Introduction
In modern biomass heat plants, most discussions about NOₓ reduction focus on technologies like SNCR or SCR. Yet in practice, many of the emission and corrosion problems observed in condensing heat plants do not originate in the reduction system itself, but much earlier — in the combustion chamber.
At the heart of these issues lies a parameter that often is underestimated: fuel moisture.
Variations in the humidity of biomass fuel, even by a few percentage points, can destabilize combustion, disrupt temperature control, force compensatory dosing of ammonia, and ultimately alter the entire chemistry of the flue gas cleaning system.
This article explores how this invisible parameter shapes the performance and lifetime of condensing biomass heat plants — and why controlling it may be the single most effective way to improve both efficiency and reliability.
The hidden challenge: variability at the source
Controlling the moisture content of biomass at its origin is practically impossible.
It depends on weather, season, storage conditions, and the type of feedstock — whether the wood comes from forest residues, pruning, or recycled sources. Even with contracted suppliers, moisture differences of 10 or more percentage points between batches are common.
Moisture sampling is equally unreliable. Laboratory tests are typically based on a single bucket taken from a truckload of 20–30 tonnes. Moisture at the top of the load may be 40%, while the bottom could be 30%. A single number simply cannot represent the true variability of a delivery.
When the fuel arrives at the plant, it is stored in large bunkers where natural drying continues. Operators may attempt to homogenize the material by mixing it, but in reality, fuel is taken from different zones — depending on bunker geometry, grab wear, and feeding patterns.
As a result, the boiler is continuously fed with a mixture of wet and dry zones, creating a fluctuating feed quality that cannot be stabilized manually.
Even if perfect homogenization were technically achievable, it would still not solve the problem.
At best, mixing could reduce short-term fluctuations — but not raise the average moisture level.
Condensing biomass heat plants are typically designed for a nominal fuel moisture of 40–45%, yet annual duration curves show that, in Western Europe, fuel moisture remains below this range for about 80% of the year.
In practice, that means the system operates most of the time under conditions drier than the design point, inevitably shifting the entire combustion and emission behaviour away from the intended balance.
Although laboratory analyses arrive one or two days later, the results are rarely used to adjust operations. Biomass is typically registered as wet, medium, or dry based on the operator’s estimation — a subjective classification rather than a quantitative control parameter.
The outcome is simple but far-reaching: unstable fuel = unstable combustion.
And since combustion is the starting point of every process downstream, all other systems — from SNCR to the scrubber and economiser — will react to that instability.
Before examining how moisture behaves within the system, it is essential to understand that once variable fuel enters the furnace, its moisture no longer acts alone. The total water present in the combustion environment is the outcome of several interlinked streams — each contributing to the plant’s overall stability and heat recovery potential.
Moisture balance in the system of a condensing biomass heat plant: three sources, one interdependent loop
In a condensing biomass heat plant, the total moisture content in the combustion process results from three main sources, which together define the plant’s overall water balance. Each of them typically contributes about one-third (≈30%) to the total water content in the flue gas. None of these sources operates independently – when one delivers less, the others must compensate, within their physical and operational limits.
a) Moisture generated by hydrogen combustion
A portion of the water vapour in the flue gas is produced directly from the oxidation of hydrogen present in the dry fraction of the biomass. This so-called chemical moisture is a fixed outcome of the fuel composition and cannot be influenced by operational adjustments. However, when the fuel is drier, the total hydrogen-derived water fraction becomes proportionally smaller in relation to the overall moisture balance.
b) Moisture physically present in the biomass
This is the physical water content of the biomass — the actual humidity of the delivered fuel. It is the most variable component, influenced by weather, storage and drying in the bunker. When the physical moisture decreases, the system receives less water with the fuel, which must then be compensated by the humidifier or by controlling air parameters.
c) Moisture introduced with combustion air (humidifier)
The third source is the water vapour contained in the primary air for combustion, often increased via the humidifier. Atmospheric air alone is too dry to maintain the required water balance, so the humidifier injects additional water vapour to reach near-saturation conditions before the air enters the furnace.
However, when the humidifier is already operating at its upper limit, or when further heating of the air would exceed design temperatures, no additional compensation is possible. At this point, the system approaches its saturation limit: the air cannot hold more water, and any additional humidification would lead to condensation before reaching the grate.
In summary, these three sources — combustion of hydrogen, moisture in the biomass, and moisture in the combustion air — together create a tightly coupled water-balance system. If one of them fluctuates, the others must adjust, but their capacity to do so is limited. This interdependence explains why maintaining consistent fuel humidity is so critical: it stabilizes the entire chain of moisture generation ,and recovery in the plant.
From persistently dry fuel to long-term combustion imbalance
In condensing biomass heat plants, the problem is not limited to short-term fluctuations in moisture but rather to a long-term, systematic deficit of water in the combustion process. For most of the year, the fuel operates below the design moisture level, causing the boiler to function in a permanently over-dry state.
The sankey diagram illustrates the moisture flows through the humidifier and scrubber, as well as the typical power split between the boiler and the condensing section under nominal operating conditions.
This chronic dryness has a cascading effect on both combustion and the energy balance of the entire plant. With less water entering the system, the combustion temperature rises above its design value, creating persistently elevated thermal NOₓ emissions. These must then be reduced downstream, which intensifies ammonia dosing and all subsequent side effects.
