Gasification

Gasifaction
Gasification is the production of combustible gases from incomplete combustion of fuel. The gases produced consist of Carbon monoxide (CO), Hydrogen (H2) and traces of Methane (CH4) and is known by various names including “Producer Gas”, “Syn’Gas” (short for “Synthesis Gas”) and “Woodgas”.

Applications of Woodgas
Although it is not a high-energy fuel, woodgas gas can be used to fuel internal combustion engines to produce shaft power. Alternatively, woodgas can be used to produce heat for boilers, dryers, ovens, or kilns. Depending on the design of gasifier and the fuel used, 60 and 75 per cent of the energy within the fuel can be supplied to an internal combustion engine or turbine. For thermal applications, efficiencies as high as 93 percent fuel to heat conversion can be achieved.
In many economic and operational respects, ‘Power’ and ‘Heat’ gasifiers are rather different technologies. The principal difference is that internal combustion engines demand a very clean gas so before it can be used as fuel, the gas must be first passed through an often complicated gas−conditioning system, which is thus an integral part of power gasifiers. In contrast, gas destined for combustion in external burners requires little or no gas conditioning. Because they do not require elaborate gas−cleaning systems, heat gasifiers are simpler and cheaper when compared to power gasifiers.
Since the 1980s, there seems to have been a continuing expectation that economic conditions should provide a strong argument for the exploitation of gasifaction using fuel sources like wood and biomass from farm crops; but despite the enthusiasm amongst dedicated groups of scientific researchers and hobbyists, wide spread implementation of micro gasifaction do not seem to have materialised. Fossil fuels by contrast have been extensively exploited over that period, apparently stretching current supply and therefore economic restraints to breaking point.

Gasifaction in the past
Gasifaction or ‘gasification’, despite its relative obscurity today, was in widespread small scale use in the 19th and early 20th century. From gasification’s first discovery at the dawn of the 19th century, gasification systems, using charcoal, peat and fossil coal as feed material, were widely used for smelting. Then large scale gasification was employed in the “gas works” of towns and cities to produce “town gas” for lighting until the introduction of natural gas and electricity. Incidentally the creosote by-products of town gas production made possible the preservation of timbers which were used to extend the modern age across the globe, both as the sleepers beneath railway lines and telegraph poles at the dawn of the information age. Even Britain’s first petrol fuelled motor cars (manufactured by Santler in Malvern from 1889 to 1922) were driven by internal combustion engines developed from an earlier model which ran on town gas.
What accounts for the historical decline of gasification? At the beginning of the 20th century, oil based fuels like petroleum and diesel gained wider use as a conveniently portable fluid fuel across Europe and producer gas systems declined. Then during World War 1 and again in World War II, gasifaction enjoyed a short-lived resurgence in popularity due to petroleum shortages. By 1945, woodgas was being used to power agricultural and industrial machines, trucks, buses, and cars and by the end of the Second World War there were as many as a million vehicles running on gasifiers.
After World War II, the return of a cheap and plentiful supply of liquid fuel and a lack of strategic necessity led to a general decline in gasification. An exception to this was Sweden, where work continued and was in fact accelerated after the ‘Suez Crisis’ in 1956 when a decision was made to include gasifiers in Swedish emergency planning. In 1989, America’s Federal Emergency Management Agency, released a set of gasifier plans to enable the emergency provision of fuel for the distribution and production of food in the event of a catastrophic disruption to the petroleum supply. Apart from that, and lab scale research for large systems, in the main, small scale gasifaction has been left to part time pyromaniacs and home inventors of which there is a thriving community online.
The current pressures on global fuel supply are, as in the past, likely to cause people to make changes to their energy procurement and consumption. Whether the changes are fuelled by anxiety over global warming or purely economic concerns, more and more people are likely to consider alternative energy sources. Solar, wind and other renewables are more accessible today than in the past but the use of wood as fuel must also be expected. A recent precedent for this was the OPEC oil crisis of the 1970s which prompted a rush for fuel diversification in oil hungry populations. In America and elsewhere this was characterised by the wide spread adoption of wood as a supplemental domestic heating fuel, predominantly in wood burning stoves. Given this past experience, technologies that improve the efficiency of wood burning systems are likely to benefit everyone by mitigating demand for finite resources. After all, biomass can only grow so fast.

