Boiler Operation, Maintenance, and Settings Matthew Baxter Technical Director, Core Biomass
How is a boiler designed to Operate? The boiler must be designed around the key characteristics of the fuel, such that the fuel can be burnt in adequate quantities to provide the required heat output, whilst maintaining a reasonable requirement for user intervention (e.g. manual cleaning). The key fuel characteristics are: ash % ash softening and melting temperature Fuel moisture content and calorific value Particle size and distribution of sizes (dust) If the design of the boiler is fundamentally not suited to the fuel type, then operating the boiler particularly at high sustained heating loads will be problematic, leading to increased maintenance requirements
How is a boiler designed to Operate? A boiler operating correctly will be capable of: Producing the rated heat output for sustained periods Running without unscheduled shutdowns for manual cleaning Producing a well burnt out ash, with minimal unburnt carbon Maintaining the set oxygen levels within a band of approx. +/- 1-2% and maintaining efficiency Keeping combustion temperatures in the desired zone Burning with emissions in line with those stated by the test report for the boiler
Lower heating value, MJ/kg Calorific Value (dry) Typical Moisture Net Calorific Net Calorific Content (%) Value (GJ/tonne) Value (MWh/tonne) MJ/kg Wheat 17 15 14.1 3.92 Miscanthus 17 25 12.1 3.37 Wood Chips 19 30 12.6 3.49 Wood Pellets 19 10 16.9 4.69 Heating Oil * 45.2 N/A 43.5 12.1 Gas Oil** 45.6 N/A 45.6 12.7 Lower heating value of wood against moisture content 20.0 18.0 16.0 14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Moisture content
800 O C Char burning Heat production Ignition 200-300 Pyrolysis Heat consumption 105 Moisture evaporation Volatiles 80-90 % Char 10-20 %
How to adjust your boiler to suit the fuel Primary air is the most important setting, and one that is normally outside of the boilers capability to vary automatically for safety reasons. Primary air should always be set first, and everything else adjusted to suit Primary air level ultimately determines power output level. So for a given fuel type, the primary air must be set correctly to burn the fuel at the required rate, and therefore obtain the correct heat output. Wetter fuels need more primary air than dry fuels, ratio to secondary air from 1:3 (dry) to 1:1 (wet). Dry and very dusty fuels in theory require less primary air however the density of the fuel results in a higher pressure loss through the grate, meaning higher primary air fan speeds can be required.experience and feel is required! Wet and dusty fuels are very problematic, as the required amount of primary air will likely lift the fuel from the bed leading to a lot of fly ash and low efficiency / high maintenance. Where to deliver the primary air? Depends on the fuel characteristics and the grate size and shape.true multi fuel boilers will give the user full control over where the primary air is delivered to the combustion grate Single fan (suction only) boilers are therefore not suitable for a wide range of fuels, as the primary air is varied automatically by the boiler controls. These boilers should only be used for very uniform / prescribed fuel types with tight size and moisture specifications
How to adjust your boiler to suit the fuel The fuel insertion setting (stoker) is set next, to match the supply of fuel to the burn rate established by the primary air setting. The boiler can normally automatically vary this setting to a limited extend in order to deal with small variances in the fuel. (lambda control) Small changes in moisture content can be accommodated for by reducing / increasing the stoker speed. More than +/-5% moisture level will require manual alteration of the primary air first. Therefore the supplied fuel must be of uniform moisture content Minor variation in size / dust content of the particles can be automatically compensated for. A change in particle size from G30 to G50 for example will require a manual change to the stoker settings (and possibly to the primary air settings).
