The bioeconomy at work: buildings made of biomass ash?

As the use of biomass in large power plants becomes more common, a problem arises: what to do with the large amount of ash that results from burning the renewable energy source? In Europe and the US (database of current co-firing projects, at the IEA Bioenergy Task 32 on Combustion and Cofiring) and China, biomass is being used more and more often to generate electricity and heat, either in dedicated power plants that exclusively burn the green resource (such as the Les Awirs plant in Belgium), or co-fired with coal in existing plants. This biomass can be divided into several categories, ranging from heavily contaminated wood (e.g. demolition wood that contains scraps of glass, steel, plastics, paint etc...) and agricultural residues (such as rice husks), to pure, clean biomass from dedicated energy crops.

Depending on the category, the resuling ash types contain different concentrations of heavy metals such as nickel, vanadium, arsenic, cadmium, barium, chromium, copper, molybdenum, zinc, lead, and selenium. Though these elements are found in extremely low concentrations, their presence warrants careful and often costly waste treatment procedures to prevent leaching into the soil.

For this reason, scientists have been searching for alternatives to landfill disposal. Amongst them is Jan Pels from the Energy Research Center of the Netherlands (ECN), who led a research team working on a project called 'BIOAS' [*.pdf/Dutch, English abstract]. While a group of scientists from the University of Leeds did similar work on rice husk ash [*.pdf] which has some importance for the developing world. Finally, scientists from the Brigham Young University in Utah worked on analysing whether biomass ash can replace cement [*.pdf] in concrete, like coal ash has been used for this purpose for quite a while now. All teams obtained encouraging results: biomass ash can be used to build houses and skyscrapers. What is more, the product can replace building materials that have a heavy CO2 footprint. Utilizing this waste stream from the combustion of biomass also boosts the sustainability of solid biofuels.

The Dutch team found a way to use biomass ash in combination with a heavy petroleum residue, the carbon of which can thus be fixed, whereas the British researchers looked at a combination of waste materials, including ash from burned rice husks, to make what they call a 'Bitublock'. They are also working on a concrete-like building material based on vegetable oil as a binder ('Vegeblock'). Finally the American team found that fly ash from pure wood and switchgrass matches the properties of coal ash, and can replace Portland cement in concrete:
bioenergy :: biofuels :: energy :: sustainability :: coal :: cement :: concrete :: biomass :: fly ash ::


The BIOAS Project
BIOAS started with the ideal scenario which says that ash from clean, uncontaminated biomass should be returned to the soil where the biomass grew so that nutrients and minerals are recycled.

However in many cases recycling is not possible, for example with ash from contaminated biomass (e.g. demolition wood), ash where the origins of the biomass cannot be traced (becoming common with increased trade of feedstocks), or in cases where the land owner does not want the ash returned (e.g. natural reserves or farm land).

In these cases, alternative forms of sustainable utilisation had to be found. ECN investigated the possibilities for the use of ash generated from clean biomass in power production and concluded that for nearly all biomass ash a technically acceptable solution can be found.

Construction materials
The largest potential lies in several kinds of construction material ranging from filler in concrete, to bricks, or even synthetic basalt. Particular kinds of ash can be used as raw material for fertilizers. Black, carbon-rich ash from gasification could even be used as fuel, to replace cokes, or as activated carbon in a multitude of applications.

However many of the solutions are more expensive than landfill disposal because of the small amounts produced and strong fluctuations in composition. Recycling ash to the soil where the clean biomass originated is already possible in Scandinavia and Austria, but in the Netherlands such specific regulations for recycling of biomass ash do not exist and are unlikely to be implemented soon. The situation could improve if large-scale imports of clean biomass begin. In this case, Dutch legislation needs updating to enable export of the ash back to the country (and soil) of origin. However, when long distance trade is involved (such as imports from dedicated energy plantations in Africa), returning the ash would become too expensive.

Fixing carbon while storing biomass ash
For the Netherlands, using biomass ash in building material is a more likely scenario. Bottom ash from biomass combustion is already used as a building material (granulate 0-40). But in the BIOAS Project, gasification ash was successfully tested as filler in a promising concrete-like building material with heavy petroleum residue as binder, called 'C-FIX'. The ECN is currently investigating other routes to produce innovative building materials from biomass ash.

C-FIX (derived from 'carbon fixation') is a product developed by Shell Global Solutions and marketed by subsidiary C-fix BV. The starting material is an extremely hard, carbon-rich residue obtained from petroleum refining. This residue is currently added to marine bunker fuels and heavy fuel oil used in power plants. Upon burning it, an extremely high amount of carbon dioxide is released, making it a very polluting fuel.

A more environmentally friendly way of using the material is to use it as a component in building materials. This way, the carbon is fixed during the lifecycle of the product and doesn't contribute to atmospheric CO2 pollution.

The properties of C-FIX range between those of cement concrete and asphalt. It is strong but flexible thermoplastic binder that resists acids and bases. Moreover, the binder can not only be combined with traditional aggregates such as sand and filler, but with other aggregates such as recycled asphalt, river sludge and waste granulates.

