Combustion technologies for biomass include fluid beds (bubbling and circulating), stokers, and suspension firing, in that approximate order. Biomass systems are inherently smaller than coal systems because of the logistics of getting biomass to a single location, and that affects efficiency. Additionally, biomass is usually more moist than coal, which also affects efficiency. These are all mature technologies with hundreds of implementations and decades of operating experience for power generation.

Gasification technologies for biomass are far less mature. Most so-called gasifiers are really pyrolyzers, that is, they pyrolyze the biomass but the amount of char reaction with CO2 or water is negligible. Traditionally, only reactors that do the latter a called gasifiers. Pyolyzers, by contrast, only thermally decompose the solid but do not react the char with CO2 or H2O. Reacting the char with O2 makes that part of the system a combustor, not a gasifier. Gasification is a slow and endothermic reaction, in contrast to the comparatively fast and exothermic oxidation reactions in combustion. Pressure and temperature both increase gasification rate, and the producer gas produced is usually destined for a Brayton cycle gas turbine and must therefore be pressurized. For these reasons, gasifiers would like to be pressure vessels. However, feeding biomass into a pressure vessel is a major challenge. 

If the lessons from coal gasification help enlighten biomass gasification, the comparison will be as follows: In theory, gasification has several advantages compared to combustion, including increased efficiency (up to a 50% increase), easier pollutant removal, and the ability to use the syngas/producer gas for products other than electricity. The pressure vessel makes the system less reliable and in most cases the gasifier wants to be oxygen blown, which makes it more expensive in combination with the pressure vessel. In practice, the efficiency gains are usually not achievable because of hot-gas cleanup and logistics issues, ash management plays a prominent role in system operation, and at the end of the day gasification is a more expensive but comparably efficient technology for power generation. Gasification is a critical enabling technology for other products, such as coal-to-liquids technologies. Compared to coal, biomass has considerably more difficult fuel preparation and feeding issue, far less energy density, and a much wider range of ash behaviors. Dedicated biomass gasifiers will probably have even greater challenges than dedicated coal gasifiers. However, biomass has a much higher volatile yield than coal. Therefore, air-blown, atmospheric-pressure biomass pyroyzers, which are often called gasifiers, are relatively easy to build and operate but have few advantages and several disadvantages compared to combustors of the same material.  

Perfect answer was given

I thought I would mention that there is also a body of work on gasification of biomass in supercritical water. This approach is interesting because it works directly on wet (very wet) biomass, and is also very fast. It is not yet proven at any significant scale, but quite a bit of research is taking place. There are significant challenges around materials due to the high temperatures and pressures as well as the corrosive properties of the supercritical mixtures.

Hello Jaswinder Singh,

I widely agree with Mr. Baxter, but especially I disagree in his definition of gasification by the reaction agent. Gasification is defined by converting a solid or liquid fuel into a gaseous fuel. This can be done by any the element oxygen containing gas, thus also pure oxygen.

Reactions with oxygen are exothermal and those with carbon dioxide and water are endothermal. Normally you will try to have a balance between exothermal and endothermal reactions to reach the desired reaction temperature. As Mr. Baxter said, reaction rates (the velocity of reaction) are far the highest with oxygen, followed by water (steam) and finally carbon dioxide, so you won't get a uniform reaction temperature trough out the process. Further more the reaction with oxygen as primary gasifying agent will form secondary gasifying agents, namely water and carbon dioxide, which changes the composition of the mixture of gasifying agents. Then there are other reactions than those of the gasification, that are taking place, e.g. water with carbon monoxide.

Reaction rates depend on the reaction, on the temperature, on the composition of reagents and products and on the structure of the solid. E.g. biomass has a far higher velocity of reaction than coal. All these parameters change during gasification.

To my opinion the main difference between combustion and gasification is, that the combustion is a complete reaction turning all combustibles to the completely oxidised form, while gasification is an incomplete reaction turning combustibles to other combustibles. The difference between pyrolysis and gasification is the presence of some gaseous gasifying agent in the case of gasification, where there is none with the pyrolysis. (But also with pyrolysis there are formed gasifying agents because of the pyrolysis reactions.)

