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MICRO-GASIFICATION Cooking applications for developing Countries

Davide Caregnato


The stages of the biomass combustion 

Drying: (endothermic T<100°C) thermal moisture vaporization Gasification: general process of converting a solid fuel into a gas (wood-gas), that can be combusted, and a solid residue, that is left behind. Combustion: (highly exothermic T>700°C) complete oxidation of the products given by the previous phases

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The gasification Consists on 2 different stages: 

Wood-gasification: pyrolysis of the wood (endothermic T>150°C), consists on a thermal degradation of the solid fuel in absence of an externally supplied oxidizing agent (carbonization). It is a process driven by heat and “char” is the solid residue left. Char-gasification: oxidation of the hot char left by pyrolysis (exothermic 700<T<800°C). Is a process driven by oxygen and ashes are the solid residue left.

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Combustion process Legend: Process

Fresh Biomass

1

Drying

H2O Vapour

Solid Gas

Biochar

Products Reagents

Ashes

Heat transfer

CO2

H2O

2

3

Char Gasification

AUTOTHERMAL HEAT 4

Dry Biomass

Pyrolysis

Combustion

Wood-Gas O2

USABLE HEAT 4


3 T's In order to have a complete and efficient combustion the 3 T's are fundamental: 

TEMPERATURE must be high enough to ignite the fuel TURBULENCE must be vigorous enough for the fuel constituents to be exposed to the oxygen of the air TIME must be long enough to assure complete combustion

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Traditional fuel consumption ď Ž

ď Ž

2.5 billions of people rely on woodfuels for most of their energy needs Too often heating generation is given by simple three stone fires

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Three-stone cooking fire Hundreds of millions of people still use this cooking method in third world Countries Drying, Pyrolysis, Gasification and Combustion occur simultaneously in an uncontrolled manner 3T's are too low and uneven in many reaction zones

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Problems related with the usage of three-stone fire 

High production of dangerous air pollutants like CO, Particulate, and smoke in general (due to incomplete combustion of many particles) Accumulation of smoke and air pollutants inside the habitation

Very Low combustion efficiency

Low cooking efficiency given by huge heat dispersion

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Effects on exposed people 

Indoor Air Pollution (IAP) from biomass fuel is a high risk factor for the development of Chronic Obstructive Pulmonary Disease (COPD) COPD is a major cause of chronic morbidity and mortality throughout the world COPD is projected to be the fourth leading cause of death worldwide in 2030 Indoor Air Pollution causes 1.3 million deaths per year (especially women an children)

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Cooking processes

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Cooking pot shapes

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Other consequences 

Combustion inefficiency of the three stone fire causes a biomass fuel consumption larger than necessary which leads to: 

Deforestation or other kinds of environment degradation due to the abuse of renewable resources Consistent increase of the fuel collecting times and efforts Negative effects given by traditional “slash and burn” technique Soil degradation

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Wood-gas stoves 

A WGS consists of a micro-gasifier combustion unit and a heat-transfer unit WGS allow a separation in space of the gas-burning zone from the gas-production zone Pyrolytic WGS produce biochar as solid remaining after combustion The technology needed to build an efficient WGS is very simple WGS are very cheap and available even for very poor people 13


Substituting traditional fires with wood-gas stoves 

Allows to burn efficiently agricultural residues and poor fuels like maize cobs, groundnut shells, palm oil fibres, palm oil kernels, rice husk, sawdust, etc.. Leads to a minor consumption of woodfuels preserving the environment from deforestation

Prevents people from indoor air pollution exposure

Improves soil fertility thanks to biochar

Represents an active free process of carbonsegregation

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Wood-gas stoves benefits

OLD “SLASH AND BURN” TECNIQUE

NEW “SLASH AND CHAR” TECNIQUE

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Micro-gasifier combustion unit 

Micro-gasifier is the most important part of a WGS It is a batch reactor, which gasifies a fixed fuel bed, and burns the produced gas apart It can reach temperatures of 900-1000°C so it should be made of metal or a refractory material There are many kinds of Micro-gasifier but the TLUD type is the most common and suitable for cooking applications

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Top-Lit Up-Draft gasifier (TLUD) 

Top-Lit: Fuel is ignited from the top Up-Draft: Air and other gases draft proceed upward Fuel is loaded as a fixed bed inside the reactor

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TLUD's conceptual variants Draft type: 

Natural Draft: gas flow is established naturally thanks to pressure and temperature gradient Forced Draft (fan assisted): gas flow is forced through a fan

Secondary air inlet type: 

Direct: gasifier consists of a single pipe Hollow spaced: gasifier consists of 2 coaxial pipes

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Single pipe vs 2 coaxial pipes FLAME AND BURNT GASES OUTLET

SECONDARY AIR INLET REACTION CHAMBER

PRIMARY AIR INLET

FLAME AND BURNT GASES OUTLET

HOLLOW SPACE

SECONDARY AIR

REACTION CHAMBER

PRIMARY AIR INLET

SECONDARY AIR INLET

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Natural-draft vs Forceddraft

EVENTUAL CHIMNEY

REACTION CHAMBER

REACTION CHAMBER

VENTURI

FAN

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Operating principle 

Biomass gasifies progressively from the top to the bottom of the fixed bed thanks to a Flaming-Pyrolysis (F-P) front moving downward Primary inlet supplies the strictly necessary oxygen amount to sustain F-P front thanks to a partial combustion of the fuel Wood-gas produced moves upward and after mixing with secondary air burns over the top of the reactor The entire process is auto-thermal and it doesn't need any external energy supply

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TLUD's combustion phases separation FLAME COMBUSTION BIOCHAR

