With fossil fuel prices rocketing and electricity costs following, what can industrial wood waste offer energy users? In many cases the cheapest cost per kWh of any fuel on the market, however these depends on the fuel, the amount required and the location. We can quickly go through the fuels available and the estimated cost per kWh.
This varies hugely, and although lots of fuel priced via a cost per ton different fuel can have huge variance in the calorific density of the fuel.
By HLHJ – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=76251034
When it comes to calorific values of wood, moisture content is the most important factor. Wood has a complex structure of xylem, phloem, and lots of other structures, but in bulk is made of long chains of the polysaccharide cellulose. When the tree is living these molecules interact with water via hydrogen bonds. Therefore, the first stage of combustion is the vaporisation of these water molecules.
Only after water has been evaporated can the cellulose be decomposed to Polyaromatic Hydrocarbons and other intermediate products that can oxidise and burn in air.
These means that the water in wood needs to heat up to 100oC and then the Enthalpy of Vaporisation energy is absorbed. This means in a 50ton delivery of 30% moisture content wood, you have been delivered 15tons of water that you have boil. This means that the supply chain around the wood you intend to burn is very important and where industrial wood waste is important.
As wood changes shape and warps with large changes in moisture content and temperature. Wood that is being used in for manufacturing needs to be dry. Wood when cut can be up to 50%-60% water. Wet wood needs to be dried, and heat treated before it can be used for structural timber, furniture. Most natural timber used in the wood working industry is between 10%-15% moisture content. Manmade boards will be less than this and can be as low as 8%.
The comparison to make is a standard virgin wood chip G50 30% moisture content fuel will have a calorific value of the order of 3.4MWh/ton, where a 10% moisture content MDF dust will be closer to 4.7MWh/ton. That is a 38% difference the energy per ton.
So, it should follow that the MDF dust is the calorific fuel and therefore costs more? However, the opposite holds true. The G50 30% moisture content chip is 50% to 100% more expensive in most areas than the MDF dust.
If this is true, why doesn’t everyone burn wood waste?
It is harder to burn wood waste. The capital cost of equipment to be able to burn wood waste is certainly higher however all the marketing focus was around beautiful photogenic virgin wood chip.
In the 2000s and the 2010s burning virgin wood chip was thought to be “clean” and burning industrial wood waste and manmade boards was “dirty”. This went further than marketing, the people who were making the rules on emissions thought so too. While biomass boilers burning virgin wood chip just had to have a laboratory type test emissions certificate for the entire product line, machines burning wood waste had to be tested onsite with the fuel that they were using all year around.
However, BEIS found in 2018 found that biomass boilers, although they had fantastic lab results on perfect fuel, in the real-world fuel was found to be less than perfect was contributing to emissions of PM2.5 and NOX. Since this point virgin wood and the “quality” of virgin wood chip going into cheap fire chambers has become of huge importance and the focus of BEIS’s efforts towards cleaning up biomass energy emissions.
The impact that this had of wood chip prices has been huge. Lots of customers has been trapped into buying cheaper machines that require expensive, nearly perfect fuel to burn cleanly.
Other factors have come into play, the Medium Combustion Plant Directive came into effect in 2019 and this put the spotlight on new biomass boilers over 1MW thermal input to pass an emission test every year on site, using the fuel it was using through the year.
There are other engineering considerations, MDF dust is far more calorie dense and produces 3 times the amount of ash. This makes designing a fire chamber challenging. Also, burning dust requires a much longer flame path and turbulence to achieve complete combustion.
As the focus on fuel quality for wood chip has increased, so has fuel costs. Now for many projects we see 3p/kWh for G50 30% chip while closer to 1p/kWh for MDF dust. These lead to staggering fuel costs.
Table 1 Example Fuel Costs for a 2.2MW Boiler
|Fuel Type||£/T||kWh/T||£/kWh||Fuel Cost per Week for 2.2MW|
|pellets 10%||£280.00||4700||£ 0.060||£22,400.00|
|g50 30%||£135.00||3400||£ 0.040||£14,929.41|
|dust 6%||£ 55.60||4900||£ 0.011||£4,266.45|
Over 50 weeks of the year this is a difference in the wood fuel prices of £350,000 a year. The comparisons to Gas and Heating Oil are huge, potentially £3-4million fuel cost difference.
It depends. We have been able to retrofit our combustion chambers and flue gas cleaning systems into existing systems with horseshoe boilers with great success. As we make our fire chambers in Northampton, we have been able to adapt our existing designs to fit into different boilers and boiler rooms. Sometimes conveying systems must be adapted and a lot depends on the space and control system that is in place.
Changing adapting a fire chamber to burn industrial wood waste is expensive but depending on the grade of fuel cost that the system is using now, return on investment can be as little as a year.
A 2.2MW system burning G50 30% moisture fuel switching to industrial wood waste could see a saving of £500,000 over a year.
Give us a call. Industrial Wood Waste boilers require a certain economy of scale. Fuel delivery need to be made by lorry and ideally in bulk. For fuel to be cheap it needs to be delivered in 40ton loads. If a delivery cannot be made in these amounts, then the cost per kWh is going to increase. We are estimating that systems over 150kW can receive fuel with a reasonable cost per kWh. System over 1MW will want to have more than week of fuel stored so will have capacity to take fuel deliveries.
For a 2.2MW system, switching from Gas to industrial wood waste could have a saving of £1.7 million a year.
Brilliant, you have a source of fuel ready for all your heating and industrial process needs.