Wood gas as engine fuel

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Wood gas as engine fuel




Mechanical Wood Products Branch
Forest Industries Division
FAO Forestry Department
The designations employed and the presentation of material in this publication do not imply
the expression of any opinion whatsoever on the part of the Food and Agriculture
Organization of the United Nations concerning the legal status of any country, territory, city or
area or of its authorities, or concerning the delimitation of its frontiers or boundaries.
M-38
ISBN 92-5-102436-7
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system, or transmitted in any form or by any means, electronic, mechanical, photocopying or
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permission, with a statement of the purpose and extent of the reproduction, should be
addressed to the Director, Publications Division, Food and Agriculture Organization of the
United Nations, Via delle Terme di Caracalla, 00100 Rome, Italy.
© FAO 1986
This electronic document has been scanned using optical character recognition (OCR)
software and careful manual recorrection. Even if the quality of digitalisation is high, the FAO
declines all responsibility for any discrepancies that may exist between the present document
and its original printed version.

Table of Contents

Preface
Chapter 1 - Introduction
1.1 Background
1.2 The present case for wood gasifiers
1.3 Overview of the contents of this publication
1.4 What to expect from a wood gasifier system

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Chapter 2 - Small wood and charcoal gasifiers for operation of internal combustion
engines
2.1 Fuelling of engines by producer gas
2.1.1 Possibilities of using producer gas with different types of engines
2.1.2 Engine power output using producer gas
2.1.3 Maximizing the power output in producer-gas operation
2.1.4 Resulting power output
2.1.5 Gas quality requirements for trouble-free operation
2.1.6 Use of Stirling engines or gas turbines with producer gas
2.2 Theory of gasification
2.2.1 Prediction of the gas composition
2.2.2 Gasifier efficiency
2.3 Types of gasifiers
2.3.1 Updraught or counter current gasifier
2.3.2 Downdraught or co-current gasifiers
2.3.3. Cross-draught gasifier
2.3.4. Fluidized bed gasifier
2.3.5 Other types of gasifiers
2.4 Gasification fuels
2.4.1 Need for selection of the right gasifier for each fuel
2.4.2 Energy content of the fuel
2.4.3 Moisture content of the fuel
2.4.4 Volatile matter content of the fuel
2.4.5 Ash content and ash chemical composition
2.4.6 Reactivity of the fuel
2.4.7 Particle size and size distribution
2.4.8 Bulk density of the fuel
2.4.9 Charring properties of the fuel
2.4.10 Assessment of the suitability of various types of biomass as gasifier fuel
2.5 Design of downdraught gasifiers
2.5.1 Processes occurring in the down-draught gasifier
2.5.2 Design guidelines for downdraught gasifiers
2.6 Gas cleaning and cooling
2.6.1 Cleaning dust from the gas
2.6.2 Gas cooling
2.7 Applications of biomass gasification
2.7.1 Production of fuel gas
2.7.2 Production of mechanical or electrical power in stationary installations
2.7.3 Mobile applications

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2.8 Health and environmental hazards associated with the use of producer gas
2.8.1 Toxic hazards
2.8.2 Fire hazards
2.8.3 Explosion hazards
2.8.4 Environmental hazards
Chapter 3 - Recent Swedish experiences with operation of vehicles on wood and
charcoal gas
3.1 Overview of development work and testing carried out at the national machinery
testing institute
3.1.1 Scope of the work
3.1.2 Gasifier for wood chips
3.1.3 Fibre glass fabric filter system
3.1.4 Conversion of diesel engines to producer gas operation
3.1.5 Tests with different fuels
3.2 Experiences gained from conversion and operation of modern vehicles
3.2.1 The need for continued practical tests
3.2.2 Conversion and operation of a Massey Ferguson 1100 farm tractor
3.2.3 Conversion and operation of a Scania truck
3.2.4 Experiences related to service, maintenance and equipment failures
3.3 Producer gas vehicles recently operated in other countries
3.4 Economic evaluation of operation of vehicles on wood gas
3.4.1 The case for wood gasifiers
3.4.2 Method used for the economic evaluation
3.4.3 Economic baseline assumptions
3.4.4 Marginal costs for the wood gasifier system
3.4.5 Economy of a producer gas tractor
3.4.6 Economy of a producer gas truck
3.5 Feasibility of using the vehicle gasifier technology for stationary applications
3.6 Operating hazards
Chapter 4 - A small wood gas power plant at a sawmill in Paraguay
4.1 Description of the wood gas power plant
4.1.1 Fuel supply
4.1.2 The wood gasifier
4.1.3 Wood gas cooling and washing installations
4.1.4 Wood gas filter
4.1.5 Engine and electric generator
4.2 Operating experience

