How Air Flow Burning Impacts Biomass Boilers

by | Jan 5, 2023 | Biomass

This article was updated on 2 May 2023

A crucial component of the worldwide clean energy initiative involves the combustion of biomass and municipal solid waste. Consequently, numerous studies have been undertaken over time to examine various facets of the biomass burning process, with the aim of optimising fuel efficiency and minimising environmental impact.

One such method involves investigating the role of airflow in the combustion process within biomass boilers or wood-burning stoves. There is a wealth of information to consider on this topic, and we’re here to provide you with a concise overview of the essential factors related to how airflow influences combustion!

stirring wood with poker in heating boiler with solid biomass fuel - a renewable source of energy

What is Air Flow?

It is the movement of air that is called airflow. For air to flow, air must exist. Flowing air has a fluid quality, which means particles naturally flow from high-pressure areas to low-pressure areas.

Airflow in ducts can be controlled by dampers. The damper controls the airflow by increasing, decreasing, or completely stopping it. The air handler is a more complex device in the modern era that can generate and condition airflow in addition to regulating airflow. These devices can be used for both a biomass boiler and a wood-burning stove.

How Does Airflow Affect Fire?

Simply put:

When more air flows through your biomass boiler (or wood-burning stove), the fire burns faster, and when there is less air, the fire burns slower.

What Will a Higher Air Flow Do?

A high air flow means:

  • Fuel may burn too quickly
  • The burn will be more intense
  • It will keep the stove glass clearer
  • Good flames are produced

Reducing the air flow means:

  • The burn time is longer
  • Improved temperature control
  • Glass is covered with more soot

Ensure that some air flows continuously through the stove glass to keep it clear. You should not completely close the vents and burn hot periodically to remove any buildup. Air washing is a feature found on many modern industrial biomass boilers and most wood-burning stoves built for smokeless areas. Consequently, the glass remains clear, and soot isn’t accumulated over time. However, as a result, regulating the burn rate becomes more difficult.

The fire will also burn less cleanly if you reduce the primary air supply, which essentially starves it of oxygen. The longer your fire burns, the more soot and tar it will produce. Therefore, with the vents open, you should let your fire burn hot and regularly. You do not need to burn for a long time – just enough to thoroughly heat the flue and eliminate any particulates that may have accumulated from slow-burning. A chimney sweep can check your flue if you have been slow-burning without burning your wood hot for a while. The chimney fire risk increases if there is a lot of tar in it.

metal chimney to get rid of air flow burning fuels in the biomass boiler

How Can You Increase the Efficiency of a Biomass Boiler?

A biomass boiler needs three main burning characteristics to start burning wood, often called the ‘fire triangle’.

  • Fuel (pellets, woodchip or logs)
  • Air Supply (both primary and secondary airflows)
  • Heat

You can ask yourself many questions to ensure your boiler is up to scratch on efficiency levels.

  1. Do you have fuel, and is the fuel good quality? (it must be clean and dry, see our guide on what makes good wood fuel quality).
  2. When the boiler is igniting, are the fans running?
  3. Is there a sound coming from the boiler when it pulls air?
  4. Is there a blockage in the air vents of the boiler?

There are plenty of other questions, but if you have any, you can contact us anytime!

How is the Combustion Air Flow to the Boiler Controlled?

The combustion air that reaches the upper part of the furnace is converted into secondary air, which prevents the emission of uncombusted gases and metal corrosion in the boilers, as well as reducing pollutants.

Depending on the fuel used, the steam created can be generated in smaller sub-processes and systems.

  • Air system
  • Flue gas system
  • Steam water system
  • Ash system
  • Fuel system
close up ventilation system

The Air System

Let’s look at this particular system considering this article is about air flow. What happens in this part of the process?

An air supply is provided to the furnace for combustion. Components supplying air to the boiler furnace make up the air system. The furnace must be kept at a pressure that is close to the surrounding atmosphere for manufacturing reasons. As a result, blowers are required to drive air into the furnace. To get air to the blowers, ducting must be installed. The noise of the air entry is often reduced by using silencers. The blowers and furnace are connected by ducting. In addition to blower operation points, dampers are commonly used to control air flow. Measurements are made of air flow, pressure, and temperature. The air is heated to improve combustion performance. When exiting flue gas heats the air, heat exchangers are often used.