Wood as a Fuel
The ‘timeless' landscape that we look out upon in the UK today is largely manmade. Generation after generation have bent the land to their use and thus it will continue. But that use has changed the land from the once ubiquitous forests to the large stretches of arable and pastoral land that surround our massive urban centres today.
At the end of the Dark Ages, Northern Europe was still heavily forested and sparsely populated, but in the following five hundred years Europe suffered a fuel shortage. From 1000-1250 the European population boomed (in those 250 years the UK population trebled) and, though there were still areas with plentiful supplies of wood, complaints about the price of wood increased. One factor during the first energy crisis was the limited transport infrastructure. Where the population was dispersed there was less of a problem but as cities began to swell so the local wood supply came under increasing pressure. Transporting fuelwood in those days was difficult and even today fuelwood is seldom transported any great distance. Wood for construction however has always been highly prized. In Elizabethan England it took approximately 4,000 oaks for each war ship. It was also used for industries such as brewing and metal smelting, along with the huge number of timber frame buildings of the time.
400 years ago, there were effectively no alternatives to wood as a fuel source even though coal has been in use as a fuel for many centuries. Shaft mining has been carried out in the UK since the 13th century but it was not until the escalation of energy consumption during the industrial revolution that coal came into large scale use. Due to ‘decreasing marginal returns’ in the face of escalating social complexity, fuel demand and diversity has gathered apace ever since. At the turn of the 20th century liquid bio fuels like alcohol and vegetable oil were developed only to be replaced (partly thanks to the Women’s Christian Temperance movement) by fossil oil. There is a vogue at present for the redevelopment of synthetic fuels like bio-diesel and bio-ethanol for distribution through the current infrastructure as well as the ever imminent messianic ‘Hydrogen Revolution’. Excluding Hydrogen from this discussion here are some significant problems with this rush for ‘sustainable’ fuels.

Bio-diesel for example could never provide competition to “big oil” because, at current diesel consumption levels, even if all American food oils were given over to fuel use it would only provide the equivalent of about 2% of American diesel demand. When it comes to bio-ethanol, (using the centralised corporate refinery model) the inefficiency of present production technologies requires as much as 80% energy input for its production. The conversion inefficiency of sugar and starch derived bio-ethanol is problematic in itself but is compounded by the inefficient use of food crops as fuel stock. Producing ethanol from abundant cellulose material like saw-dust, rice-husk and straw could make ethanol a profitable fuel, but here there is the problem of the highly crystalline materials and binding lignins of such crops, which require enzymes and mechanical methods to break them down before fermentation. To solve this problem there are patent protected proprietary technologies, in the form of genetically modified organisms, capable of directly fermenting cellulose to ethanol. Let us assume then that there are sufficient safeguards to prevent these novel organisms from escaping into the soils of forests and farmlands in the real world, this approach still has the drawback of employing technologies beyond the economic reach of most, thereby maintain the high economic cost of fuel.

