Dobson Gasifier

Omnifueled Gasifier/Air-Water-Heater Energy System


by Larry Dobson

This gasifier design is the culmination of my 38 years of work in the field. It will solve many of the problems that now plague those modeled after the old WWII Imbert gasifiers. While many recent improvements have been made by dedicated gasifier enthusiasts, it is still an evolving technology, with much scope for new approaches and improvements in the utilization of huge quantities of locally-available waste biomass fuel sources. Biomass- and waste-fueled energy has the potential to contribute much more to global green energy demands. To learn more on this subject, read my Department of Energy report on “Biomass Energy, State of the Technology, Present Obstacles and Future Potential” at

The above drawing is a 120,000 Btu/hr biomass-fueled gasifier/hot-water-heater. It can be a stand-alone gasifier, heating water by cooling the gas, to be used to fuel a genset or other gas appliance; or it can be a waste/biomass-fueled hot-water furnace, burning the gas internally, with high efficiency and cleanliness of burn.

This is a completely new design, based on the best of my previous prototypes. Most of my earlier prototype gasifier/combustor systems were based on a rectilinear side-draft gasification flow through a narrow fuel column into a close-coupled combustor surrounded by a ceramic heat exchanger that preheated gasification air, even using fuels with up to 2/3 their weight in water. For more details on my previous work visit my website.

This prototype embodies optimized gravity flow principles that follows more elegant thermodynamics than previous approaches, through an interweaving of concentric shells and spiral ducts. This latest design, embodied in both a gasifier/hot water heater and a gasifier/hot air heater, named Roundy Water and Roundy Air, are sized for a large household, 120,000Btu/hr, 35kWe, which will heat water for heating needs or generate electricity through a genset fueled from the gasifier.

Roundy will put out substantially more than 32kWe when responding to an internal combustion engine vacuum. I expect a very large turn-down ratio, at least 20/1 in combustion mode, with very high efficiencies in condensation mode, perhaps 95% of the High Heat content of wet fuels. Most furnaces are rated at Low Heat (dry weight basis) efficiency, discounting the moisture in the wood, since condensation does not occur, thus losing the heat of vaporization. So a 100% efficient furnace is still passing steam-heat out the stack. Since green biomass can often be half water, and the other half of bone-dry wood combusts with more Oxygen from the air into more than half its weight of additional water vapor! This represents a significant increase in energy available from condensing the woodgas or exhaust. This and the exhaust-scrubbing advantage from condensing the exhaust is the reason the heat-exchanger on this furnace is so large (150 sq.ft.).

This unit will create a high quality gas, clean enough after the condensing heat exchanger (CHX) for the engine without further cyclone separators or filters. There will be an operating range where the gas is the cleanest, and another condensing range where efficiencies of both gas and hot water production are highest, and an upward range where exhaust is too hot to condense. These parameters will all be tested to optimize this technology for manufacturing.

Any fuel that will fit in the hopper and produce a combustible gas can be gasified ~ logs, chips from the tree trimmers, bark, sawdust, corn cobs, leaves, wood-, straw-, municipal solid waste-pellets, green and wet biomass (up to 2/3 water in the furnace mode), household and farm waste, etc. Ideally, anything that will burn can be vaporized or gasified and turned into a fuel.

A quality fuel gas contains mainly Carbon Monoxide (CO) and Hydrogen (H2), combustible gases, along with the 79% Nitrogen (N2) from the air and the considerable water vapor (H2O) generated by the combustion chemistry as well as held in the fuel. Generally, the Nitrogen Oxide (NOx) content is very low, and soot, hydrocarbons and other smoke pollutants are non-existent. The steam will condense on grains of fly-ash and the cleansing rain will scrub the gas clean in the condensing HX.

The difference between ideal and practical has been significant in the history of gasification, largely because waste fuels are not homogeneous and measurable like gasoline and natural gas. The pieces gasify at different rates, and the difference in moisture, ash, elemental composition, slagging temperature, oxygen and CO2 penetration of the coal-bed, feed considerations, etc. present many uncontrolled variables. These challenges have been grist for my explorations into this technology. Roundy has been designed from the following

Observations & Discoveries

This gasifier/furnace is designed around these observations and discoveries, with the following features:

Air/gas flow characteristics:

Air and gas flow throughout this system is governed by fundamental principles of gravity, temperature and density.


Air/gas flow through the system

Refer to the above drawing for orientation.

Warm regards,
Larry Dobson
[email protected]

For more information, visit Larry's website:
Fundamental Form | Biomass Gasifier Breakthrough
Fundamental Form | Energy From Waste

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