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4.2.1 Fuel consumption
4.2.2 Manpower employed and operating procedure
4.2.3 Safety
4.2.4 Environmental impact
4.3 Economic evaluation
4.3.1 Capital investment
4.3.2 Operation and maintenance costs
4.3.3 Fuel costs
4.3.4 Total operating costs
4.3.5 Costs for electricity generation for family homes of sawmill personnel
4.3.6 Comparison with other alternatives for the electricity supply
Chapter 5 - A small gasifier power plant in Sri Lanka
5.1 Description of the plant
5.1.1 General system layout
5.1.2 The gasifier
5.1.3 Cyclone
5.1.4 Impingement separator
5.1.5 Glass fibre cloth filter
5.1.6 Cooler
5.1.7 Engine and alternator
5.1.8 Start-up fan and flare
5.1.9 Safety devices
5.1.10 Auxiliaries
5.2 Operational procedures
5.2.1 Start-up
5.2.2 Closing down
5.2.3 System maintenance
5.3 Operational experience
5.3.1 Operating record and observations on the performance
5.3.2 Disturbances of the operation
5.3.3 Desirable modifications
5.4 Economic evaluation of electricity generation costs at Giriulla mill
5.5 Concluding remarks
Chapter 6 - A 1.4 MW wood gas fuelled power plant in Paraguay
6.1 Historical background of the gasifier installations
6.2 Wood-fuel supply and preparation
6.3 Description of the down-draught gasifiers
6.4 Electricity production with the gas engine alternator sets
6.5 Operating experiences
6.6 Profitability of using wood gas at Loma Plata

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Chapter 7 - The future of wood gas as engine fuel
7.1 Prerequisites for extensive use of wood gasifiers
7.2 Industrialized countries
7.3 Developing countries
7.4 The need for international cooperation
Appendix 1 - Calculation of the power output of a producer gas engine
Appendix 2 - Design calculation of downdraught gasifier
Table of conversion factors and symbols (used in this manual)
References

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Preface
Wood gasifiers played an important role in the past in the substitution of oil-based fuels in
internal combustion engines, but fell into disuse after the Second World War because of their
economic and technical disadvantages as compared with relatively inexpensive imported
fuels.
Since the middle of the 1970's the increase in oil prices has led to a renewed interest in wood
gasification technology, especially in countries dependent on oil imports but with adequate
supplies of wood or other biomass fuels or, as in the case of Sweden, where the technology
is maintained and developed as a matter of policy.
Research into the technology of gasifier/engine systems has provided modern designs which
work reliably at a level of technical skill appropriate to rural applications in developing
countries. Such systems are economic in certain conditions found in many developing
countries, but the technology and manufacturing facilities are not widely available and their
commercial utilisation is limited.
In "Wood Gas as Engine Fuel" FAO presents a summary of modern wood gasification
technology and the economics of its application to internal combustion engines. Texts on
different aspects of wood gasification, prepared by specialists, are the basis of this
publication.
FAO gratefully acknowledges the co-operation of B. Kjellström of the Beijer Institute,
Stockholm; H. Stassen of the Twente University of Technology, Enschede, Netherlands; D.
de Silva of the Ceylong Institute of Scientific and Industrial Research; N.E. Cañete of the
Sociedad Cooperativa Chortitzer Komitee, Paraguay and R. Thun of the Technical Research
Centre of Finland.
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1.1 Background
Coal, wood and charcoal gasifiers have been used for operation of internal combustion
engines in various applications since the beginning of this century. The utilization peaked
during the Second World War when almost a million gasifiers were used all over the world,
mainly vehicles operating on domestic solid fuels instead of gasoline.
It is important to keep in mind that small gasifiers have been used quite extensively in the
past and that they have played a very important part in reducing or eliminating the need for
fuel imports in some countries. There is no need, however, in this publication to go deeper
into the history of the development of small gasifier technology. Those who are interested in
the subject are advised to study the reviews made by the Swedish Academy of Engineering
Sciences (43) Kaupp and Goss (20) Skov (36) Bailey (3) Earthscan (12) or the National
Academy of Sciences in U.S.A. (32).
Interest in the technology of gasification has shown a number of ups and downs over the last
eighty years, as is strikingly illustrated in Fig. 1.1 which reproduces a histogram that plots the
number of wood gasification reports referred to in "Chemical Abstracts" since the year 1900.
It appears that interest in gasification research correlates closely with the relative cost and
availability of liquid and gaseous fossil fuels. The histogram shows that the number of articles
was at an all-time peak in 1979, following a period of low activity in the 1950's and 1960's.
There is an important difference between the earlier periods of high interest and the present
situation, however. Earlier, the increase in scientific activity corresponded to an increased
number of gasifiers in practical use. The recent increase in scientific interest has not yet
resulted in much practical and commercial activity. The reason will be discussed in the last
chapter of this publication.
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1.2 The present case for wood gasifiers
After the double fuel crises of 1973 and 1979, the harmful effect of high and rising oil prices
on the economies and development efforts of oil-importing developing countries have
become apparent. There has, as a result been increased interest in indigenous, renewable
energy sources, of which biomass in the form of wood or agricultural residues is the most
readily available in many developing countries.
Figure 1.1 Wood gasification citations in Chemical Abstracts

