A. Basics and background
It is assumed you know that pyrolysis is a process that transforms biomass into biochar, volatiles and heat. If the char is gasified and the volatiles completely burned, that would be clean combustion to water, CO2, ash and heat. The hundreds of different combustion devices, some excellent and some quite poor, with special characteristics for different types and conditions of biomass, can be classified in different ways to help us determine which are best for which objectives.
Except for charcoal-as-fuel production, for centuries combustion engineering has focused on maximizing the heat by minimizing any residual carbon. But in recent decades, the retaining of carbon as biochar has attracted interest in intentional pyrolysis (versus unintentional in forest fires, etc.).
Intentional pyrolysis for biochar almost exclusively occurs in confined non-combustible spaces commonly called kilns or ovens that are essentially “cavities” (chambers, including pits) that exclude or limit access of oxygen (in air) to hot biomass in order to avoid or limit the combustion of the created biochar. The few “non-cavity” exceptions include the “Top-Down-Burn (TDB)” method and similar Conservation Burns.
The technologies (methods and devices) for pyrolysis can be described in many ways:
- Size: (micro to industrial) (See image below with 7 orders of magnitude of pyrolysis)
- Oxic or anoxic: (the presence / allowance of some oxygen (in air) inside the chamber
- Batch or continuous (or semi-continuous)
- Speed of the pyrolytic moment: (Fast or slow pyrolysis)
- Manual or mechanized (or some degree between)
- Automated or person-controlled
- Air control
Forced air (FA) or natural draft (ND) - Open or closed top
- Mobile or stationary (or some degree between)
- Directional (movement of the air or of the biomass inside the kiln. [Source: Paul Anderson; used in multiple presentations.]
- Cost: (Free to Multi-million dollars)
- Emissions
- Materials of fabrication)
- Distinctive components of fabrication
- Skills needed for operation
- Biomass characteristics
- And more
Of the several basic types of kilns, our focus is on low-cost relatively simple pyrolysis technologies.
Retorts have existed for centuries: Biomass is in an enclosed anoxic container that is heated from the outside, first with an external fire and later with the burning of the combustible volatiles emitted.
Top-Lit UpDraft (TLUD) devices originated in the 1990s and utilize an oxic downward migratory pyrolytic front (MPF) for exceptionally consistent clean combustion, especially for cooking stoves.
Flame Cap (FC) pyrolysis is a 21st century innovation that is well suited for the panel innovations and is central to this discussion.
B. Flame Cap (FC) pyrolysis
Originating in Japan and with early 2000s work by Makato Ogawa and associates including Akira Shibata and Steve McGreevy, the Flame Cap (FC) technology is associated with many names: Moki kiln, Top-Fed Open Draft (TFOD), Kon-Tiki by Hans-Peter Schmidt and Paul Taylor, Moxham kiln, Flame Curtain, and Cavity Kiln pyrolysis. There are numerous specific designs and implementations, including pits and trenches in the earth as well as steel cones, inverted pyramids, troughs, vertical cylinders (rings), and horizontal barrels. From 2015 to present, Anderson has been involved with Flame Cap devices, including his 2020 invention of the Rotatable Covered Cavity (RoCC) kiln, the only FC kiln with a top cover of any type. .
Flame Cap (FC) pyrolysis focuses on relatively thin layers of intermittent incoming biomass being exposed to pyrolytic temperatures radiated downward from a “cap” of flames that is the burning of the created pyrolytic gases, sometimes burning between and around the uppermost pieces of biomass. The created char descends into the lower cavity area where there is minimal or no entry of air (O2) and no further pyrolysis nor combustion.
A major component and factor for all kilns is the walls that define the cavity. Materials, shapes, sizes, assembly, costs, mobility vs. permanence, emissions, efficiency, safety and special features must be considered. Existing FC kilns are known to function, but with limitations about scaling up to reach much larger quantities that could justify mechanical assistance to reduce labor costs.
C. Prior art of low-cost FC pyrolysis kilns with 3D (rigid) construction
Being chambers or cavities with fire, all constructed FC kilns have a non-combustible bottom and walls that are typically sheet steel (but sometimes of ceramic materials or earth). When joined together the metal sheets create three-dimensional rigid cavities with structural bottoms, thereby losing their “flat panel” nature.
1. A major group structural-bottom kilns is based on cylinders that are either horizontal (such as rotary kilns, heated auger kilns, some retorts, and RoCC kilns) or vertically upright, including TLUD cookstoves/barrels, fire rings, and Kon Tiki kilns that include sloping walls, becoming cone shaped. Hans-Peter Schmidt and his co-developer Paul Taylor are the originators of modern-day Kon-Tiki kiln designs (Fig. 1) with structural bottoms that have popularized the Flame Cap (FC) technology with use since 2014 by early adopters for production of biochar. https://www.pinterest.com/ithakainstitute/kon-tiki-kiln/ Some inspiration is attributed by them to the charcoal-producing Moki cone kilns of rural Japan, but Schnidt and Taylor’s designs feature the sealed (structural) bottom that allows quenching by flooding from the bottom.
The other major group includes essentially box-shaped kilns, being 6-sided parallelepipped (box) structures that can include inclined sides. Most do not have a top, allowing the emissions and heat to move upward unrestricted. But all have a bottom that excludes or restricts the entry of air. The bottoms are (typically) welded steel sheets or mortared bricks/tiles, thereby establishing permanence (i.e., not to be disassembled). These have substantial weight and 3-dimensional volume that favor large and (mainly) stationary installations. These are “structural-bottom” kilns (a.k.a. “box” kilns).
Examples include the Trough Kilns (Figs. 2 and 3) made in Thailand by Warm Heart Foundation that allow for lengthy biomass to be inserted without much pre-processing (chopping). According to communications (1 July 2024) with Dr. Michael Shafer, it works well, produces good biochar, but “… is in limited use because of cost, power, and welder required.” Size and weight make it difficult to transport. Excellent details by Dr. Karl Frogner are at https://www.warmheartworldwide.org/flame-cap-trough.html .
Another example is Kelpie Wilson’s Oregon Kiln. (Fig. 3, photo from Kansas Forest Service). Wilson is a world leader in biochar production, especially in wooded areas. Her earlier technology (derived from Schmidt and Taylor’s work) included the “Oregon kiln” (Fig. 3) that is square with 4 sloping (trapezoid) sidewalls and a sealed (welded) bottom. A very informative video is:
Two examples (Fig. 4) of closed-bottom trough flame cap kilns by Paul Anderson in 2016 used corrugated roofing sheets (CRS). His report at https://www.drtlud.com/?resource=prt16820 includes nine useful references and his discussion of fabrication as well as operation of trough kilns.
