Written by Paola Piazzi

Anyone who designs or operates a biomass plant — a small company boiler, a district heating network or an industrial cogeneration plant — sooner or later faces the same question: which filtration system should I choose?

There is no one-size-fits-all answer: the right choice emerges by answering, in order, a series of technical questions about your own plant. This guide walks you through the process step by step, from the initial assessment to identifying the most suitable technology.

• Step 1 – What is the air flow rate of my plant?
• Step 2 – At what temperature do the flue gases exit?
• Step 3 – What fuel do I use?
• Step 4 – What regulatory limits must I meet for NOx and acid pollutants?
• Step 5 – What constraints do I have on space, maintenance and automation?
• The four technologies compared
• Which filter for which plant?
• FAQ

Step 1 – What is the air flow rate of my plant?

The air flow rate to be treated (m³/h), linked to the plant's output in MW, is the first criterion that narrows down the range of possible solutions.

• Small plants (roughly <2 MW) with limited plant-room space → compact, modular solutions.
• Medium-to-large plants (district heating networks, industrial cogeneration) → larger vertical structures, often assembled from multiple modules.

Note down the estimated flow rate: it will be the first parameter to share with our engineers during sizing.

Step 2 – At what temperature do the flue gases exit?

The temperature of the flue gases leaving the combustion process determines the materials and protections the filter needs.

• Moderate temperatures, above the dew point but without high peaks → simple mechanical structures, without heavy insulation.
• High temperatures (up to approx. 250°C) → insulated structures are needed (e.g. rock wool with aluminium cladding) to prevent condensation and maintain efficiency over time.
• Very high temperatures, when the chemical reaction of a reagent is also required (e.g. urea for NOx abatement, optimal at around 350°C) → technologies designed to operate reliably under these conditions are needed.

If you don't know this value precisely, a flue gas analysis during the design phase will determine it.

Step 3 – What fuel do I use?

The fuel composition affects the dust load and process variability, and it is often the deciding factor between the available technologies.

• Homogeneous woody biomass (virgin wood chips, pellets) → more predictable particulate load.
• Heterogeneous fuels (agricultural waste, mixed residues, waste) → variable moisture and formation of acid fumes or complex particulates, which put less flexible filters under stress.

The more variable the fuel, the better the chosen technology must handle fluctuations in pollutant load without losing efficiency.

Step 4 – What regulatory limits must I meet for NOx and acid pollutants?

If the plant is subject to strict limits on NOx or acid gases (SOx) as well as particulate matter (PM), you need a technology that integrates — or allows the integration of — DeSOx/DeNOx neutralisation systems.

• Limits on NOx/SOx → consider integrating hydrated lime, sodium bicarbonate or urea/ammonia.
• Particulate-only limits → you can opt for solutions focused exclusively on dust abatement.

This step is often decisive for district heating and cogeneration plants, which are subject to stricter environmental regulations.

Step 5 – What constraints do I have on space, maintenance and automation?

Beyond the technical process parameters, the plant's practical operating constraints also matter.

• Available space: low or decentralised plant rooms favour compact, horizontal structures.
• Maintenance staff available: if they are not specialised, it is best to favour simple mechanical solutions with quick, front-access interventions.
• Desired level of automation: large plants benefit from a centralised electrical control panel that automatically manages dosing, temperature thresholds and bypass.

At this point you have everything you need to compare the available technologies.

The four technologies compared

Bag filter

The bag filter (baghouse filter) separates dust by passing the flue gases through filter fabrics, which are periodically regenerated by a compressed-air pulse-jet cleaning system. In the high-temperature version, such as our Pulco Air HT, the rock-wool-insulated structure with aluminium cladding allows stable operation up to approx. 250°C, staying above the dew point even with heterogeneous fuels.

It is the most flexible solution for handling variable fuels — mixed biomass, residues, waste — thanks to the possibility of integrating a pre-abatement chamber, customised filter media and DeSOx/DeNOx systems to neutralise acid gases and nitrogen oxides. The modular structure, with bags from 2 to 8 metres long, adapts to different plant layouts, and top access simplifies scheduled maintenance.

We cover this in detail in "Combustion fumes: how to manage them with a bag filter" and in the application case of a company in the agri-food sector.

Horizontal metal cartridge filter

Designed for small solid biomass plants (typically below 2 MW), the horizontal metal cartridge filter is a fully mechanical solution with no on-board electronic components. The stainless steel cartridges are arranged horizontally and are regenerated by a mechanical cleaning system.

Its main strength is compactness. The modular design (25-cartridge modules, from 1 to 4 per unit) handles airflows from 1,500 to 6,000 m³/h, and the filter can also be installed in low-ceiling or decentralised plant rooms. No multicyclone is needed upstream: the filter works on its own, so the overall footprint is smaller and there is one less component to install and maintain. It is the right choice when you want to keep specialist maintenance and energy consumption to a minimum, without having to manage complex electronic control panels.

Electrostatic precipitator (ESP)

The electrostatic precipitator works on a different principle from the previous ones: rather than retaining dust on a physical filter medium, it charges the particles electrically as they pass through a field generated by discharge electrodes and collecting plates, and then collects them on the earthed plates.

Its main advantage is its efficiency on fine and ultrafine particles (even below 0.3 microns, such as soot), which is particularly relevant in woody biomass combustion. With no fabric or cartridge to replace periodically, filter spare-part costs are reduced, in exchange for a larger electronic and electrical component in the plant (high-voltage power supply, dedicated control panel).

It is a well-established technology for combustion, suitable for both small and higher-output plants, and is often proposed as an alternative to the bag filter when low long-term filter maintenance costs are a priority.

Quick comparison table

Which filter for which plant?

Some practical guidelines, always to be verified case by case with our engineers:

• Small company boiler, limited space, simple operation → horizontal metal cartridges.
• Plant with heterogeneous fuel or regulations requiring DeSOx/DeNOx → HT bag filter.
• District heating network or cogeneration with strict NOx targets → catalytic ceramic candles.
• Priority on low long-term filter spare-part costs, very fine particulate → electrostatic.

In practice, many plants combine several technologies in sequence — for example an upstream cyclone separator for coarser particles, followed by the main filter — to optimise performance and component life.

FAQ

Can I switch from a bag filter to an electrostatic precipitator on the same plant?

In many cases yes, but it requires a specific engineering assessment of space, flow rate and the existing electrical panel. Our design team can analyse the feasibility.

Is a DeSOx/DeNOx system always needed alongside the filter?

No. It depends on the fuel and on the emission limits set by the regulations applicable to the plant: not all configurations require it.

What is the difference between a bag filter and an electrostatic precipitator for a biomass plant?

The bag filter retains dust on a physical filter medium and is more flexible with heterogeneous fuels and with DeSOx/DeNOx integration. The electrostatic precipitator charges dust electrically and collects it on plates, offering higher efficiency on fine/ultrafine particulate and no filter medium to replace, in exchange for a more significant electrical component.

What is the cheapest solution overall?

There is no single answer: you need to assess the total cost over the plant's life cycle (initial investment, maintenance, spare parts, energy consumption), not just the upfront outlay.

How long does a biomass plant filter last on average?

Service life depends on the technology, the dust load and the maintenance regime: the filter fabrics of bag filters and metal cartridges are wear parts subject to periodic replacement, whereas electrostatic precipitators have no filter medium to replace.

Still not sure which filter is best for your biomass plant? Contact us for a personalised technical assessment: we analyse flow rate, fuel and project constraints to propose the most efficient configuration.