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Alternative energy news, and information about renewable energy technologies.

Oct 06

Thermo-chemical Conversion Technologies for Woody Biomass Utilization

Posted in Biofuels

Woody Biomass There are many ways to generate electricity from biomass using thermo-chemical pathway. These include directly-fired or conventional steam approach, co-firing, pyrolysis and gasification.

1. Direct Fired or Conventional Steam Boiler

Most of the woody biomass-to-energy plants use direct-fired system or conventional steam boiler, whereby biomass feedstock is directly burned to produce steam leading to generation of electricity. In a direct-fired system, biomass is fed from the bottom of the boiler and air is supplied at the base. Hot combustion gases are passed through a heat exchanger in which water is boiled to create steam.

Biomass is dried, sized into smaller pieces and then pelletized or briquetted before firing. Pelletization is a process of reducing the bulk volume of biomass feedstock by mechanical means to improve handling and combustion characteristics of biomass. Wood pellets are normally produced from dry industrial wood waste, as e.g. shavings, sawdust and sander dust. Pelletization results in:

1. Concentration of energy in the biomass feedstock.
2. Easy handling, reduced transportation cost and hassle-free storage.
3. Low-moisture fuel with good burning characteristics.
4. Well-defined, good quality fuel for commercial and domestic use.

The processed biomass is added to a furnace or a boiler to generate heat which is then run through a turbine which drives an electrical generator. The heat generated by the exothermic process of combustion to power the generator can also be used to regulate temperature of the plant and other buildings, making the whole process much more efficient. Cogeneration of heat and electricity provides an economical option, particularly at sawmills or other sites where a source of biomass waste is already available. For example, wood waste is used to produce both electricity and steam at paper mills.

2. Co-firing

Co-firing is the simplest way to use biomass with energy systems based on fossil fuels. Small portions (upto 15%) of woody and herbaceous biomass such as poplar, willow and switch grass can be used as fuel in an existing coal power plant. Like coal, biomass is placed into the boilers and burned in such systems. The only cost associated with upgrading the system is incurred in buying a boiler capable of burning both the fuels, which is a more cost-effective than building a new plant.

The environmental benefits of adding biomass to coal includes decrease in nitrogen and sulphur oxides which are responsible for causing smog, acid rain and ozone pollution. In addition, relatively lower amount of carbon dioxide is released into the atmospheres. Co-firing provides a good platform for transition to more viable and sustainable renewable energy practices.

3. Pyrolysis

Pyrolysis offers a flexible and attractive way of converting solid biomass into an easily stored and transportable fuel, which can be successfully used for the production of heat, power and chemicals. In pyrolysis, biomass is subjected to high temperatures in the absence of oxygen resulting in the production of pyrolysis oil (or bio-oil), char or syngas which can then be used to generate electricity. The process transforms the biomass into high quality fuel without creating ash or energy directly.

Wood residues, forest residues and bagasse are important short term feed materials for pyrolysis being aplenty, low-cost and good energy source. Straw and agro residues are important in the longer term; however straw has high ash content which might cause problems in pyrolysis. Sewage sludge is a significant resource that requires new disposal methods and can be pyrolysed to give liquids.

Pyrolysis oil can offer major advantages over solid biomass and gasification due to the ease of handling, storage and combustion in an existing power station when special start-up procedures are not necessary.

4. Biomass gasification

Gasification processes convert biomass into combustible gases that ideally contain all the energy originally present in the biomass. In practice, conversion efficiencies ranging from 60% to 90% are achieved. Gasification processes can be either direct (using air or oxygen to generate heat through exothermic reactions) or indirect (transferring heat to the reactor from the outside). The gas can be burned to produce industrial or residential heat, to run engines for mechanical or electrical power, or to make synthetic fuels.

Biomass gasifiers are of two kinds – updraft and downdraft. In an updraft unit, biomass is fed in the top of the reactor and air is injected into the bottom of the fuel bed. The efficiency of updraft gasifiers ranges from 80 to 90 per cent on account of efficient counter-current heat exchange between the rising gases and descending solids. However, the tars produced by updraft gasifiers imply that the gas must be cooled before it can be used in internal combustion engines. Thus, in practical operation, updraft units are used for direct heat applications while downdraft ones are employed for operating internal combustion engines.

