How do biomass boilers work?

You can see how a biomass boiler transforms solid fuel into heat energy on the video above. This video can also be seen on our YouTube channel: How do biomass boilers work ?

Main components of a biomass boiler

Components of a biomass boiler

A biomass boiler usually includes the following equipment:

1. Biomass silo
2. Fuel conveyor
3. Burner
4. Boiler
5. Control automation
6. Flue gas cleaner and chimney
7. Ash-conveyor
8. Hydraulic aggregate

Biomass boilers may differ in size, output, and specifications. The following describes a mid-sized moving-grate stoker-type Säätötuli hydronic biomass boiler.

Biomass silo

The silo stores the fuel for the boiler. This is where the fuel (usually woodchips or pellets) is delivered to the heating plant. The choice of the silo is essential for a functional biomass heating system. You need to find the perfect compromise between your budget and the size of the silo. The bigger the silo, the smaller the frequency you will need to fill it, but the bigger the investment.

Biomass silos can be classified in four different types:

  • Pellet silo. It is simply a bin with a conical bottom. Pellets are standardized in size and have a smooth surface that will make them drop down by gravity in the cone. That kind of silo cannot be used with woodchips as they would get stuck inside.
  • Rotary silo discharger (with or without spring arm). The bottom of this silo is equipped with a disc that will transfer the fuel to the silo auger. The disc can be fitted with a spring arm to extend the swept area. This kind of discharger is mainly for small silos like the Säätötuli STRONG.
  • Silo discharger with rakes. It is a sturdier silo solution that allows to build quite big biomass silos. The rakes will move to transfer the fuel to the silo auger. Rakes are usually made with thick steel and move with hydraulic actuators like with the Säätötuli Hydrobar (shown here), or with a mechanical movement like with the Metri2 and Kaks2 silo dischargers.
  • Hydraulic scraper station. It is the go-to solution for huge biomass silos as it allows to build silos able to accommodate several semi-trailers of solid fuel. The scrapers work on the same principle as the rakes and transfer the fuel to an auger or a conveyor. Scrapers can be manufactured sturdy enough to allow the delivery trucks to drive on top of them to dump the fuel.

When designing your silo, size is not the only thing to consider. It is of utmost importance that the silo can handle the type of biomass fuel you want to use. Usually, the better screening and quality control your fuel has passed, the more expensive it will be. On the long term, investing extra in a silo technology able to handle lesser quality fuels may end up in big savings on fuel on the lifetime of your boiler.

Last thing to consider when choosing your silo is how are you going to fill the silo and what fuel-handling equipment do you have available. A small pellet silo can be loaded manually with pellet bags. If you have a loader, make sure that you do not buy a silo that is too high for your loader or too small for the width of the bucket.

Biomass silo discharger with hydraulic rakes

The silo discharger on this picture is designed with low-quality woodchips in mind. The fuel may contain some bigger chunks, twigs, or parts of branches. The silo auger is designed to crush twigs and small branches when rotating. The entrance of the tube is fitted with a round blade that will cut any woodchunk that is too big to enter the tube. You can see how this work on our video of the Hydrobar biomass silo discharger.

Fuel conveying system

Biomass silo auger with a drop to the burner auger

The biomass fuel is transferred from the silo to the burner by a fuel conveying system. Bigger boilers usually have chain conveyors. For the small and mid-size boilers, the conveying is usually done with augers.

A direct auger between the silo and the burner is technically possible. This low-cost solution has however been abandoned in the beginning of the 2000’s.

Building a conveying system with multiple augers allow to add an extra security layer against backfire. For example, on the picture above, the silo auger will drop the fuel to the burner auger. A lever sensor controls the silo auger to maintain the fuel level in the drop. Another sensor controls the temperature of the burner auger. Should that temperature rise to an alarming level, the control automation will then stop the silo auger and force-feed the burner auger. That will evacuate all the fuel in the burner auger to the boiler’s firebox, including the fuel that was backfiring. The empty space between the silo auger and the burner will avoid the fire to propagate to the silo.