If you have a closer look at combustion, it consits of different steps, namly drying (if the fuel consists water), pyrolysis, gasification and finally combustion, as the combustion only takes place with the gaseous products of the pyrolysis and gasification of the biomass, if you have an excess of oxygen.

The problem with gasification of biomass comes with its incomplete nature. Not only the carbon matter of the biomass reacts incompletely to combustibles, but also the pyrolysis products react incomplete. This is the cause for so many problems which are different from the gasification of coal. Biomass has a high content of volatile matter (something around 80 % on dry ash free basis, which is several times that of hard coal). These volatile matter are causing canzer, are poisonous, are corrosively and give damage to engines by condensation.

With combustion it is comparatively easy to have the three T's of combustion: time turbulence and temperature for complete combustion. With gasification it is far more complicated.

As gasifyers you normally have a fluidised bed, which minimizes local differences in distribution of temperature and composition, and, in case of batch reactors a fixed bed, or in case of continuous reactors a vertically moving bed. With coal you also have entrained flow reactors, but for biomass it would only work with the powdered char coal or a slurry from char coal and pyrolysis oil. Most other types are, as Mr. Baxter mentioned, mainly for pyrolysis as they have a bad gas solid contact or time and/or temperature are to low.

You normally will have the problem to clean the gas, if you want to use it for something else than to burn it. Then you have to cope with high temperature gas cleaning to get rid of tars and with dedusting.

The high pressure gasification metioned by Mr. Pye is somewhere around the critical point of water and somewhat different. Quite some work has been done since mid of the 80th.

By, Jörg Ho

Normally gasification has not been used directly to produce power, but to produce carbon monoxyde and hydrogen which can be used for other energetic uses, such as town gas, or to produce light hydrocarbons after Fisher Tropsch synthesis.

The key difference between combustion and gasification is that the former refers to complete thermal destruction of the fuel (biomass in this case) into CO2 and water vapor (H2O). The heat of combustion is then used to convert water into steam that runs turbine generator to produce electricity. The latter technology however refers to a partial thermal decomposition of the biomass into CO and H2 (Syngas) plus other gases such as H2S and particulate matter. This Syngas is then cleaned from the acid gases and particulate matter and directed to a water-gas shift (WGS) reactor to convert CO to CO2 and produce more H2 gas.

There are two option to produce electricity from the Gasifier: 1) Directly burn the Syngas to convert water in steam and finally electricity via the turbine generator or 2) Separate the Syngas into H2 and CO2. Then, H2 gas can be either combusted in ICE engine to produce mechanical energy or electric power. Alternatively, the H2 gas be used as a feedstock in PEM fuel cells to produce electricity.

As you can see, gasification requires a purification step followed by water-gas shift reaction.

PS: I should also mentioned that H2 and CO can be burned directly without the need for WGS reaction which is meant to convert CO to CO2.

Hope this helps answer your question.

Professor Yehia Khalil

Yale University

USA

The most elemtary difference is that with combustion, power is generated where the biomass is converted. With gasification, no (electrical) power is made but a carrier of energy that can be transported via piping or whatelse, anyhow to be used for power generation at a different location. This may be more efficient if biomass is availalble/abundant at a site where there is no power demand - so the choice for either technology must include this fundamental economical consideration.

The basic difference is in combustion biomass is burned(oxidation) in presence of air . The product will be heat output and a flue gas containing sensible heat. Carbon dioxide will be the main constituent of the flue gas. In gasification biomass is heated either in absence i.e pyrolysis or partial oxidization to supply the heat necessary to pyrolyze the biomass. Carbon monoxide and hydrogen are the important product of gasification. In combustion we are getting heat output in situ whereas in gasification we may burn the flue gas in our convenience. For biomass gasification will be more advantageous because biomass volume will be large and transportation may be costly, but if we can gasify biomass in situ than it can be burned at a centralized place carrying the gas through pipelime. 