At the same time, the condensing section of the plant loses performance. The heat exchanger in the scrubber produces less recoverable power because of reduced condensation, which directly lowers the total heat output of the line. Since the control system maintains a constant nominal line output (boiler plus scrubber), the boiler compensates automatically by increasing its firing rate. This leads to an operational imbalance where the boiler delivers more heat than it was designed to, resulting in higher flame temperatures and thermal stress.
The consequences are multi-layered:
- sustained operation at elevated combustion temperatures,
- increased formation of thermal NOₓ,
- distorted temperature profiles within the furnace and SNCR reaction zone, leading to unreacted ammonia or additional NOₓ formation,
- potential thermal damage in refractory linings due to uneven or excessive heat load.
Operators typically have only a few pre-set combustion configurations for different fuel categories, and even these cannot dynamically compensate for gradual seasonal changes in moisture. Manual adjustments are limited to combustion parameters such as Primary Air (under-grate air), Secondary Air, and Tertiary Air, as well as their distribution across combustion zones and the grate speed, which determines the progression of the fuel bed through its drying, devolatilization, and burnout phases. These adjustments influence flame stability and burnout length, but in most medium-scale boilers, mechanical design limitations and slow-responding dampers make such control coarse and imprecise.
Even careful operator intervention can only partially mitigate these negative effects — it cannot eliminate them. In practice, operators are responsible for the entire plant operation and cannot continuously fine-tune combustion variables in real time. Therefore, true stabilization cannot rely on manual control. Automated systems must take over modulation of key parameters based on real-time fuel quality and moisture input. Only then can the process maintain consistent flame temperature, stable NOₓ formation, and predictable thermal loading across the furnace and the condensing section.
SNCR: the link between unstable combustion and scrubber chemistry
As discussed in the first part of this article series, the Selective Non-Catalytic Reduction (SNCR) process has well-defined temperature limits and reaction dynamics. When combustion remains stable, SNCR performs efficiently — reducing NOₓ within its optimal thermal window. However, when the process drifts outside this range due to persistently dry fuel and elevated flame temperatures, the system begins to work at its operational limits.
In over-dry operation, excessive combustion temperatures shift the SNCR reaction zone upward and shorten the residence time of flue gases within the effective temperature band. When this happens:
- part of the injected ammonia oxidises to form additional NOₓ instead of reducing it;
- the reaction zone becomes spatially unstable, with hot spots and cold spots in the duct;
- and the total residence time becomes too short for full reaction, resulting in ammonia slip.
Operators often respond by increasing the ammonia feed rate, assuming this will restore performance. In practice, this exacerbates the problem: more reagent is injected into an already unsuitable temperature environment, and even less of it reacts effectively. As a result, unreacted ammonia passes downstream, entering the scrubber of the condensing section.
Thus, the SNCR does not cause the imbalance — it amplifies the consequences of poor combustion stability. The unreacted ammonia that escapes this stage becomes the bridge to the next phenomenon — the chemical transformation within the scrubber water.
Disturbed scrubber chemistry — the real catalyst of corrosion
The scrubber’s purpose is to remove residual pollutants and recover heat.
But when ammonia slip becomes significant, it interacts with sulfur compounds in the water circuit, changing the chemical equilibria and pH balance.
Excess ammonia:
- raises the process water pH, interfering with NaOH control,
- shifts the SO₂ ↔ SO₃ equilibrium,
- and promotes the formation of ionic sulfur–nitrogen compounds such as ammonium bisulfate (NH₄HSO₄).
These compounds are highly soluble and are continuously recirculated through the humidifier, creating a feedback loop of reactive chemistry.
In this environment, corrosion in heat exchangers accelerates — not because of condensation itself, but because of the chemically active condensate enriched with ammonium and sulfur species.
It is crucial to emphasize that condensation is beneficial and should remain part of any efficient biomass heat system.
The problem arises when the condensate becomes chemically contaminated — a direct consequence of poor combustion stability and uncontrolled ammonia dosing.
Breaking the chain: towards stable combustion and clean chemistry
The conclusion is straightforward:
Most emission and corrosion problems attributed to SNCR are in fact symptoms of unstable combustion caused by inconsistent fuel quality.
By ensuring stable fuel moisture at the boiler inlet, plants can:
- maintain a stable combustion temperature,
- keep the SNCR within its optimal reaction window,
- minimize ammonia slip,
- and preserve balanced scrubber chemistry with clean condensate.
Achieving this requires real-time monitoring and active control of fuel moisture — not manual estimation or delayed lab analysis.
An automated system capable of measuring, predicting, and compensating for drying effects in the bunker could maintain a constant, predictable fuel quality.
Such systems are already being developed by WEProS, combining process engineering with data-driven optimization to deliver measurable improvements in efficiency, emissions, and plant lifetime.
Final takeaway
In condensing biomass heat plants, the battle against NOₓ and corrosion does not start in the scrubber — it starts at the fuel feed.
By controlling the most fundamental variable — moisture — operators can stabilize combustion, simplify emission control, and protect their heat recovery equipment.Sometimes, solving complex chemical and environmental problems begins with something as simple as a drop of water in the right place.