Doug Willams, Fluidyne, August 23, 2007
While this discussion is to encourage the beginner to make producer gas in the most simplistic manner, there are very sound arguments against making charcoal at cost to the environment. I missed out a very important issue which Mark questioned regarding residue volatiles, and remembered that I had written about this years ago on file, relating to designing reliable charcoal gasifiersThanks to a timely visit from my daughter to type out some extracts,(and $20), you get to see what I said about charcoal gasifiers 18 years ago to people on the ground in developing countries, and fully engaged in gasification activity. Nothing has changed, but a wider appreciation of global warming emphasises the need to use our biomass resources in less wasteful practice, than making charcoal for gasifier fuel.The bottom line of my presentation stated: If the quality of charcoal could not be controlled to ensure that all volatiles were carbonized, then the gasification process had to have a tar cracking capability. In this respect, the design had to be the same as for a wood gasifier, therefore it was a waste of the fuel resource to make charcoal, and loose the energy content of the volatiles. The arguments against this, continually return to the cost of labour, to cut the wood into fuel blocks by hand, which last week, I did myself at the rate of 50-60 kgs/hr, the equivalent of about 16 litres of diesel fuel. (worth $1.05/ltr)Even in New Zealand, that is a good return for a worker with no skills to be employed for the job, and if done on contract, rural people will bury you in blocks given the opportunity to earn cash money. This leads onto the simple fact, that if a gasifier is not being used to conduct an activity that creates a financial return, then there is no point in using the technology. Alternatively, if you wish to create a standard of living that is currently provided by fossil fuels, then you need to reallocate your time to become a survivor, and face the realities, and responsibilities it takes, to cut the mustard! There is a price to pay to join the "Black Hands Gang" (:-)
Extracts from a Paper I wrote on conceptual designing of charcoal gasifiers in 1989.INTRODUCTION In preparation of the material for this workshop, most of the modern papers on engine gasification have been reviewed. This modern material is of course based on historical work by earlier investigators of the technology, adding to, but rarely improving on the quality of knowledge available.For whatever reason, researchers papers concentrate on the theoretical mathematical modelling of each and every aspect of the various chemical processes. None to my knowledge make reference to the physical phenomena that accompany the chemical equations. Gasification is a continuous process of staged phenomena that can be managed to perform consistently given that the phenomena is created in the correct sequence.It is this physical behavior that determines gasifier design, and the design which provides the conditions to handle the phenomena.While the knowledge of gasification is widespread, the understanding of converting it into a reliable technology appears to be missing from the comprehension of many involved with the technology.It has been written that gasification is not an art but a science. While this may be true, what has not been appreciated is that the fabrication of steel to manufacture a gasifier is an art form unto itself.This material pays particular attention to the gas cooling, cleaning systems.No formulas are offered in this work which is based on the collection of information from Fluidyne wood gasifiers over many thousands of operating hours.Doug WilliamsMarch 1989
1. The ChoiceThe gasification of biomass is going to play an ever increasing role in providing energy for developing economies.Given the right conditions, gasification is a powerful economic tool, but the wrong conditions can create an ecological disaster.To implement gasification projects in any country, the choice of gasification fuel has to be determined by availability. Most projects will use wood or charcoal, and this is the choice that has to be made before a project begins. The Greenhouse Effect is now starting to affect the world climates as a result of fossil fuel combustion and atmospheric pollution. Can the manufacture of charcoal continue to be encouraged?Rapid depletion of tropical rain forest continues unabated and soil erosion contaminates rivers, ruining water supplies.As designers and builders of gasifiers, our choice of whether to carbonise 3 - 5 tonnes of wood to yield one tonne of charcoal, has long term environmental repercussions. Taking the easy route to problem solving, has historically proven to be the most costly in the long run. Can we afford the cost?
2. DEVELOPING THE CONCEPT
2.1 Purpose of the design Before any gasifier design can be formulated, the purpose of the design must first be derived.When a gasifier is used in one place with a single fuel and source of supply, the operating parameters of the gasifier can be determined and closely controlled. From a design point of view this has proven to be the greatest weakness, since most of the design and manufacture has evolved in the safety of a developed country. Even the fuel woods and charcoals used for development are those readily available and rarely are similar or compatible to those found in the countries that have to use the gasifier.Most of the problems that are experienced with the gasifiers can be tracked right back to the beginning of the project by the objectives being founded on the wrong concept.Because a gasifiers use cannot be guaranteed to be restricted to one set of operating parameters, the design must be compromised and provision made for variability.
2.2 VariabilityVariability is a specification in itself for it has to cover the uncontrollable aspects of gasifier operating conditions. These can be summarised as follows:Location Altitude Ambient Temperature Humidity Fuel Species Moisture content of the fuel Quality of fuel preparation For any gasifier to operate without problems within these variables, the design must be able to handle them all in the very worst situation.
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