Technical newsletters about emerging issues & our latest research

How much more attention do biomass boilers really need?

The comparison to the ‘stoker’ associated with coal fired steam engines may be unfair and extreme but it does go some way to painting a picture of the difference between biomass and gas fired boilers.

Biomass (wood chip or pellet) is a solid fuel which requires more manual intervention than natural gas.

Generally a site representative needs to attend to wood chip deliveries to clean up any wood chip that might spill outside the store. Pellet deliveries can be performed by the delivery driver alone (much like heating oil deliveries).

Biomass boilers, whether wood chip or pellet, need to regularly have the ash removed. Automatic ash removal systems only automate the process of removing the ash from the boiler and filling a wheeled bin. This bin still needs to be moved out of the boiler house manually for waste contractors to dispose of it. The heat exchange plates should also be cleaned regularly (every 3rd or 4th time the ash is removed) to maintain the efficiency of the boiler.

Someone (not quite a ‘stoker’ but a designated boiler operator) needs to be contracted to attend to the boiler on a regular basis to ensure the ash is removed. There also needs to be a site representative present for wood chip deliveries. Finally, a waste contract needs to be arranged to collect the ash.

How often does someone need to attend to the boiler?

How often these tasks need to be performed depends on the operating hours of the boiler. Most biomass boilers in new build developments are sized as small as possible whilst still being capable of delivering the Carbon Dioxide emissions reductions required by planning policies. This means that they need to be run at full power for long periods in the year to deliver the savings committed to. They therefore need regular de-ashing, often at least fortnightly.

What about deliveries?

The regularity of deliveries is a function of the boiler size, store size and running hours. In new build developments where the boiler was installed to meet planning policies the store is often as small as possible and the boiler size results in running hours needing to be high (as covered above). Therefore deliveries also need to be regular. These can need to be as regular as weekly in the heating season and less frequently in the warmer months.

What needs to be considered at design stage?

Boiler house designs need to accommodate ground level access or a lift or ramp from basement plant rooms so that the ash can be removed.

For small boilers (less than 100kW) a larger fuel store would reduce the number of deliveries required because these would generally not have a store that could take a full load. With larger boilers, with stores that can take a full load, increasing the store size further would not reduce the number of deliveries.

Wood chip needs to be tipped into a store which ideally should be located underground with direct access for delivery vehicles. There are suppliers that have hook lift lorries that can supply chip into ground level stores. Specifying such an arrangement would limit the number of suppliers that could service the site and therefore may result in increased fuel costs.

Wood pellet can be blown, by what is effectively a powder lorry, up to 25m in a pipe. This means that the fuel store can be located at ground level, or underground, anywhere that is less than 25m from an HGV loading bay. The pellet can be piped by a combination of flexible pipe provided by the delivery lorry or fixed pipes installed at site.

What are the implications of blowing pellets long distances?

However, blowing wood pellet over long distances (20-25m) requires the delivery vehicle to rev very high in order to provide the power to blow the pellets so far. This results in much noisier deliveries and possible disturbance to nearby residents. The pellets will break down more due to the speed and distance they travel. This will result in more dust in the store which will need to be cleaned out (annually in the case of 25m delivery distance).

There is also an issue with guaranteed quality of the pellets supplied. If the pellets are likely to break down excessively (due to long delivery lengths) this can result in problems with the biomass boiler. Pellet suppliers may therefore not guarantee the quality of their pellets will meet the specification required by the boiler manufacturer. A lack of such a guarantee could cause problems if the manufacturers warranty needs to be invoked.

Are biomass suppliers reliable and will they continue to be in 5, 10, 25 years?

Since pellet is compressed it has a higher energy content and therefore a higher value per lorry load. This means that the delivery cost is a smaller proportion of the cost of a delivery and it is therefore worth transporting pellet over larger distances (nationally by road or even international shipping). Therefore users can rely on a national supply of wood pellet fuel. Wood pellet is also often produced from the waste sawdust produced in milling operations. Therefore, as long as sawn timber is required, there will be a supply of wood pellet.

Wood chip has a much lower energy content per load and therefore is only worth transporting over shorter distances. Only local supply is viable (generally less than 50 miles) so long term security of supply is more of a concern.

So surely there’s no question that pellet supply is more?

Yes, but, wood pellet is a more processed fuel so it costs more per unit of energy supplied. So the heat produced costs more to occupants than heat produced from wood chip. In fact, at current prices, heat supplied using wood pellet costs more than natural gas.

Posted on February 22nd, 2010