For whom it may be of interest. An article of me in relation with gasification in spanish: "Conversión de hidrocarburos gaseosos a líquidos - La sintesis Fischer-Tropsch: El resurgimiento de una tecnología para producir combustibles limpios" (Conversion of gaseous hydrocarbons into liquids - The Fischer Tropsch sunthesis: The resurrection of a technology for the production of clean liquid fuels)

https://docs.google.com/file/d/0B4t62vBUtHSMOTc3NWNlNTItM2Q1OC00YTcwLTgzOWEtOTI3ZGNiYTBmZTNi/edit

The main difference is in combustion technologies  excess air is required to combust the fuel completely otherwise CO may appear which reduces the conversion efficiency

in the case of gasification technologies system must be designed in a way  that restricted the quantity of air is supplied ( for partial combustion) to produce producer gas which after cleaning contains (CO +H2 ) also called syngas which can be utilized in many ways, main applications are:

(a) Electricity generation 

(b) Hydrogen production

Bubbling fluidized beds (BFB) have a combination of gasification and oxidation due to the design of the process.  Combustion air is divided into two separate streams:

• Primary air is fed into the bottom of the fluidized bed section.  Biomass fuel is fed into the bed section and primary air flow rate is less than stoichiometric quantity of air required to completely oxidize the fuel. 
• Secondary air is introduced above the fluidized bed which operates with a dilute phase of solids.  Secondary air flows through nozzles at different levels in the lower section of the furnace.  It oxidizes the gases (CO, H2 plus N2, CO2 and H2O) exiting the bed section.  Secondary air provides more than the stoichiometric air flow rate (excess air). 

The BFB section of fluidized bed boiler operates with less than stoichiometric quantity of air flow rate so the chemistry in the bed section is similar to a gasification reactor.  Heat release in the bed section needs to be matched by the design of the boiler, gas velocity, surface area and choice of refractory materials.  The supplier must have a solid understanding of heat transfer rates.

NOx emissions out of the upper furnace are reduced because a) oxidation occurs at relatively low temperature so atmospheric nitrogen is not oxidized and b) oxidation of fuel bound nitrogen is lowered by staged combustion of the fuel (and presence of CO, H2O and H2).

The BFB boiler must be designed correctly to allow for disengagement of solid particles from the bed that are naturally carried upward with the gas exiting out through the upper surface of the fluidized bed (entrained solids).  These solids must return back into the fluidized bed. 

If the boiler is designed incorrectly the solids do not return to the fluidized bed.  Entrained solids can be carried up with secondary air.  (This occurs by design in a circulating fluid bed boiler, but a CFB boiler also includes a cyclone that returns solids to the bed section).  If entrained bed solids are allowed to be carried up with secondary air in a BFB boiler there will be undesirable consequences:

• Difficulty in maintaining bed level,
• Excessive amount of make up bed material requirement,
• Erosion of boiler tubes,
• Erosion of back end equipment,
• Possible scaling of boiler surfaces and back end equipment,
• Premature shutdown of the boiler for maintenance due to tube failure. 

I would consider this as a summary on why gasification can be used in some applications:

• Gas can be used for power production and base for other chemicals;

• Low value products such wastes can be transformed in more valuable materials;

• Separation of a cleaner component (gas) from solid that can be contaminated;

• Various and heterogeneous materials can be processed and gasified

• Solid fraction is reduced by volume.

Combustion: It is an exothermic reaction in which fuel is burnt generally in the presence of atmospheric air or sometimes only by oxygen alone. The heat energy produced is usually converted into an useful form of work.

Gasification: It is a process of obtaining syngas (producer gas) which usually contains carbon monoxide, carbon dioxide and hydrogen. This process is attained by reacting a fossil fuel with controlled amount of oxygen or steam at temperatures usually above 700 deg.Celsius

Pyrolysis: It is also known as thermal crackling which is thermo chemical decomposition of organic material at higher temperatures under the absence of oxygen. It is widely used in chemical industries to porduce charcoal, methanol, etc.

It's a quite informative discussion. Combustion, gasification and pyrolysis are biomass conversion technologies having their own significance depending upon working principle and end uses.