Deforestation and Reforestation
Today in the so called developed world wood is no longer a primary fuel, (America is estimated to obtain only about 3% of its energy supply from all forms of biomass) but western culture does make extensive use of wood in construction, for furniture and in the production of packaging and paper. In the ‘Developing world’ woodfuel currently accounts for about two thirds of total energy consumption, though it can be as high as 89% as it is in Laos. Half of humanity, by and large the poorer half, continues to cook over wood fires. Indeed at present nearly half the world's wood supply is consumed as fuel and with population pleasures on resources 2 billion people are facing fuelwood shortages.
In the 70's the "fuelwood gap theory" implied that woodfuels were consumed on a non-sustainable basis. The observation was that in many countries the consumption of fuelwood was larger than the sustainable supply from forest land from which it followed that deforestation and forest degradation were largely due to fuelwood consumption. However it has since been observed that over 60% of fuelwood originates from non-forest sources and the supply from these non-forest sources appears to have been sufficient to "fill the gap". Yet forests in developing countries are shrinking by more than 15 million hectares a year.
It is not necessarily the use of wood as a fuel that causes the rapid decline of forests. Just as in Medieval Europe, the main causes of global deforestation today are the pressure for more farmland and large scale commercial logging for industry. People who depend on woodfuel try to maintain the trees that provide it they don't want to kill them off. For centuries Pollarding and Coppicing practices have supplied fuel wood in convenient dimensions for burning. This fuel source comes with the added benefits of fast regenerative growth from strong roots, soil protection and diverse habitats for wildlife. It seems that traditional woodland husbandry is not intuitively understood by today’s urban population who reflexively picture large logs from felled trees when they think of wood as fuel. This kind of cultural inertia is likely to be difficult to displace without practical demonstration of the advantages of small dimension biomass as fuel. Wood is by no means the only biomass fuel suitable for gasification, agricultural by-products like straw, husks, hulls and shells are an extremely attractive alternative.

Charcoal
It is also worth mentioning charcoal, which, though still derived from wood, results in a gas with less tar. During World War II charcoal was the preferred fuel for vehicles running on woodgas but it has a major disadvantage over wood in terms of the energy consumed in its production. The process of conversion from wood to charcoal accounts for about 50% of the original energy in the wood and in traditional production processes can be as high as 80%. Having said that char can the deliberate by-product of low temperature gasification and as such the “used” fuel from cooking or heating can go on to be used as fuel for power gasifiers. But Char need not be consumed as a fuel. As a soil additive Char can result in a 50 to 80% reductions in nitrous oxide emissions and significantly increases the efficiency of traditional chemical fertilizers. Experiments with “terra preta”, as this char enriched soil is known, claim to have seen yields for some crops more than double.



Gasification in a Nut Shell
The gist of the process is as follows. Heat from combustion is applied to biomass fuel. The heat performs three different functions depending on its intensity. Firstly, in the coolest part of the system, the fuel dries out. Then as the fuel heats up further, a process called pyrolysis takes place. In pyrolysis gases are driven out of the biomass leaving behind predominantly carbon in the form of char. The hottest process, know as Reduction takes place when the cocktail of really hot gasses from pyrolysis swirl round the super hot char and the two inter-react with one another in full blown gasification to create “Woodgas” which with any luck can then be put to work.
Gasification is a very efficient method for extracting energy from carbon rich organic materials and there are many technologies for achieving it; but here we will only discuss gasifiers that use combustion to provide the heat they need and ‘breath’ air to provide the oxygen for that combustion.
The process of gasification is a thermo chemical process, which uses heat to convert carbonaceous materials into gaseous components. The gas is referred to by several different terms including “Producer gas”, “Syn-gas” and “Woodgas”. Woodgas is the name I will try to stick to because Woodgas excludes fuel sources derived from fossil or liquid fuels and the point here is that we want to make use of “waste” biomass.
When “woody” fuels are heated to temperatures of approximately 600-1000°C, the solids undergo thermal decomposition, and become gas-phase products and these typically include carbon monoxide, hydrogen, methane, carbon dioxide, and steam. Along with these gases solid char (charcoal) and tars are also formed. The remaining products of these processes are ashes.
The chemistry of gasifaction is complicated and there are several separate processes involved and though they are all distinct phases they will tend to overlap and intermingle depending on scores of factors but that’s what makes it so interesting.


The more detailed, step by step, explanation is as follows.

