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How Does a Wood Gasifier Work?

A wood gasifier is a system that converts wood into a combustible gas called wood gas or producer gas. This wood gas can then be used to generate heat, power engines, and fuel vehicles. Wood gasifiers work by using high temperatures to carry out the gasification process, where wood is broken down into simple gases like hydrogen, carbon monoxide, methane and others.

What is Wood Gasification?

Wood gasification is the process of turning solid fuel like wood into a mixture of gases by heating the wood in a low oxygen environment. This process is called pyrolysis. The main components of wood gas are nitrogen, hydrogen, carbon monoxide, methane, and other hydrocarbons.

During gasification, the wood is heated to very high temperatures, between 700-1200°C. At these temperatures, the compounds in the wood start breaking down and release volatile gases. The solid mass remaining is mostly carbon, called charcoal. The hot gases produced during pyrolysis pass through the charcoal and become wood gas.

How Does Pyrolysis Work?

Pyrolysis occurs in stages as the temperature increases:

  • Drying (100-200°C) – Moisture evaporates from the wood.
  • Torrefaction (200-300°C) – Hemicellulose breaks down releasing CO2, CO, H2O and acetic acid.
  • Pyrolysis (300-700°C) – Lignin and cellulose break down releasing CO, CO2, CH4 and other hydrocarbons.
  • Gasification (700-1200°C) – Tar and methane react with charcoal producing H2, CO and other gases.

So pyrolysis converts the solid mass of wood into hot volatile gases and a carbon-rich charcoal residue simply by heating in the absence of oxygen. The charcoal plays an important role during gasification.

Gasification Reactions

As the pyrolysis gases pass through the charcoal, they undergo several reactions that convert them into the simple gases that make up wood gas:

  • Combustion – Heats up the charcoal for other reactions.
  • Boudouard reaction – CO2 + C ⇌ 2CO
  • Water-gas reaction – C + H2O ⇌ CO + H2
  • Water-gas shift reaction – CO + H2O ⇌ CO2 + H2
  • Methanation reaction – C + 2H2 ⇌ CH4

These gasification reactions between the hot pyrolysis gases and charcoal generate the combustible components of wood gas – hydrogen, carbon monoxide and methane.

Key Components of a Wood Gasifier

A wood gasification unit consists of several components that work together to produce wood gas from wood. The main parts are:

Fuel Hopper

This is the entry chamber where the wood fuel is loaded. It has an airlock that minimizes air from entering the lower parts of the gasifier.

Throat

Also called the pyrolysis zone. This is where drying and pyrolysis take place. Temperatures reach 500-700°C releasing the volatile gases.

Combustion Zone

Charcoal burns creating heat for the endothermic gasification reactions. Air intake is controlled to keep this zone hot enough (>700°C).

Reduction Zone

The pyrolysis gases react with the hot charcoal here to form wood gas. Temperatures are 700-1200°C.

Ash Zone

Inorganic material and char left after gasification collect here as ash. Air inlets at the grate allow ash removal.

Cleaning System

Wood gas contains tar, ash and soot. The gas must be cleaned before use in an engine. This system cools the gas and removes particulates.

Gas Holder

Acts as a buffer storing the cleaned wood gas before it is supplied to the engine or burner. Helps even out fluctuations in gas production.

This basic design allows a wood gasifier to convert wood into combustible gas through drying, pyrolysis and charcoal-based gasification reactions. The gas produced is cleaned and stored for end use.

How Do You Start and Operate a Wood Gasifier?

Operating a wood gasifier involves several steps to get it up and running safely and efficiently:

Load Fuel

  • Use recommended fuel – typically dry firewood cut into pieces.
  • Load fuel into hopper up to specified level.
  • Maintain airlock seal to prevent air flowing into reduction zone.

Lighting Material

  • Place kindling, paper or firestarter cubes in throat.
  • Light and allow to heat up throat (~20 minutes).

Air Intake

  • Set air dampers to supply correct air amounts.
  • Primary air ignites the fuel bed.
  • Secondary air controls combustion zone temperature.

