easier and cheaper to use.

Record what changed. When gas quality shifts, the answer is usually in what happened upstream a few days earlier.

Frequently asked questions

Why does my gas quality keep changing?

Because it comes from a biological process with a variable input. Feedstock, temperature, loading rate and process health all move it.

Which contaminant causes the most damage?

Hydrogen sulphide is the most common problem and it attacks internals and oil. Siloxanes are less common and the deposits they form are harder to deal with.

Do I need siloxane removal?

Only where the feedstock contains silicone compounds, which is typical of landfill and some wastewater applications. Confirm with gas analysis rather than assumption.

Should I size the engine on average gas quality?

No. Specify against the expected range, and confirm the engine tolerance for both maintained output and derated operation.

Is one large unit or several smaller units better?

Where gas production varies, multiple units generally run at better load and keep generating during maintenance on one of them.

How do I know the treatment is still working?

Measure downstream of it, not just upstream. Adsorbent media looks unchanged after it is exhausted.

Analyse the gas across a range, not on one day

A single gas analysis specifies equipment for a condition that will not persist. The range is what the design needs.

See biogas power generation, biogas treatment solutions, service and maintenance, or contact us.