Solid fuel (smokeless coal) efficiency

[Marcin]

A bit of an unusual topic, as these are very rare in the UK.

I went back to Poland to commission, test and tune the 20kW solid fuel boiler and the domestic heat exchanger for the hot water tank (300litres). Old heating system with steel 2-inch pipes and cast iron segmented rads

We were warned, that the boiler can struggle a little to heat 210m2 (most likely 24kW would be better for 5 bedrooms, lounge, kitchen, bathroom and the toilet).

On arrival - the house was very cold, an elderly mother is afraid of lighting fire.. (oops, a huge design fault). 12 deg inside, 300litres DHWT @mains cold.

There is an automatic coal delivery: every 1min 40 sec, spiral turns for 8 seconds, air pump set up to 15 (in scale 0-30).

I decided to torture (aka stress test) the system and kill some legionella bugs. Target: 75 Cels on rads and 75 inside DHWT (temporarily - I will reduce it appropriately), so after some tweaks, delivery 9 seconds, every 1:20min, air pump 15

Few questions though.

1. Ashes are approx 10% of the burnt coal volume. Barely any slag present. Good/bad?

2. I do not understand what is the purpose of the valve pictured. It has to be there as per manufacturer's blueprint, but what for? Is there a problem when cold water gets inside the boiler, hence the bypass, to slowly seep the warmth into the CH system?

3. System struggles to get to my desired deperatures (75 and 75). looks like 55CH/65DHW. Should I give it more air and fuel? Wait until the bulding warms up a bit more and then to tune the settings?

House got warmish in about 6 hours (16 cels), currently holds 19 degrees in all rooms, Tap hot at 65 after 24h. Used 25kg of coal.


I do get that this technology is widely frowned upon, but for less than £10 per day, it is quite tricky to heat the big house.

Any advice would be appreciated.

 

[gmartine]

Dont know much about the system but...

it looks like a fairly crude furnace and much depends on quality of fuel but 10% is ok.

Three way mixing valve being used as bypass needs adjustment?

Do you need to heat all the rooms and 300 litres of water store and for how many residents?

 

[John.g]

You only used 25kgs fuel/24hrs?, using the info below, CV of coal is 6.94kwh/kg so heat input was 7.23 kwh/hr., assume 75% boiler efficiency = 5.4kw boiler output (from a 20kw boiler?), you need a new fireman.
Any idea of the flue gas temperature?.

"The coal selling by LS POLAND is characterized by a high calorific value exceeding 25 000kJ / kg, with a low moisture of about 10%, low sulfur content of 0.3% and ash of about 6%."

 

[Marcin]

You only used 25kgs fuel/24hrs?, using the info below, CV of coal is 6.94kwh/kg so heat input was 7.23 kwh/hr., assume 75% boiler efficiency = 5.4kw boiler output (from a 20kw boiler?), you need a new fireman.
Any idea of the flue gas temperature?.

"The coal selling by LS POLAND is characterized by a high calorific value exceeding 25 000kJ / kg, with a low moisture of about 10%, low sulfur content of 0.3% and ash of about 6%."

Im using Sztygar Ultra: Groszek Plus (dawniej ekogroszek) Ultra Sztygar - https://www.wegielsztygar.pl/product-pol-113-Groszek-Plus-dawniej-ekogroszek-Ultra-Sztygar.html So, it is 28-26 MJ (or if you prefer 28000-26000kJ), so 26MJ gives about 7.2kWh/kg.

Still, many consider about a kg per hour as high consumption.

Yes, the boiler is rated 20kW, but it's the max.

In "work" mode, you can set up air blower, length of the coal delivery (in seconds), and frequency how often it should load.

For example, if I would set up the blower to the maximum and set up delivery every minute, and for 10 seconds, I would probably change the deflector into plasmous stage. But it would waste huge amount of coal nonsensically. The whole trick of using it, is to burn the coal as slowly/effectively as possible.

I cannot reliably measure the flue gasses now, but - I can touch the steel pipe for a second, and the walls with the chimney shaft are cold (as they should)

 

[Marcin]

Dont know much about the system but...

it looks like a fairly crude furnace and much depends on quality of fuel but 10% is ok.

Three way mixing valve being used as bypass needs adjustment?

Do you need to heat all the rooms and 300 litres of water store and for how many residents?

Apparently, coal boilers should not receive a full load of cold water returning from radiators. It causes sooth and moisture to slowly damage the steel heat exchanger and makes te burning less efficient. That is why those mixers are being used. Took me a while to understand that idea.

After 3 days of heating, the temperature has stabilised across the house - all around 20-21 cels, which was desired.

300 litres DHW storage is way too large for one person, but every few months during family gatherings there are about 6 adults and 3 kids using it.