FLAMING-PYROLYSIS FRONT

UNBURNT BIOMASS

HOT WOODGAS RISING

PYROLYSIS + PARTIAL COMBUSTION DRYING

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Flaming-Pyrolysis front 

Is the reaction zone where simultaneously occur Partial Combustion and Pyrolysis The wood-gas is produced in this zone Thanks to its oxidative exothermic component it provides the heat needed by Pyrolysis and Drying In this zone fuel assume a bright red colour typical of oxidative reactions

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F-P front shape and movement ď Ž

ď Ž

ď Ž

It has the shape of a thin disk extended along the entire horizontal section of the chamber (its thickness depends on primary air amount, fuel calorific value and fuel bed porousness) F-P front moves downward from the top to the bottom of the fuel packed column F-P front velocity varies from 3 to 20 mm per minute depending on primary air amount, fuel calorific value and fuel bed porousness

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Gas flow Legend:

WOOD-GAS + SECONDARY AIR MIXING ZONE

Fresh air Pre-heated air Hot wood-gas

WOOD-GAS PRODUCTION ZONE

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Temperature gradient T [째C] USABLE HEAT GENERATION

700-1100

350-600

AUTOTHERMAL HEAT GENERATION

600-850 20-150

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Fuel container sizing 

Diameter of the fuel container (D): gasifier power is proportional to the surface of fuel available for oxygenation (heating power is proportional to D²) Length of the fuel container (H): process duration is proportional to the length of the fuel column Typical gasifier dimensions for cooking purpose are: D=10-15 cm, H=20-30 cm (producing a power output of about 2-5 kW and a duration of 1-2 hours depending on fuel type) 27


Primary air inlet 

Consists of a series of holes placed at the bottom of the fuel container It must provide at least the minimum necessary amount of air to sustain the F-P front It should be adjustable to allow the variation of the primary air amount (more oxygen from the primary causes an increase of the generated power)

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Secondary air inlet ď Ž

ď Ž

ď Ž

There are many possible different shapes suitable for the secondary air inlet depending on reactor features It has to ensure a good mixing of secondary air and the hot wood-gas rising from the fuel It must carry as air as possible in the mixing zone in order to grant a complete combustion of the burning gases (at least 6 times more than the primary)

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Section of 2 examples of secondary air inlets OUTER COAXIAL CYLINDER

SECONDARY AIR HOLES

SECONDARY AIR BAFFLES

HOLLOW SPACE

Legend: Secondary air 30


Gasifier's top CAP ď Ž

0,6D

D ď Ž

The top constriction is important to convey all the outgoing gases and to allow a good mixing with the second air, giving stability to the flame (that is necessary to prevent interruptions of the upward gas flow) Diameter of the top hole should be about 0,6D

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Feedstock requirements 

Fuel bed has to be porous enough to allow air to pass through the space between the fuel particles Particles diameter should be at least 3mm and should not be more than 20mm depending on biomass features In most cases fuel is not naturally sized for a gasifier so it has to be previously prepared by resizing Moisture content exceeding 20% will reduce efficiency of combustion and in many cases can stop the burning process

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Biochar 

It is the residual solid product of the process

It is a kind of very porous lightweight black carbon

Biochar residual is about 15-20% of the weight of the starting loaded biomass At the end of the process the flame goes out spontaneously but to prevent char gasification is necessary to cool it down or to seal it in the absence of oxygen

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Emissions 

Little smoke is produced during the ignition phase while after the end of the process consistent amount of tarry smoke is produced for several seconds During the steady condition smoke is totally absent Measuring CO is indicative of all hydrocarbon's unburnt species (including particulates) CO emissions varies from 60 mg/m³ to 200 mg/m³ (compared to 11% of oxygen content), 10 times less than a traditional stove

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Testing stoves at University of Udine Many tests were made in order to establish a link between stove design and emissions. Tests features: ď Ž

ď Ž

Stoves loaded with wood pellets were placed inside the combustion chamber of a modified boiler which convoyed all the produced gases in a chimney where measuring instruments were placed Instruments measured CO,O2, CO2, HnCm, NOx, SOx

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Some important deductions 1 ď Ž

ď Ž

Placing a small chimney at the top of the gasifier increases the stove power and reduces the ignition time but at the same time increases CO emissions Primary air control is important to reduce ignition time (providing more air at the beginning) and to reduce emissions (providing less air than possible during the steady condition time)

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Some important deductions 2 

Primary air inlet holes should be made as lower as possible A small metallic sectional net should be placed a few centimetres above the primary air inlet Heat transfer unit design is very important to contain emissions Top constriction should be horizontal to create a good turbulence in the mixing zone

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Advantages given by micro-gasification 

Simple technology and low costs of production

High combustion efficiency with low fuel consumption

Very low emissions

Possibility to use poor biomass or agricultural residues as fuel Possibility to save biochar for many purposes Stable combustion process during a long time (even 2 hours with no human needed intervention) 38


Problems connected with micro-gasification 

Often different types of fuels needs different gasifier features Sometimes fuel size is not immediately suitable and it needs preparation (it can take several minutes) At the end of the process the stove produces a toxic tarry smoke for several seconds To reload the stove with a new fuel charge is necessary to stop the process and repeat the ignition Producing biochar not all the calorific value of the fuel is exploited

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Next steps 

Study a new type of heat transfer unit with the aim of produce fewer emissions than possible Do many tests on emissions with different types of fuel Improve the knowledge of the needs of rural target populations Study the stoves behaviour directly on the field Project a versatile stove which can be adaptable for a wide range of different fuel types

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Thank you!

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