Drying:
Assuming that the fuel contains moisture, which in wood will range from as little as 10% for kiln dried wood but can be higher than 45% in damp wood, then that moisture must be driven off before combustion can take place. In the case of wood there is water between the fibers as well as within the cells of timber. To drive this water out requires temperatures up to about 200ºC and this is where the first cost (in terms of energy loss) is incurred in any biomass combustion process. Though steam is still a gas, the problem is, as far as we are concerned, it’s not a “useful” gas as fuel.

Pyrolysis:
In Pyrolysis, volatile gases are driven out of the fuel at temperatures up to about 700ºC. From 200ºC to 280ºC, carbon dioxide, acetic acid (which can give rise to corrosion of the gasifier) and the last of the water is driven off. What one might call the ‘real’ Pyrolysis then takes from 280ºC up to 500ºC where some methyl alcohol is generated and large quantities of tar, which are condensable vapors, and non-condensable vapors in the form of methane and carbon-monoxide are liberated. Between 500 to 700ºC the gas production continues on a smaller scale but with a greater proportion of hydrogen.
The remaining solid residue after Pyrolysis is char the carbon in which is essential for Reduction.

Oxidation/Combustion:
Combustion reactions theoretically yield temperatures of 1450ºC though fire is a complicated beast and radiation, convection and conduction along with endothermic reactions drastically reduce the temperature. Carbon (from the char) is the dominant fuel in biomass gasifiers though other gaseous fuels will inevitably burn as O2 is introduced to the combustion zone. As the O2 contacts and reacts with the incandescent charcoal they produce carbon monoxide which then occupies the spaces between the charcoal pieces where, in turn mixes with incoming Oxygen and itself burns to form carbon dioxide which serves to exclude O2 from the surface of the Char until it is displaced by incoming gas flow. This tends to create a stable CO2 CO balance dependant on what is known as Superficial Velocity (more of which later).

Reduction:
Reduction (some people call it reaction) occurs in an oxygen depleted environment where the activated carbon (char) reacts with gases and vapors to produce combustible products such as hydrogen and carbon-monoxide. The reduction (or gasifaction) process is carried out in temperatures somewhere between 800 and 1000oC. Two different flavors of chemical reaction govern the temperature in the reaction zone. Endothermic reactions (meaning they require heat to drive them) cause cooling. While Exothermic processes (which produce heat) increase the temperature. The objective is to reach a balance of exothermic to endothermic reactions and balance the heat produced by one with the heat required to sustain the other.
When the reduction zone is hot enough, the tars passing through it undergo “thermal cracking” where heavier molecules are depleted of their hydrogen to produce "light", hydrogen-rich products. Water too can be cracked apart by heat but doesn’t really want to crack into Hydrogen and Oxygen in any significant proportion in the presence of charcoal. For example; in order to dissociate 20% of steam passing through the reduction zone into hydrogen and O2 the gas would need to remain in the reduction zone at 1100ºC for about half a second but in a typical down draft gasifier the residence time is likely to be about a fifth of a second. There is also the “water gas shift” reaction where water and carbon monoxide shift to produce CO2 and H2 this again this is an endothermic reaction which will tend to reduce reaction zone temperatures. This brings us back to the fact that the dryer the fuel, the better the efficiency and output gas is likely to be.

Output gas
When oxygen for combustion is derived from the air nitrogen dilutes the product gas and the gas is thus comparatively low-energy. Low-energy gasifiers are best used in situations where the heat content of the gas is not a critical issue such as co-firing applications, cooking, heating, and internal combustion applications.
The Woodgas leaves the gasifier hot, wet and dirty, but can be burned immediately if heat production is the objective. For other applications such as internal combustion it will require cooling, drying and cleaning. Using ballpark proportions the Woodgas is likely to be about 60% non fuel, the diluting gas as a proportion of the whole is about 10% carbon dioxide and 50% Nitrogen. This leaves the remaining 40% which is the useful fuel gas with an approximate 50/50 mix of carbon monoxide and hydrogen (not including the methane and ethene which will probably make up about 1% of the overall total.)

