Don't be fooled by the numbers

During a selection process, when equipment is selected, proposals elected, many points are evaluated. If you аre one to look at boiler performance values, and we think you should be, when selecting a proposal or an equipment vendor then avoiding some traps could be helpful. In this part 1, we will focus on one of the key performance indicators, efficiency.

Whether you impose, or not, stringent non-performance penalties, one thing is for certain, you are stuck with the equipment installed and have to work with it for the whole life cycle. So, taking the preliminary precautions when deciding on the purchase order is always a good choice.

Efficiency

There are two terms often confused or misrepresented: boiler efficiency and boiler plant efficiency. Boiler efficiency defines the quality of the combustion and еnergy conversition process in the boiler unit itself. While boiler plant efficiency also includes any downstream heat recovery equipment and is generally a higher efficiency value to the former.

A boiler unit generally consists of a furnace and heat exchanger (boiler), while downstream heat recovery equipment are generally economizers (condensing or non-condensing).

One of the main factors that impact efficiency is biomass moisture content (MC). The MC should ultimately define the type of combustion unit and combustion itself. So, of course, burning high MC biomass is different than low MC biomass, hence the equipment and investment differ as well.

Low(er) efficiency could also indicate a drop in boiler output. Not getting the power you need when you need it, might be a problem.

For example, low MC biomass (i.e. dry) tends to be burned in simpler units or furnaces with water cooled surfaces, fixed grates, smaller grates, this means smaller furnaces w/o partial masonry, more compact units and less control instrumentation.

High or higher MC biomass (i.e. wet) tends to be burned in dry furnaces (adiabatic) which help maintain hot conditions to evaporate all the water without impacting boiler output/performance, moving grates, larger grates, this means larger fully bricked furnaces (heavy masonry), staged combustion air dosing, combustion air preheating and economizers.

Sometimes, in order to show higher performance figures (efficiency) vendors will use downstream heat recovery economizers (on the border between non-condensing and condensing) which would only operate in a tight range of operating conditions, not always, but sufficiently to achieve higher efficiency values for performance verification while getting higher marks in the vendor selection process.

These are not condensing economizers, as they would normally be based on their location, but are installed to operate in a certain limited range of operating conditions as to avoid flue gas condensation. This means that when working with wet biomass (high MC) the economizer would not be in operation and the plant would lack the declared efficiency and power. Non-condensing means that the temperature of the flue gases is kept above the dew point temperature. For example, for biomass with 55% moisture content, the dew point temperature is 63 °C.

In order to get a realistic comparison, instead of apples to oranges, make sure you are comparing boiler efficiency vs boiler efficiency or boiler plant efficiency vs boiler plant efficiency and not get them mixed up, because boiler plant efficiency will always hold a higher value and have an advantage in the evaluation process.

This is sometimes obvious when the efficiencies greatly differ between several vendors. This indicates a different scope or superior technology. Deciding which one it is and how it impacts the capex is important.

Having someone with technical proficiency on your team, definitely helps sort this out.

In conclusion, know the vendors, their technology, scope, experience and chances of realistically achieving the performance values offered.

Low efficiency could indicate, to name a few:

• The wrong technology for the application,
• Undersized equipment, especially the furnace,
• No combustion air preheating,
• No cleaning system,
• No downstream heat recovery (i.e. economizers),
• Inadequate control and visualization system.

Written by: Viktor Radic, dipl.ing.