A characteristic of the energy system in many developing countries - in particular in rural
areas - is that internal combustion engines are widely used in stationary applications such as
electric power generation and operation of water pumps and mills. Technologies such as
gasification, which allow utilization of biomass fuel in such engines after minimum
preparation, are therefore of particular importance.
In industrialized countries internal combustion engines are mainly used for vehicles.
Electricity generated in large central power stations is used for most of the stationary
applications.
These different structures of energy systems explain why there appears to be fairly small
interest in using biomass gasifiers for operation of internal combustion engines in the
industrialized world, whereas several developing countries are either introducing small
biomass gasifiers or are in the process of evaluating the technology.
Charcoal gasifiers dominate the present re-introduction of small gasifiers for engine
operation in developing countries. They are the basis of the systems used in the Philippine
programme and in Brazil, see (5). Much of the indigenous research and development now
carried out in developing countries is also concentrated on charcoal gasifiers in view of their
good prospects for early commercialization.
As illustrated by Figure 1.2, the utilization of charcoal gasifiers does, however, imply higher
demands on the biomass resources, resources which are indeed already over-exploited in
many developing countries. On the other hand, at least some designs of charcoal gasifiers
are less likely to cause operational trouble than wood gasifiers or gasifiers for agricultural
residues. This is because one of the potential problems with the latter, the excessive tar
content in the gas, is virtually eliminated by the removal of most of the volatiles in the
production process for charcoal.
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Experience from the Second World War shows, however, that properly designed wood
gasifiers, operated within their design range and using fuels within the fuel specifications
(which may differ between designs), can provide a sufficiently tar free gas for trouble-free
operation.
One of the objectives of this publication is to make decision-makers more aware of the
possibilities of using wood gasification as a substitute for gasoline and diesel oil, without
unreasonable increase of the demand on the natural resource.
1.3 Overview of the contents of this publication
In this publication, an overview of gasifies technology and the main design rules for
downdraught wood gasifiers are presented together with some accounts of recent
experiences from practical operation, and assessments of the economy of the technique.
The possibilities of fuelling different types of engines by producer gas, the theory of wood
gasification, gasification fuels, gasifier types and their design rules are presented in chapter
2. This chapter should not be considered as a complete design handbook for downdraught
wood gasifies systems, but as a guide to those who wish to be able to assess the suitability
of a particular gasifier system for a particular engine. Health and environmental hazards
associated with the use of producer gas are also outlined.
Figure 1.2 Comparison between the amounts of gasoline which can be substituted by
producer gas if wood or charcoal is utilized in the gasifier

The economy of using wood gasifiers for different stationary applications is discussed in
chapters 4 to 6. These chapters also include accounts of recent operating experiences.
Use of gasifiers for operation of modern vehicles is discussed in chapter 3, on the basis of
recent Swedish experiences. This chapter is designed to assist in evaluating the feasibility of
wood gasifiers for road vehicles or tractors.
Even if the results presented in chapters 3 to 6 are valid in the strict sense, only for the
particular circumstances of the cases described, they should be useful as indications of what
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