This safety feature is completed with automated sprinklers or aerosol extinguishers placed in the fuel conveying system.

Extra-safety can be added by adding hydraulically operated locks in the drop, thus creating a physical barrier between the silo and the burner. When using only pellets or high-quality woodchips, the fuel conveying system can be fitted with a rotating lock.

Burner and boiler

Moving-grate biomass burner connected to a hydronic boiler

This is the heart of any biomass heating system. The burner will transform the biomass fuel by combustion into hot gases and ashes.

A lot of different biomass burners are available on the market, each with different specifications and abilities to burn different kind of solid fuels. Säätötuli manufactures stoker-type moving-grate biomass burners. They are sturdy and heavy burners with a lot of accumulated heat. That heat accumulation in the ceramic and cast-iron parts of the burner allow it to process fuels with higher moisture content. That type of burner will not choke on moisture as the stored heat will evaporate the moisture out of the fuel without losing too much efficiency.

The burner will generate a flame from the biomass fuel. That flame will develop inside the fire-chamber of the boiler. The fire chamber of the biomass boiler is lined with water.

The moving grate will stir the fuel from time to time to evacuate the ashes so that they do not accumulate in the burner.

Hot combustion gases flow through the tubular heat-exchanger, cold water enters from the bottom of the boiler and exits from the top

Like the burners, there is a wide variety of concepts for biomass boilers. Säätötuli’s ASME-certified hydronic biomass boilers are fitted with vertical heat-exchange tubes. The hot gases from the combustion will flow through these tubes, transferring their heat to the water that is on the other side of the tube walls.

The cold water enters the boiler from the bottom, goes through the tubular heat-exchanger and goes out at the top of the fire chamber, at the hottest point of the boiler.

Boiler pump avoids condensation inside the boiler

Biomass fuels may have high moisture contents. The moisture will vaporize during the combustion and flow with the flue gases inside the boiler. If this moisture-saturated smoke arrives on a cold surface, the moisture will condensate back into water. Therefore, if the water that enters the boiler is too cold, there is a high risk of condensation and water in the last parts of the heat-exchange tubes at the back of the boiler. That would result in corrosion and rust inside the boiler, diminishing the boiler’s life expectancy of several years.

To avoid that problem, a pump will automatically mix a little bit of the heated water into the cold water that enters the boiler, thus keeping the temperature above the dew point.

Steel will retract with cold and expand with heat. Even if these dimensional changes are not visible, they will generate metal fatigue and wear. To optimize your biomass boiler’s life expectancy, Säätötuli advises to always keep the boiler near the maximum temperature it is designed for and to avoid any temperature changes. Controlling the temperature of the water entering the heat network should never be done with the boiler thermostat as it can be seen in some places in Canada. The heat-transfer section below gives some advice on how to design your hydronic heating network.

Ashes resulting from burning the biomass

Automatic ash-removal system with metal springs going up and down in the heat-exchanger

Biomass combustion generates ashes. A part of these ashes is made with small particulate matter and will flow with the smoke. These ash particulates will then deposit on the way, notably on the surfaces of the heat-exchanger’s tubes.

The tubes will then accumulate ash particulates and lose partially their ability to transfer the heat from the gases to the water. A layer of ash is a good insulation material.

To fight this issue, three factors will combine:

  • A quality burner that is regularly maintained will have a good and complete combustion. It will generate a lot fewer fine particulates than a burner that has not been maintained or that has a design flaw. The better the combustion, the less particulate matter in the smoke.
  • Vertical heat-exchange tubes will accumulate fewer ash particulates than horizontal heat-exchange tubes. Even if they are small, ash particulates are subject to gravity and will drop downwards. In a vertical heat-exchanger, the particulates will drop down the tube into the ash box instead of dropping on the bottom of the tube like in horizontal exchangers.
  • An automated ash-removal system can be installed. Mainly two types of ash-removal systems are on the market. The mechanical removal systems like on the picture above will move metal springs inside the tubes to keep them clean. The other solution is to send compressed air pulses inside the tubes to remove the ashes.

