La microgassificazione secondo Davide Caregnato

MICRO-GASIFICATION Cooking applications for developing Countries

Davide Caregnato

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The stages of thebiomass 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

AUTOTHERMALHEAT

Pyrolysis

CharGasificationAshes

H2O Vapour

Wood-Gas

O2

USABLEHEAT

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3

Process

Solid

Gas

Products

Reagents

Heat transfer

Legend:

CO2 H2O

Drying

Combustion

Dry BiomassBiochar

Fresh Biomass

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1

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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 fuelconsumption

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

Drying, Pyrolysis, Gasification and Combustion occur simultaneously in an uncontrolled manner

3T's are too low and uneven in many reaction zones

Hundreds of millions of people still use this cooking method in third world Countries

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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

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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 carbon-segregation

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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 conceptualvariants

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 vs2 coaxial pipes

FLAME AND BURNT GASES OUTLET

FLAME AND BURNT GASES OUTLET

SECONDARY AIR INLET

REACTIONCHAMBER

PRIMARY AIR INLET

HOLLOW SPACE

REACTIONCHAMBER

SECONDARY AIR

SECONDARY AIR INLET

PRIMARY AIR INLET

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Natural-draft vs Forced-draft

FAN

VENTURI

REACTIONCHAMBER

REACTIONCHAMBER

EVENTUAL CHIMNEY

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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

DRYING

PYROLYSIS + PARTIAL COMBUSTION

HOT WOODGAS

RISING

COMBUSTION

UNBURNT BIOMASS

BIOCHAR

FLAME

FLAMING-PYROLYSIS FRONT

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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

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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Pre-heated air

Hot wood-gas

Gas flow

Fresh air

Legend:WOOD-GAS + SECONDARY AIR MIXING

ZONE

WOOD-GAS PRODUCTION

ZONE

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AUTOTHERMAL HEAT GENERATION

USABLE HEATGENERATION

Temperature gradient

600-850

T [°C]

20-150

350-600

700-1100

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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)

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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

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

Legend:

Secondary air

SECONDARY AIR HOLES

HOLLOW SPACE

OUTER COAXIAL

CYLINDER

SECONDARY AIR BAFFLES

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Gasifier's top

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

0,6D

D

CAP

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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 atUniversity of Udine

Many tests were made in order to establish a link between stove design and emissions.

Tests features:

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 importantdeductions 1

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 importantdeductions 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 bymicro-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)

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Problems connected withmicro-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!