Fuel
The fuel stock used for gasification will ideally be the determining factor in terms of gasifier design. As with most fuel systems, biomass or otherwise, having an abundant source of raw materials should be the beginning of the process rather than a secondary consideration. The assumption made here therefore is that you are considering gasification to make use of an existing fuel supply. In which case, for the most part, the fuel will determine the design of gasifier chosen and in the interests of efficiency any processing of the fuel required to improve gasification will be kept to a minimum.
Thermal processes are critical to successful gasification, but these in turn are dependent on the fluid dynamics of the gasses within the system. The inter-reactions between solids and gasses at each stage of the process revolve around the ratio of one with the other and fuel size has a crucial role to play in this. Acceptable fuel sizes depend to a certain extent on the energy density of the fuel and on the reactor designs but as a rule of thumb gasifiers operate most effectively on wood in chunks ranging from 4 x 4 x 8cm. down to 0.5 x 0.5 x 1cm.
When the dimensions of the fuel stock are too large free convection leads to uncontrollable combustion conditions but control improves with a reduction in the size of the fuel, to a point. At the other extreme, small pieces can collapse together to obstruct the interstitial gas circulation resulting in an unacceptable pressure drop over the reduction zone, failing that, they can become entrained in the output flow, leaving excessive proportions of dust in the gas.
The passage of fuel through the system can sometimes become impeded even with ideal stock sizes. There are two results of fuel jamming together, known as Bridging and Channeling. Bridging describes structures of interlocking fuel that bind against the reactor sides and each other to create a span that prevent the descent of fuel from above the bridge resulting in a void beneath. Channeling describes a similar interlocked structure but this time the result is a shaft of free flowing gas which concentrates reactions in a localised zone around the channel and creates uneven conditions within the gasifier. Both of these problems can be overcome by agitation of the fuel in the system, mobile gasifiers benefit in this respect from the vibration of the entire unit when in motion but static systems may require mechanical mixing which adds to the complexity of the system.


The choice of gasifier type will depend on the available input fuel, its size, moisture content and ash character, and also on the intended application for the output gas. As more designs are developed the terminology to describe gasifier types has to adapt to keep up. There are many acronyms used to describe variations within gasifier categories but for our purposes and to keep it simple let us confine ourselves here to discussing three categories of gasifier, each of which burns from the bottom. These categories are defined by the direction of air flow (or draft) within the gasifier and are; Downdraft, Updraft and Crossdraft.
Updraft gasifiers are fed air from the bottom and the gas comes out through the top of the fuel bed.
Downdraft gasifiers reverse this and the air passes downward through the combustion and reduction zone.
Crossdraft (predictably) have air feeding in from one side and gas exiting the other.

Up Draft
With updraft gasifiers, air passes upward in a counter-current direction to the downward flow of fuel. Exothermic combustion reactions between the incoming air and the outgoing charcoal at the bottom of the bed drive the gasification process just above the combustion zone. As the hot gas rises it transfers its heat to the Pyrolysis zone before going on to heat and dry the upper layers of fuel.
The woodgas produced by an Updraft gasifier leaves the system at a relatively low temperature with few particulates but tends to be high in Tar because the majority of the vapours released in Pyrolysis are not drawn through the reduction zone. Char residues are low and ash is swept along with the solids in the opposite direction of the gas flow and is eventually withdrawn from the bottom of the gasifier. In sequence as the fuel descends it undergoes drying, pyrolysis, gasification and then combustion before the ash and small amounts of char exit from the grate at the bottom. Because of these characteristics Updraft systems are sometimes called ‘Char-burning’ / ‘Tar-making’ gasifiers.