Automatic ash removal with two augers

Most of the ashes will however drop below the burner. They can be automatically removed with augers that can transfer them to an ash bin.

The quantity of ashes made by a biomass boiler will vary depending on the type of biomass fuel that is used.

A quality wood pellet will have an ash content around 0.5%. For woodchips, ash content will depend a lot on the quantity of bark that enters the mix. Ash content of woodchips usually sets between 0.5 and 2%.

Agricultural residues and peat may have high ash content. For example, peat’s ash content is around 4 to 8% and hay may have up to 9% of ash content.

To burn biomass fuels with high ash content, the quality of the biomass burner becomes essential. Burning high ash content fuels in a burner that is not designed for them will in most cases choke the burner.

High ash content will also generate clinker inside the burner. Clinker is a lava-like material that will appear when ashes melt inside the burner. A moving grate burner can diminish clinker formation by stirring the burning fuel regularly. In the case of extremely high ash content fuels, Säätötuli will add smoke recirculation on the burner. This will diminish the combustion temperature on the burner grate and therefore avoid clinker formation.

Smoke generated by biomass combustion

The cyclone will clean the smoke from particulate matter

Smoke generated by biomass combustion will flow through the boiler. After the boiler, the flue gas (smoke and steam from the moisture) will be evacuated to the chimney.

Depending on local regulations and the type of biomass fuel, the boiler may be equipped with a smoke cleaning system. In most cases, the boiler will be equipped with a cyclone. In places where the regulations are particularly strong, the use of a multi-cyclone will provide an effective way to clean almost all particulate matter from the flue gas.

In any case, the stack of a modern biomass boiler should not show any visible smoke nor generate any odour of burnt wood. In cold conditions, only pure white clouds should appear at the chimney. Those clouds are made by the humidity of the fuel that was transformed into vapour in the burner and that condensate back into water in cold weather.

Heat transfer

Hydronic heating network

Once the water is heated by the boiler, it is sent to the hydronic heat-network. The main pump of the network will send the hot water from the boiler to the network. The heat-network may include several buildings (even far away one from the other). You can learn more on this on our Wiki about biomass-fired municipal district heating.

The water will go through the different heat-exchangers of the network (radiators, air heaters, heating floor…) and circulate back to the boiler.

Automated shunt controlling water temperature in a hydronic heating network

As stated above, the biomass boiler should always be kept near the maximum temperature it was designed for. However, you may not want the water in your heating floor for example to be too hot as it may damage the tubing and it would certainly not be comfortable.

The solution is to add an automated shunt to control the temperature of the water going into the heating loop. That shunt will automatically mix cold water from the return line to the hot water from the boiler. You can then precisely choose the temperature you want to send to your heat network and still keep your boiler at the high temperature it needs.

The boiler can be connected to several heating loops on the network. The temperature of each loop can be controlled by a shunt and have its own setting. For example, a factory can set a low temperature for a loop that goes to the heating floor of the offices and a high temperature for the loop for the air heaters of the workshop.

Control automation

The control automation oversees the opeation of the biomass boiler

The control automation oversees running the motors, fans, and hydraulic actuators of the biomass heating plant.

The control automation on the picture above is the C210 model. It is manufactured with standard electrical components. As a fuse or an electric relay may fail, a control automation that has standard components instead of proprietary control boards will be easy and fast to repair, as most of the electricians will have those components on the shelf.

The C210 control automation has a PLC with a color touchscreen. The intuitive software shows each component of the boiler plant in a schematic that is easy to understand. The simple color-code allows to see directly if everything is running smooth: green items are running, yellow items are ready, red items have a fault that needs an intervention. Touching the item will show parameters. If the item is red, a troubleshooting window will open when the item is touched, telling what operations should solve the problem. Optionally the automation can be connected to the internet for remote operating.