Monitor Temperatures

  • Use thermocouples to monitor drying, pyrolysis and reduction zones.
  • Temperatures indicate if reactions are proceeding correctly.

Remove Ash

  • Ash chokes the reactions so must be cleared regularly.
  • Open ash gate slightly to allow ash to fall into collector.

Gas Production

  • Gas production begins 20-30 minutes after lighting.
  • A stable flame indicates good quality gas production.

Cleaning System

  • Pass gas through cyclones, wet scrubbers or fabric filters to remove particulates.
  • Tar residues require additional filtration or cracking methods.

Test Gas Quality

  • Check composition using gas analyzers. CO and H2 should be highest.
  • Higher hydrocarbons indicate incomplete gasification.

Careful operation keeps the gasifier performing optimally and producing good quality wood gas.

Types of Gasifiers

There are four main types of gasifier designs based on the direction of airflow through them:

Updraft Gasifier

Air flows up from the grate through the fuel. The gas outlet is at the top so the gas must pass down through the charcoal resulting in thorough cleaning. Ash removal is relatively easy but tar content in gas is high.

Downdraft Gasifier

Air and fuel flow downwards. The throat and oxidation zones are above the reduction zone allowing easier start-up. Tars are cracked in the hot combustion zone giving cleaner gas but ash removal is harder.

Crossdraft Gasifier

Air is introduced horizontally into a cylindrical chamber. Fuel enters from the top and flows down through the hot combustion zone for gasification. High ash fusion temperatures are required to prevent melting.

Fluidized Bed Gasifier

Fuel is introduced into a bed of hot sand that behaves like a fluid when air blows through it from below. Highly efficient gasification but requires consistent fuel sizes. Ash and char are mixed so continual removal is needed.

The downdraft design is most popular for wood gasification as it gives clean gas while also being easy to start and operate. Updraft units are simpler but need extensive gas cleaning. Crossdraft and fluidized bed gasifiers are more complex in operation.

How is Wood Gas Used?

The wood gas produced can be used for various thermal applications and power generation:

Fuels Internal Combustion Engines

The most common application is powering Internal Combustion (IC) engines in vehicles, generators, pumps, etc. The wood gas is filtered and fed into the air intake of the engine. Minimal modifications to the engine are needed but the power output is reduced by 50-75% compared to gasoline.

Heating and Cooking

The wood gas can be burned directly for heating or cooking applications, providing a substitute for fuels like LPG. The gas requires only rudimentary cleaning so the equipment is simpler.

Combined Heat and Power (CHP)

Large gasifiers can be integrated into systems that produce both heat and electricity. The gas powers an engine-generator set while recovering heat using exchangers. Overall energy efficiency is high at around 80%.

Syngas Production

Further cleaning and reforming of wood gas produces syngas – a mixture of mainly H2 and CO used in the manufacture of fuels, chemicals and fertilizers.

Biomass hybrid gasification

Adding a small portion of biomass like animal manure or agricultural waste along with wood allows production of methane through anaerobic digestion. This biomethane can supplement the wood gas.

Wood gasifiers allow decentralized and renewable energy generation from wood in various applications. The ability to power engines and provide heat, electricity and gas makes it a versatile technology.

What Are the Benefits of Using a Wood Gasifier?

Some of the top advantages of wood gasifier systems are:

Renewable and Carbon Neutral

Wood is a renewable and sustainable fuel. As long as replanting occurs, wood supplies are indefinitely renewable. The carbon released during combustion was absorbed during tree growth, so wood energy is carbon neutral.

Efficient Use of Wood Energy

Wood gas can substitute for fossil fuels and power machinery efficiently. Around 60-75% of the energy in the wood gets converted to usable wood gas energy. Overall energy efficiency is high when the technology is integrated into CHP systems.

Low Emissions

Wood gas combustion produces very low soot or particulate emissions compared to burning solid fuel. Unburnt hydrocarbons are also minimal since the gas combusts fully. CO2 emissions are considered neutral.