Large scale applications of gasifiers include comprehensive versions of the small scale updraft and downdraft technologies, and fluidized bed technologies. The superior heat and mass transfer of fluidized beds leads to relatively uniform temperatures throughout the bed, better fuel moisture utilization, and faster rate of reaction, resulting in higher throughput capabilities.

Written by Salman Zafar, Renewable Energy Expert.

  • Michel

    I like the idea concept listed above of co-firing the coal with the biomass. It seems like a quick way to get coal fired plants to be at least a bit more environmentally friendly in a very quick way.

  • Salman

    Its true, indeed. Co-firing is the simplest and the easiest method of utilization of woody biomass.

  • Nflight

    I prefer the working practice of Biomass Gasification. Some friends of mine asked for my assistance on converting the remaining lost heat into something useful. The proclaimed efficiency is now close to 90%, and with that we are also able to heat a greenhouse, and extract hydrogen for resale in the same operational gasification process. Any Excess heat is transferred to a district heating/cooling setup for the offices. Any biomass we use we must push the limitations beyond fossil fuel thinking. We only have the biomass to use once and then regrow it several months to 20 years later.

  • Salman

    Biomass gasification is definitely the best option to recover energy from woody biomass. Its very good to know that it is now practically possible to achieve 90% thermal efficiency from BG. You may find more information on the efficiency of biomass gasification and waste-to-energy at my blog http://www.energyblogs.com/wte1

  • Robin

    Are there any commercially available or reliable self-build designs for wood gasification electrical generators for residential applications?

    Thanks in advance for any leads or ideas.
    Robin

  • Salman

    Please visit this website for more information, Robin.
    http://www.gasifiers.org/

  • Nflight

    Salman that web site is not functioning but, Robin This web site works well and has plenty of different manufacturers.
    http://www.fluidynenz.250x.com/

  • Blake

    Hey check out http://www.Victorygasworks.com to get really insightful information on gasification. They even have the first gasifier supply store with things that can help test your results or make a gasifier. It’s a cool online community as well for biomass gasification or wood gasification. Robin and Nflight, there are free plans there and if you want, you can buy a video of one of the gasifiers being made.

  • Notrandom

    I see this working for farmers…
    1 – There are other sources for biofuel; corn was obviously a bad mistake. But in a politicians world, corn is where the moneys at! – I say that’s not good enough for the rest of us. Then we have algae, switch grass, wood chips and the best alternative – “HEMP”. So the first question would be, when comparing amount of waste (Sulfur, CO2, etc) how much waste is produced by each and every option? Which one of these produces the least amount of waste?

    2 – It takes energy to make energy – the downside with corn is it took too much energy to make. Corn is naturally a hard crop to grow. Hemp on the other hand grows everywhere. It is drought-tolerant and a nitrogen-fixing “weed” that improves soil integrity. Meaning, more farm land in the future and possibly an end to famine. So we know In addition to its potential to solve our energy crisis, it would also impact: soil degradation, deforestation and desertification. But everything comes at a cost… when comparing the alternatives for biofuels, what are the production / energy ratios? You can’t possibly consider using a fuel which takes more energy to make than it produces?

    I am positive if you do your research, you will find Hemp would be the best source for biofuel. (And I am always willing to be proven wrong)

  • stovepipe

    There is a good source of potential energy available from straw on farms. Areas with a lot of moisture produce more straw than they can deal with, and it gets burned or requires more fuel consumed to work into the soil. It’s a byproduct and should be utilized as an energy source. As well, switchgrass or other perennial biomass crops can be grown on marginal land unsuitable for grain and oilseed cropping.

  • http://www.powerecalene.com Gene R. Jackson

    With our complete package of the GTL process we can use almost any biomass with a free carbon. We do not need to “Grow” our feedstock. Garbage, or manure, or the beetle killed trees are just examples of what we can use to make “Ecalene(tm)”.

    Low cost and a very effecient fuel that will be used in all motor vehicles.


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