It’s the third installment in this series, and now that I have been working with the burners for a month or so, I thought it would be fitting to review the challenges and successes I have experienced. I have only really tinkered with two burner systems – the double-barrel retort and the simple, yet archaic, smother burn system (in a barrel or pile). However, I would only use the smother burn method when I did not have sufficiently dry feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More to hand. In this article, I will explore how the double barrel system is, in principle, best in class, whilst having some practical teething issues. 

WHY the Double Barrel System?

I was drawn to the double barrel burner system for its intriguing promise of managing its own pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More process – an almost self-sufficient way of creating char from biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More. The appeal was clear: the prospect of loading up the barrels in the morning, lighting the fire, and letting the system run its course without constant intervention, only to return later and find a fresh batch of biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More ready for use. In theory, it sounded like a low-maintenance, almost set-and-forget solution, which, for anyone who has wrestled with more hands-on methods, is a compelling proposition.

Of course, if experience has taught me anything, it’s that even the simplest-sounding systems can come with unexpected complications. My curiosity was first sparked by Bob Wells’ insightful series on grassroots biochar production – a fantastic entry point for anyone interested in the DIY side of pyrolysis:

Armed with this inspiration, I set out to construct my own double barrel retort, eager to explore whether this much-lauded method could live up to the hype, and whether it could truly deliver a balance of efficiency, simplicity, and quality. What follows is an honest account of my journey through initial setup, first burns, and the daily realities of making biochar with this system.

HOW Does it Work?

The double barrel system, as the name suggests is made from two steel barrels: a smaller one nested inside a larger drum. The inner barrel holds the biomass and is sealed, apart from small vent holes at the bottom. The outer drum acts as a combustion chamber, filled with kindling or scrap wood and set alight. As the outer fire heats the inner barrel, the feedstock inside starts to pyrolyse – essentially cooking the contents of the inner drum in the absence of oxygen – turning it into char rather than ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More. To ensure adequate air flow to the combustion chamber (the outer barrel cavity), air intake holes should be drilled on the sides. Although I would advise cautious experimentation with this, as too large holes will bring in too much oxygen, and too few will hinder the burn process.

The feedstock is loaded into the inner chamber, filling it to approximately 80% capacity to ensure adequate gas flow from the material and even heat transfer. This is the same reason woodchips can struggle in this system, as there is not enough space between the pieces. 

The outer chamber is then loaded with wood, surrounding the inner barrel, but not underneath it. One impracticality I found was that my inner drum lid clip was quite large, and so I had to load it and then place it in the larger drum. This could be physically challenging for some users with heavy feedstock, but can be remedied using metal weights to hold the lid on. Despite it sounding counterintuitive to typical fire lighting approaches, a fire is then lit on top of the inner barrel.

This fire then begins to spread down the side of the barrel, burning the material in the outer chamber. This process begins to heat the material within the inner barrel, causing the reactive organic compounds within the feedstock to be released. Because they are in a sealed environment, the pressure pushes this gas out of the release holes at the bottom. This then acts as fuel to continue the burn process. This then continues, until the feedstock has become char, and it has no more organic compounds to release, stopping the burn process and making the system self-regulating.

A problem I have encountered is getting the inner barrel contents hot enough to cause their natural compounds to release and thus feed the burn, once the outer material is expended. The obvious solution is to burn material on top of the inner drum for a longer period, but this can detract from its self-regulating strengths. A potential approach to get around this, however, is to place some evenly sized dry wood around the outer lid’s rim, propping it open temporarily, and building a larger initial fire. Once this fire has raged for a sufficient amount of time, the timber burns through and the lid falls back onto the barrel. However, the safety of your own set-up and surroundings should be considered here.

WHAT’s the Verdict?

My current evaluation of the system is mixed. There are some benefits to using the double barrel retort that other ‘simple’ pyrolysis systems cannot achieve, but there are also practical challenges that the unit faces in my experience using it. I hope to refine these over time and make some adjustments as I continue to experiment. To close, here are some pros and cons to the method:

Pros:

• Minimal Waste/Emissions: The system is still far less wasteful than a standard smother burn, by utilizing the feedstocks natural compounds to fuel itself. There is still some wasted material and emissions from the outer chamber burning, but I have found this is far less than the amount lost to ash during a smother burn. However, testing a Kon-Tiki kiln burner could improve smother burn waste levels.
• Self-Regulating: When working at optimal levels with good feedstock, the system can be self-regulating and this is very appealing to people that are seeking to trial biochar or use it in heating systems that will burn for longer periods without continuous manual inputs.
• Similarity with ‘Advanced’ Systems: The other benefit, which I find most useful from an educational standpoint, is that it is the most comparable ‘simple’ burner system to understand more ‘advanced’ ones. I take this definition of ‘advanced’ to be any system that extracts syngasses and/or bio-oils (Adhikari et al., 2024).

Cons:

• Feedstock Size: Feedstock size has to be uniform, and not too large to ensure natural gasses are released early enough and consistently. Otherwise, pyrolysis will occur unevenly, leaving you with some uncooked material.
• Time: Burn times seem to vary quite drastically depending on the feedstock, so it is hard to estimate how long different material will take to burn. This isn’t a problem for my circumstances but could be for someone working to a commercial timeframe. 
• Moisture: This burner system has very little tolerance for moisture or sappy feedstocks. For the inner material to begin releasing it’s natural gasses, it often has to be extremely well seasoned. Again this isn’t a problem for some people, but I have a quick feedstock turnover supply, coming from gardening and coppice waste, so I would need a system that can handle this better. This is why a system like this would need adequate adaptation for forestry applications, unless sites were returned to annually.
• Initial Smoke Output: Whilst the double burner system begins to burn quite cleanly once the gases get released, the initial combustion that occurs is still quite smokey. The use of a heat funnel does seem to mitigate this slightly, but it ought to be remembered when utilising it – particularly on a windy day.

References

Adhikari et al. (2024) Comparative analysis of biochar carbon stability methods and implications for carbon credits, Science of the Total Environment 914 (169607). DOI: https://doi.org/10.1016/j.scitotenv.2023.169607