Uses Waste Wood Resources

Wood waste from lumber mills, forest management activities and agriculture plantations can be used as fuel. This both displaces fossil fuels and deals with waste biomass sustainably.

Flexible Scale

Gasifiers can be designed at sizes from a few kW to several MW. This allows decentralized small scale systems for remote areas as well as community or institutional power generation.

Versatile Energy Applications

Wood gas can power or replace a range of heat engines, cooking equipment, generators, etc. It can also be reformed into other gaseous fuels expanding its uses even further.

Advantages like sustainability, efficiency and flexibility make wood gasification an attractive renewable energy technology.

What are the Challenges With Wood Gasifiers?

Some of the main challenges involved with wood gasifier systems are:

Tar Production

Tars are produced during pyrolysis that must be removed as they condense and clog pipes and engines. Managing tar through cooling, wet scrubbers or catalysts increases system complexity.

Fuel Handling

Fuel wood must be properly sized, dried and continuously fed into the gasifier. Wet fuel can cause problems during pyrolysis. Maintaining feedstock supply requires labor and infrastructure.

Gas Quality

The composition of wood gas varies with factors like fuel composition, air flow, temperature etc. Contaminants like tar, soot and ash affect gas quality. Monitoring and optimizing operation is important.

Skilled Operation

The technology requires trained technicians to maintain temperatures, check systems, do troubleshooting and repairs, analyze gas composition etc. Lack of skilled labor can affect deployments.

Ash Management

Ash from the feedstock mineral matter builds up and requires regular removal otherwise it will choke the gasifier. Failed components like grates need repair or replacement.

While wood offers many benefits, the technology has specialized operational needs that require awareness, monitoring and skill to manage. Proper training and maintenance can ensure successful long-term operation.

Frequently Asked Questions

How much fuel do wood gasifiers need?

Fuel consumption depends on the gasifier size and output. As a rough estimate, 1 kg of bone-dry wood produces 1-2 kWh of wood gas energy. Gasifiers above 100 kW capacity may consume several tons of wood per day.

Can any type of wood be used?

Preferred fuels are hardwoods like oak, maple, beech etc. Softwoods work too but may lead to more tar. Moisture content should be <20%. Avoid painted, treated or laminated wood as it releases toxic substances.

Is wood gas toxic?

Wood gas contains CO which is toxic and corrosive when inhaled. The gas must be used with proper piping and engines designed for it. Normal safety precautions should be taken as with any gaseous fuel.

How long do gasifiers last?

Well designed and maintained gasifiers can run for decades. Key components like grates and filtering systems may require replacement over the equipment lifetime. Proper start-up and shutdown avoids damage from thermal stresses.

Can wood gasifiers power electric generators?

Yes, wood gas can power reciprocating engines coupled to generators to produce electricity. The engine output is lower than if running on diesel but enough to generate usable electricity for homes or businesses.

Conclusion

Wood gasification provides a renewable, efficient and low emission option for heat and power generation from wood. It produces a combustible gas through drying, pyrolysis and gasification of woody biomass. With the right design and operation, wood gas can replace fossil fuels in a range of engines and thermal equipment.

Challenges exist around consistent fuel supply, tar removal and maintaining skilled operators. However the environmental and efficiency benefits make wood gas an attractive technology to recover and utilize energy from wood sustainably. With further development and deployment, gasifiers could play a greater role in renewable energy transitions, decentralized power access and waste biomass utilization.

Laura Kassovic

Laura Kassovic, a former engineer at Intel SOC, now dedicates her efforts to mentoring startups in the realms of Wearables and AI. As a co-founder of New Tech Brake, she spearheads a wireless sensing solution enterprise catering to diverse applications including product development, research, location tracking, and people monitoring, as well as asset and cargo supervision. The platform empowers developers to craft an array of innovations such as fitness trackers, temperature-monitored cargo systems, medical trial tools, smart running garments, or even straightforward transmission of unprocessed accelerometer data to cloud-based repositories.

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