Charcoal quality

[Woodworks]

9 hours ago, openspaceman said:

Hotter drives off more volatiles and increases the fixed carbon content. So hotter means less yield but higher carbon content. Carbon has a higher calorific content per kg than wood.

 

 

OK so the fact I will let my retort get up to 700C towards the end of the burn could be why this chap found our charcoal so good. We simply make a more energy dense charcoal than most.

 

"Retorts tend to self limit their temperature because the process goes back to endothermic above about 440C as the structure of the char matrix begins to change and  most of the evolution of the hydrogen and oxygen containing species has finished so the exothermic reactions of the initial pyrolysis products splitting and cracking has finished." 

 

Any chance you could break this bit down for numpty's to understand?  You say it will tend to self limit but it wants to sit up around 600C-650C for much of the burn without careful air control. The temps are taken in the firebox mid hight of the charge chamber.

Edited May 10, 2018 by Woodworks

[openspaceman]

2 hours ago, Woodworks said:

OK so the fact I will let my retort get up to 700C towards the end of the burn could be why this chap found our charcoal so good. We simply make a more energy dense charcoal than most.

700C is very hot for a steel walled retort because at that temperature the steel will oxidise on the outside at least.

2 hours ago, Woodworks said:

 

"Retorts tend to self limit their temperature because the process goes back to endothermic above about 440C as the structure of the char matrix begins to change and  most of the evolution of the hydrogen and oxygen containing species has finished so the exothermic reactions of the initial pyrolysis products splitting and cracking has finished." 

 

Any chance you could break this bit down for numpty's to understand?  You say it will tend to self limit but it wants to sit up around 600C-650C for much of the burn without careful air control. The temps are taken in the firebox mid hight of the charge chamber.

Most of the mass loss is in the temperature range 330C to <500C above 800C the char is almost pure carbon and only 15% of the dry weight of wood used, depending on species. Batch retorts (i.e. those that you initially heat with other means and then they burn their own offgas to sustain the temperature) have given off most of the gaseous pyrolysis products by ~450 C so there is no more fuel to sustain the pyrolysls, as above about 460C the char undergoes a change which requires energy input  it tends to not exceed this temperature in the absence of other heat input. This lower temperature char thus still contains some tars which need to be burnt in a flame else they smoke which makes it poor for indoor cooking (indoor use of charcoal is unwise because of the risk of CO but it happens in many less developed urban areas where a wood fire and associated smoke en mass is unacceptable) . From the little I have seen of imported hardwood charcoal  it typically is much denser than ours and made at higher temperature, so it is harder to light, burns more slowly and gives off little or no smoke.

 

Incidentally it is the fat dripping onto the hot coals, being partially burnt and then condensing on the food that makes flame grilled meat a source of VOCs and free radicals which are best not eaten regularly.

[Vigen Tigen]

Hi, if it's not too late to pick up this thread, it would be great to get your thoughts on the following as I have recently made my own charcoal retort and struggling a little bit with understanding working temperatures. We currently cook the mixed batch of wood at around 500° and after about five hours get a reasonable looking barrel of charcoal, although there are some brown tinges to the resulting charcoal. Should I be burning longer and is it possible to start the burn at a high temperature say around 700° and then reduce to around 450 to 500° (similar to cooking root veg on a stove ie bring up to boil, then simmer to cook)

[openspaceman]

How are you measuring your 500°C and  which side of the retort?

 

Charcoal is quite a good insulator so getting heat into the middle can be a problem if you want to exceed 450°, this is one reason we used a type of kiln.

 

The brown may not be un pyrolysed wood ( brown end, brand etc) but offgas that has been cracked as it was evolved from within the log.

[Vigen Tigen]

I'm  monitoring the heat using a barbecue style gauge with a long probe that goes through the insulation and is towards the top end of the retort as you can see in the image. 

It's interesting what you say about the pyrolysis process as there are definitely no brown ends but some of the charcoal has a brown/bronze tinge, mainly on some pieces of bark.

My last burn (third so far with this retort) lasted around 10 hours. Each of the previous two burns were around 5 to 6 hours. There was no noticeable difference in terms of quality of charcoal by extending the burn time which was really interesting as I was expecting to achieve that crystal light tinkle sound, which none of the burn so far has given me. Should I be concerned about this? 

 

Edited February 6 by Vigen Tigen
new image

[openspaceman]

So the thermocouple measures the temperature at the top outside of the retort?

[GarethM]

Can you actually overcook charcoal.

 

I assumed as it's oxygen free its more a case of cooking it without wasting extra time ?.

[openspaceman]

With slow pyrolysis as  you heat it beyond about 450°C , when the charcoal is about 25% of the dry weight of the original wood, more tars are volatilised, these contain carbon, hydrogen and oxygen, until you reach a point at above1000°C when less than 15% of the weight of the original wood remains the char is almost pure carbon and ~10% ash.

 

Fast pyrolysis, where a wood particle is subjected to a temperature rise of more than 1000°C in a second, converts nearly all of the wood to gases and ash.

[GarethM]

1 minute ago, openspaceman said:

With slow pyrolysis as  you heat it beyond about 450°C , when the charcoal is about 25% of the dry weight of the original wood, more tars are volatilised, these contain carbon, hydrogen and oxygen, until you reach a point at above1000°C when less than 15% of the weight of the original wood remains the char is almost pure carbon and ~10% ash.

 

Fast pyrolysis, where a wood particle is subjected to a temperature rise of more than 1000°C in a second, converts nearly all of the wood to gases and ash.

I get the fast Vs slow pyrolysis, as that's with added oxygen its effectively a gasification log boiler.

 

But you're charcoal retort keeps the two separate, carbon can't burn without Oxygen.

 

Presumably Vigen isn't getting it hot enough then ?.

[judastreefairy]

1 hour ago, GarethM said:

Can you actually overcook charcoal.

 

I assumed as it's oxygen free it’s more a case of cooking it without wasting extra time ?.

Bloody hell marra what planet you on, it’s charcoal, I assume if you had a brain cell you’d be dangerous