Key Components of a Biomass Boiler Set-up

You may be surprised by the intricacies that go into a biomass heating system and the set up a business needs in place to manage it and get the most out of the firewood you’re burning for your business.


This is a tank that stores water within the wood burner heating system. Heat energy is collected and stored to allow flexible use throughout the day.

Biomass Boiler

Heat is generated by burning biomass fuel in a wood burner (whether this is a biomass boiler, wood burner, multi-fuel stove, etc). An ash collector and removal system are typically included in the boiler. The grate in the combustion chamber supports the fire bed, a hot air igniter lights the fuel, and fans supply combustion air.

Biomass Fuel

Biofuels grew specifically for fuel (e.g. miscanthus, short rotation coppice), such as wood pellets, wood chips, and logs.

Boiler House

Buildings or areas containing primary components of biomass systems. This includes the boiler, the accumulator, the chimney, and the controls.

Chimney and Flue Gas Clean-up System

An assembly that removes combustion products from the boiler and vents them to the atmosphere. There are likely to be additional filters and cyclones in a biomass system to collect particulates in the flue gas before it is vented to the atmosphere.

Fuel Feed System

Boiler fuel delivery method using biomass fuel. Auxiliary equipment such as augers, gravity feed systems, conveyors, and moving floors is included.

Fuel Storage

An underground or aboveground storage area for biomass fuel. There may be hoppers, silos, bunkers, or fuel delivery areas involved.

Heat Exchanger

It transfers heat from the fire bed and flue gases to a fluid that transfers heat (usually water), while keeping the heat source and the heat delivery system separate.

Heating System

An arrangement of pipes that returns cooler water to the biomass boiler after heating is extracted from it for space heating and hot water use. To match the heat demand to the boiler’s capacity, it is best practice to design the heat distribution system and boiler system together. In order to signal demand to the boiler, the small scale or commercial heating system uses room thermostats.

System Controls (including air flow burning!)

The system’s main components are electrical components, which monitor and control different aspects of it. Fuel feed rate and combustion air flow controls (for both the primary air vent and the secondary air vent) are usually present to control the hot water temperature output and monitor combustion conditions.

Other system controls can include:

  • Maximum heat output controls for a sufficient temperature and an efficient fire.
  • Primary and secondary combustion controls.
  • Air vent control to release smoke and reduce emissions and air pollution from burning wood.
fir pellet held by hands

Operating Principles of Biomass Systems

There are four primary phases to the control regime of any biomass boiler:

1. Start-Up Sequence

Boilers start their operation in succession, starting with the ignition of fuel and continuing until combustion is complete.

2. Steady Operation

This involves a warm-up time for the boiler followed by a steady operation with an oxygen content around 10% in the flue gas.

3. Shut Down Sequence

When combustion in the boiler ceases and oxygen levels in the flue gas rise, the shutdown sequence begins. The firebed may still be present at the end of the period, but there is no significant combustion.

4. Off Period

A boiler’s off period begins when it is cooled down and ends when it is heated up. As a boiler operates near its maximum output, its efficiency is highest and pollutant emissions are lowest. When the boiler is frequently switched on and off (on / off cycling), the emissions are the highest and the efficiency is the lowest.

How Can Woodyfuel Help with Air Flow Burning?

We are a reliable and local wood supplier, so take the opportunity to work with us.

We can answer any questions you might have about using wood as fuel and how it works in relation to your industrial biomass burner; after all, we are the experts!

There’s also plenty of other reasons to stick with us.

hand holding small tree ready for planting and can be used for biomass fuel when it has grown

As part of our commitment to sustainability goals and low running costs, we can help your local organisation run on renewable wood fuels.

Our trading partners benefit from optimal RHI payments, reduced CO2 emissions, and support for the local environment.

We have many years of experience in the business of wood fuel supply and provide you with the best wood delivery service.

Both our wood chips and wood pellets are organic and within the right specification, especially with moisture content, for your wood boiler to keep performing at a high standard. Having the right moisture content works hand in hand with optimal and cleaner air flow burning.

We believe businesses are the beginning of setting the right example to the world of how to live more sustainably. Choose our ethical and sustainable business to work with to make our world a better place now!

Wood chip, pellet, brash

Call 03333 447234 to arrange your wood fuel supply today

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