Horizontal Bed Kiln

An Innovative Approach to Thermally Decompose Biomass

The CarbonZero horizontal bed reactor is based on a very reliable industrial oven design that is used in everything from commercial bakeries to glass recycling plants. Feedstock such as wood chip is heated in 3 stages, in successive compartments:

  • drying at about 180° C
  • wood vinegar extraction at about 280° C, and
  • syngas extraction / pyrolysis at about 400° - 750° C

This staged process of thermo-decomposition has several advantages. First it allows for the easy separation of water, wood vinegar and syngas, as each are outgassed at successive temperature gradients. Second, a staged process is much more energy efficient, because drying and wood vinegar extraction occur at just the temperature needed and no more, and condensation of water and wood vinegar to re-extract dry heat can then occur with only minimal cooling.

Horizontal bed kiln stages
The thermo-decomposition of biomass in stages has multiple advantages

Unique to this design is the fact that it has very few moving parts, all of which bear minimal loads and are all very simple. Critically, none of the bearings are in the hot zones of the device. The shafts that support and drive the conveyor have bearing that are housed in the peripheral framework, operating at ambient temperatures. The risk of mechanical or structural failures typical in other pyrolysis designs is completely avoided. In addition, very little energy is needed to transport tonnes of material through the device per day. For our base capacity unit, about 7 kw would be sufficient.

Also unique to this design is its simple and highly effective heat distribution approach. The feedstock particles are spread out in a relatively thin layer, and hot oxygen free gas, at the desired temperature, is injected from below. Each particle is heated to the exact target temperature in this way, producing a precise result, perfectly baked biochar. For various feedstocks and precise biochar specifications, it is possible for the operator to vary the temperatures in each of the stages, the conveyor speed, and the volume / velocity of hot gas being pumped through each section.

Our Horizontal Bed Design Compared to "Traditional" Auger and Rotary Kilns
For its throughput capacity, our horizontal bed kiln is also very economical to manufacture, maintain and operate compared to other types of kilns.
Rotary kilns are massive steel tubes designed to tumble biomass to evenly distribute heat to the particles.

A common alternative is a rotary kiln, similar to a giant cement mixer, where the entire heated vessel is rotated to evenly heat and mix the feedstock particles. The walls of the kiln must be very thick, to bear the entire weight of the biomass, steel vessel, and insulation (if it is insulated). The bearings of a rotary kiln are massive, and must be designed to withstand high temperatures. Because the entire kiln rotates, loading biomass and unloading char during continuous operation, injecting hot dry gas, and extracting vapors without allowing air to enter the vessel all become complex engineering challenges that are expensive to implement. And of course the energy needed turn the kiln plus tonnes of feedstock thousands of times per day will be substantial.

One of the main issues with this design is that the residence time of the feedstock cannot be regulated with any degree of precision. The internal vanes guide the biomass through the vessel, but some tends to fall back and some fall forward. So by the time the biomass traverses the entire kiln, there is typically a 50% variation in residence time. Hence the feed rate must be significantly slower than optimal to wait for the delayed particles to be fully processed, and that many particles will be over-processed.

To develop a staged heating process with this approach, to make it more energy efficient and productive, would require 3 separate vessels all with separate intake and outtake mechanisms for solids and gases, bearings and drives. This becomes even more expensive to build, operate and maintain than a single vessel design.

Repair and maintenance costs for rotary kilns tend to be high. Just taking it apart, or lifting it a bit to replace the bearings, can be a challenge because it is so heavy. And of course, all that steel is expensive to begin with. This approach to distribute heat might be fine for a clothes dryer, but when scaled up to tonnes of biomass, it is too primitive to have the versatility and financial performance we need for our product line.

Another common alternative is an auger kiln, where the feedstock is fed through a large auger assembly and hot gas is injected into a jacket surrounding the auger tube. The idea here again is to mix the feedstock to distribute heat evenly to the particles, but it is an expensive way to do so, and the main problem with auger based pyrolysis designs is that they often jam. Forcing the evolving gases through the feedstock and char in a tight space often causes tars to re-condense and stick particles together, called “bridging” in the industry. Bridging progressively blocks the flow of gas and heat until the auger jams on a compressed tar / wood chip / char mix. Once the auger jams, the solids within it continue to cool, and it can be a major maintenence task to disassemble the auger to free it, or worst case replace the whole assembly.

Auger kiln model
Model of an auger kiln
Auger assembly of a Pyreg biochar kiln installed near Stockholm, Sweden.

There are dozens of innovations found in the patent literature that attempt to solve the problem of bridging, including clutch assemblies to prevent the auger from breaking when it jams, auger housings fitted with a series of hatches to allow an operator to clear these blockages when they occur. Pyreg, whose unit is shown above, operates their kiln at a fixed temperature no lower than 750° C in an attempt to ensure that tars will not condense in their auger assembly. Required high temperature pyrolysis is common for auger kilns.

Most auger pyrolysis kiln manufacturers must modify their auger assemblies multiple times to overcome various failures. These designs generally become reliable through multiple iterations of trial and error, and once a particular design works, it cannot be scaled to a larger capacity. Just like the rotary kiln, a large capacity auger kiln requires a lot of energy to turn tonnes of feedstock thousands of times every day. The metal components required to mix the feedstock are expensive and prone to failure because of their exposure to high temperatures.

Similar to rotary kilns, the repair, maintenance and fabrication costs for auger kilns all tend to be high. In particular, they are prone to auger assembly failures and bridging. The approach used to distribute heat, mixing the feedstock, consumes significant mechanical energy, and the risk of bridging limits control of processing temperature. Again, auger kilns do not have the versatility and financial performance we need for our requirements. Hence we had to find an another approach.

Compared to the alternatives, the horizontal bed design is simple, very economical, and very reliable. Bridging cannot occur because the evolving gases have plenty of space above the feedstock bed to flow out of the device, rather than being forced to flow through the feedstock. Heat is distributed evenly to all particles in the simplest manner possible, by spreading them out, rather than turning tonnes of material thousands of times per day. And because of the simplicity of the design, an economical staged pyrolysis process of water vapor extraction, wood vinegar extraction, and syngas extraction becomes vastly easier to accomplish than with a rotary or augur kiln.

The simplicity of the horizontal bed biochar reactor is intentional and each aspect of the design has been considered to provide a reliable, economical and financially performant device that will produce biochar, condensates and energy, all of which can provide revenue.

Our base capacity unit will process about 550 kgs of wood chip per hour (dry weight basis) 690 kgs @ 20% moisture content, producing 170 liters of dilute wood vinegar (4% acetic acid), 165 kgs of char, and 500 kw of chemical energy in the syngas stream. These figures are indicative and will vary according to feedstock type, moisture content, and process conditions of temperature and residence time that are applied.

Capacity can be increased by lengthening it (adding sections to the centre) and/or running units in parallel. There is sufficient excess syngas to operate a commercial genset, provide process heat, or provide heat to additional horizontal bed units to torrefy biomass rather than char it. Waste heat at 50° - 80° C is available from the condenser for a pre-dryer. Drier feedstock can be processed at higher throughput rates.

The horizontal bed kiln is being developed and manufactured in partnership with Seneca Farms Biochar in Odessa, New York, USA. Please contact us for further details.

Rob Draxler
Rob Draxler with some of the first biochar produced at Seneca Farms Biochar. More to come ...