Down Draft
With downdraft gasifiers, air passes downward in a co-current direction to the downward flow of fuel. At the bottom of the grate, just before the woodgas leaves endothermic gasification reactions between outgoing char and pyrolysis gases take place. These reactions are driven, by the heat from the combustion zone which burns above. Tars in the gas are broken down or burned as they pass through the hot reaction zone and
Downdraft systems are sometimes called ‘Tar-burning’ / ‘Char-making’ gasifiers. The woodgas produced by a downdraft gasifier leaves the system at a much higher temperature with low tar levels because the pyrolysis vapours are drawn through the hot reduction zone. The level of particulates in the gas tends to be low but depends on gas flow rates. The gas is also more likely to contain alkali vapours formed in the reaction zone. In sequence as the fuel descends it undergoes drying, pyrolysis, combustion and then gasification before the gas, ash and char exit at the bottom of the grate.

Cross Draft
With crossdraft gasifiers, high velocity air is jetted, through a single nozzle, into the fuel. Within the fuel mass a small volume high temperature reaction zone is contained within the insulating effect of the surrounding fuel and ash. There is considerable turbulence within this hot zone and convective circulation within the surrounding fuel bed where pyrolysis takes place. Because of the high temperatures the nozzles of cross draft gasifiers tend to require, either some form of cooling, or to be made of temperature tolerant materials.
Crossdraft gasifiers are the simplest systems in construction but this apparent advantage is offset by their comparatively demanding fuel requirements. Due to the very high operating temperatures slagging can cause problems so for trouble free operation fuels will ideally deposit little ash. Low tar fuels (such as charcoal) are also preferred because there can be a tendency for the fuel stock from the surrounding pyrolysis zone to drop in to the reaction zone causing the sudden release of uncracked tars.



Superficial velocity / Hearth Load
Superficial Velocity is the rate at which air passes through the reduction zone and is synonymous with Hearth Load. Effectively all gasifaction processes are dependent on the Superficial Velocity in the gasifier and it is possible to compare the superficial velocity across gasifiers of any scale because it is relatively trivial to calculate. The hearth load is the volume of gas passing through the reduction zone divided by the area of the smallest section of the hearth.
The Superficial Velocity of the gas in the hearth has a fundamental effect on the oxidation processes and heat transfer rates and so governing the combustion, the pyrolysis of volatiles, the cracking of tars, and the gasification of the charcoal and therefore on the quality of the output gas.
A low Superficial Velocity causes slow oxidisation of Char resulting in low temperature combustion due to the sequestration of O2 by the Carbon Monoxide in the gas surrounding the char. Pyrolysis conditions in slow flow rates can fall to around 600°C before the char ceases to oxidise and the whole process halts. Low temperatures such as these produce yields of charcoal (around 20-30%), and a wood gas with large volumes of unburned heavy tars, and high levels of hydrocarbon soot. At lower temperatures the reduction equilibrium leans toward exothermic, carbon dioxide producing reactions.
By contrast; a high Superficial Velocity causes very fast pyrolysis and reduces charcoal yields to less than 10%, with a correspondingly high volume of ash. The temperature of gases in the pyrolysis zone climbs to about 1200-1400ºC and at higher temperatures endothermic reactions tend toward the production of more carbon monoxide. Due to thermal cracking the Tar outputs at these high superficial velocities can be less than 1000ppm though the resulting gas has less energy.

Complicating Products of Gasification
Ash and tar are the two largest inconveniences to the smooth running of gasification systems. Ash can have a strong influence on the internal conditions of the gasifier as well as on the output gas, tars can cause major plugging and fouling as well as disposal and handling problems.

Tar
“Tars” are a complex mix of oxygenated constituents made up of hundreds of chemicals of which approximately 200 have been identified so far. They are released during pyrolysis as gases and/or vapours which are then subject to combustion, thermal cracking and polymerization reactions.
Tars in the product gas can be tolerated (in fact add to the heating value of the gas) where the gas is used as a fuel in closely-coupled direct heat applications but as they cool they become subject to condensation which result in tar deposition on cool surfaces within the equipment and upon soot entrained in the gas flow. These deposits can cause mechanical ware in engines and or obstructions in heat exchanges and flues.
The composition of the tar is complex and highly dependent on the fuel type as well as the gasification conditions encounter. Temperature and time in the reactor play a significant role in quality and quantity of output tars.