How to adjust your boiler to suit the fuel The moving grate speed is crucial to correct combustion, and is very useful if burning difficult fuels. Moving the grate allows the fuel pile to be kept from getting too thick, without having to add additional primary air. Excessive grate movement will open up the fuel pile, and cause hot spots on the grate which can form clinker. It could also cause a large amount of airborne ash, or partially burnt material to enter the de-ashing screws wasting energy and causing a hazard. A moving grate is a necessity for fuels that: Clinker (the grate must also be fully cooled!) Are very dusty Are very wet Are not very uniform
How to adjust your boiler to suit the fuel The Secondary and Tertiary (if fitted) air supplies are the final part to set, and are critical in obtaining low emissions. The boiler controls will normally alter these settings by fairly significant amounts to achieve the desired oxygen level in the boiler Lambda sensor is therefore critical to clean combustion, and to some extent efficiency. The lambda sensor is the first thing to inspect / clean or replace if necessary before adjusting any boiler. To know for sure it is working correctly, you need to have a flue gas analyzer to check it against. Secondary air speeds must be sufficient to mix the gasses well enough to achieve clean combustion. For this reason, boilers should not be set to modulate too low in output, as the secondary fan speeds will drop too low resulting in bad emissions Position of air injection, and direction is also important to mix the gasses well for complete combustion and low emissions.
BREAK
Daily Checks Visual check of the fire getting to know what good combustion looks like. Correct shape of fuel pile on the grate / visual check of smoke emissions. Cleaning the boiler grate as necessary. Ash emptying bins, checking levels in automated systems. Always ensure that seals are good and everything is fitted tight afterwards Checking boiler control parameters: Fire temperature key indicator of future problems Flue temperature Indicates if the heat exchanger is performing correctly. Low temperatures indicative of long term problems Flow / Return temperatures indicate problems with flow such as blocked strainers which can lead to overheating Pressure Water loss and refilling in cycles is not normal
Fuel contaminants and Grate wear
Clinker
Monthly Checks Check inside combustion chamber and clean air holes in the grates. Clean any ash piled on ceramic surfaces or brick walls to avoid clinker formation. Inspect for any wear to the metal grates or damage to ceramic linings or arches (a sign of overheating). Check and clean as necessary the flue gas pathways through the entire boiler system, from the combustion chamber to the chimney exit to avoid blockages. Grease bearings particularly high temp such as flue fan or moving floor shafts. Inspect / tighten chains and fan belts. Remove and clean the oxygen sensor Clean any ash out from areas that are not auto de-ash (e.g. underneath the grate, or on top of ceramic arches) Clean fans, particularly if they are sounding out of balance
Central Ash extraction
Basement ash handling system
Lambda Sensor
Dirty flue fan, out of balance
Annual Service Most of the annual service should already be covered on a monthly basis as and when required. It is not recommended to leave all the cleaning to a single annual service. Recommended to have an engineer who is familiar with the particular machine carry out annual servicing. Parts that are wearing can therefore be scheduled for replacement prior to failure there are many parts that will eventually need renewing such as: Spring arms on agitator systems Ash augers / stoker augers Individual grate pieces (normally wear faster at the back) Ceramic linings / arches and lintels Sensors, particularly combustion temperature and oxygen If you plan to change fuel, get a combustion engineer booked to come and recommission to the new fuel before you start running it, a lot of problems can be avoided.
Common indicators of problems Black ash / hot ash high in carbon (wasted energy) - Ash should be dark grey to white in color not black. Fuel set up is wrong. Possibly too much airborne material or excessive grate movement Burning back or smoking back - indicates a blocked air route through the boiler. stop and clean all the way through from combustion chamber to chimney top. Check rotary valve / BFP seals Overheated combustion chamber (Clinker / Cracks / Burnt out grates etc.) Fuel used is the wrong type or the setup of the boiler is badly wrong Condensation / tar Fuel is too wet, or the load is too low (long periods with no load). Wet fuel and low load is a combination to avoid. Over temperature faults Indicate circulation problems e.g. blocked strainers, poorly designed heating controls etc. or inadequate buffer volume for the boiler size and loading. Lack of adequate heat output The boiler is not correctly set up for the fuel being burnt, or is incapable of producing the rated output on the fuel used Ignition failures fuel is too wet or too dusty for auto ignition, or the ignition program is set up incorrectly.
Overheated ceramic lining
Overheated ceramic lining
Overheated ceramic lining
Damage to combustion chamber from overheating
Auger Chain coupling damaged by oversize fuel
Failed Rubber flaps (Rotary valve)_
Burn back Melted wires / sensors