Temperature / Tar Correlation from 450°C on the left to upward of 950 °C on the right.
Mixed Oxygenates Phenolic Ethers Alkyl Ethers Heterocyclic Ethers Polyaromatic Hydrocarbons(PAH) Larger PAH

NOTE: PAHs are mutagens classified as a "Known Human Carcinogen" by the International Agency for Research on Cancer (IARC).

Low temperature conditions tend to produce mixed oxygenates and phenolic ethers which may have a brown and watery (60% water) appearance. It seems that Tar products formed at temperatures up to 800C can be relatively safely handled using minimal safe practices such as eye protection, gloves, and appropriate clothing. Tars formed at high temperatures tend toward heterocyclic ethers and polyaromatic hydrocarbons of high molecular weight having a black and highly viscous character (at about 7% water).
Tars exposed to temperatures of above 800 °C or long/repeated residence times within the gasifier may also contain refractory polyaromatic products. These ‘refractory tars’ will require a higher level of care in handling and disposal.

Tar Treatment
There are broadly two approaches to the tar control problem under research.
The first attempts to minimize the amount of tar that leaves the gasifier through gasifier designs and catalysts added to the gasifier-bed. Tar destruction can be accomplished with heat alone at temperatures above 1200 °C or with the use of catalysts between 750-900°C. These approaches are obviously attractive because they increase conversion efficiencies and reduce the need for collection and disposal of tars.
The second solution is to process the tar after gasification, either by physical removal, secondary catalytic conversion, thermal conversion or chemical treatment.
The simplest of which is the re-injection of tars into the gasifier. This would allow the tar to react further and form additional gas-phase products which is attractive with highly oxygenated tars. The consideration here is that these tars will undergo further dehydration and condensation reactions as they are reheated and will produce more refractory tars. In a feedback system like this the balance between ‘Mixed Oxygenate Tars’ and ‘Polyaromatic Hydrocarbon Tars’ would need close monitoring to avoid the buildup of highly aromatic, refractory tars by repeated reactor exposure.

Ash
After complete combustion the remaining oxidized mineral content of the fuel is known as ash. Ash can have a major impact on the operation of a gasifier.
Ash levels in the woodgas are therefore dependent on the mineral content of the biomass feedstock and gas flow rates. As the fuel is gasified, inorganic matter from the feedstock will either remain in the gasifier bed or become entrained in the product gas.
When ash fuses together it forms conglomerations known as slag (or clinker) which can inhibit or even prevent the movement of fuel and even if it doesn’t fuse together ash can create sheltered areas where gasifaction reaction are inhibited. Slagging can be overcome by lowering operation temperatures to below the melting point of the ash, or by maintaining temperatures above ash melting point. When the latter method is employed the molten material will need to be accommodated or ejected by the gasifier design.



Soot
Fine char particles released from the fuel and entrained in the gas stream as an aerosol are often thought of as soot but strictly speaking soot is a result of gas-phase oxidization where two CO give rise to one CO2 and one carbon which is then likely to precipitate out of the gas stream and build up as soot. Filtration of hot gasses will therefore be effective in removing entrained carbon leaving the hot CO to pass through to form later soot depositions. Rapid quenching can be employed to prevent gas reverting to soot and CO2 for cold gas application and this quenching or any gas cooling systems will tend to capture tars at the same time. The soots from gasification contain polycyclic aromatic hydrocarbons (PAHs - see health warnings) which are lipophilic and so mix more easily with oil (as in tar) than with water. It is thus difficult to account for soot precipitation separately from tar deposition.

Corrosive Outputs
The high mineral alkali content of some biomass feed stocks must also be considered. Sodium and Potassium salts can vaporize at moderate temperatures of about 700 °C and may then condense on cooler components in the system. Unlike easily separated solid particulates vaporized alkalis will remain in the gas mixture at high temperature. If not accounted for in the design of the system then the condensation of alkalis on heat exchangers or other cool areas may lead to corrosion and should be anticipated.







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