The BIOAS project studied the possibility of using biomass ash as an aggregate for C-FIX, and results were encouraging. The test material conformed to Dutch norms on leaching of macro and micro elements. Five different building blocks made from different types of biomass ash also showed excellent physical properties.

The conclusion of the project was that biomass ash can be used successfully as a building material composed of binders such as C-FIX.


Bitublock and Vegeblock
C-FIX relies on a heavy petroleum product, the carbon of which is fixed. However, in another development, researchers from the University of Leeds found that ash from rice husks can be used safely as a concrete filler, not unlike coal fly ash, which is already used for this purpose.

The team led by John Forth worked at developing a building block made almost entirely of recycled glass, metal slag, sewage sludge, incinerator ash, and pulverised fuel ash from power stations, including ash from rice husks.

Dr Forth, from the School of Engineering, believes his Bitublock has the potential to revolutionise the building industry by providing a sustainable, low-energy replacement for around 350 million concrete blocks manufactured in the UK each year. "Our aim is to completely replace concrete as a structural material", he explained.

Bitublocks use up to 100% waste materials and avoid sending them to landfill, which is quite unheard of in the building industry. What's more, less energy is required to manufacture the Bitublock than a traditional concrete block, and it's about six times as strong, so it's quite a high-performance product.

The secret ingredient is bitumen, a sticky substance used to bind the mixture of waste products together, before compacting it in a mould to form a solid block. Next the block is heat-cured, which oxidises the bitumen so it hardens like concrete.

This makes it possible to use a higher proportion of waste in the Bitublock than by using a cement or clay binder. The Bitublock could put to good use each year an estimated 400,000 tonnes of crushed glass and 500,000 tonnes of incinerator ash.

Meanwhile, a 'Vegeblock' is also under development, based on using vegetable oil as the binder. This would make for the greenest of all concrete-like building materials. The researchers found that waste vegetable oil can easily be mixed with recycled aggregates at ambient temperatures to produce a very workable, easily compactable product. Contrary to the Bitublock, he visual appearance of Vegeblocks is highly attractive in that the units reflect the colour of the aggregates used in the manufacturing process.


The Vegeblock's color changes according to the type of vegetable oil that is used during its manufacture

Curing is required to fully oxidise the vegetable oil and hence stabilise the block. However, due to totally different chemical composition of vegetable oils as opposed to bitumens (mineral oil derivatives), the curing regime is far shorter. Typically curing a Vegeblock only consists of heating for 12 to 24 hours at 120 to 160 °C. The properties of the Vegeblock are at least equivalent to concrete blocks.


Biomass ash as a replacement for cement in concrete
Shuangzhen Wang and Larry Baxter from the Department of Chemical Engineering at the Brigham Young University recently presented their "Comprehensive Investigation of Biomass Fly Ash in Concrete" at the Advanced Combustion Engineering Research Center's congress.

They first looked at the strength and microscopy of coal ash concrete, then at the strength and kinetics of concrete with a biomass fly ash filler, and finally at the durability of the material. The analysis looked at five different forms of concrete based on fly-ashes obtained from co-firing coal with respectively switchgrass and saw dust from pure wood, in different ratios.

Their conclusions on biomass fly ash look as follows:

• Equal strength to that of pure cement concrete from 1 month to 1 year after mixing.
• Significant pozzolanic reaction up to one year in concrete.
• 3-6 times the strength of coal ash samples with Ca(OH)2.
• Comparable strength with Ca(OH)2 even to pure cement.
• Quantitative kinetics has been derived
• Matches or outperforms coal ash in reducing ASR expansion

This means that biomass fly ash, in this case derived from pure wood and switchgrass, can potentially be used as a replacement for Portland cement in the production of concrete.

Conclusion
Without taking things too far, developments in using biomass ash for construction materials are very encouraging, which opens opportunities for the developing world. There, large streams of agricultural residues (such as rice husks) as well as the potential for dedicated biomass crops is available. If this renewable energy resource were to be combusted in dedicated and efficient biomass power plants there, an important component of affordable and reliable building materials would become available and the sustainability of solid biofuels would be enhanced.

Image: the Sears Tower in Chicago, long the tallest building in the US, was built from concrete containing coal fly ash. Will a green Sears Tower ever be built from concrete based on biomass fly ash?

More information:
ECN: Askwaliteit en toepassingsmogelijkheden bij verbranding van schone biomassa (BIOAS) [*.pdf] - April 2004.

BioEnergy Network of Excellence: "A House built of biomass ash" [*.pdf], Newsletter, Volume 1, Issue 3, July 2005.

John Forth, "Non-Traditional Binders for Construction Materials" [*.pdf], IABSE Henderson Colloquium, Cambridge, 10-12 July 2006 Engineering for Sustainable Cities.

Eurekalert: New homes rise from rubbish - April 2, 2007.

C-FIX website.

Shuangzhen Wang, Larry Baxter, Comprehensive Investigation of Biomass Fly Ash in Concrete: Strength, Microscopy, Quantitative Kinetics and Durability [*.pdf] - Brigham Young University ACERC annual conference, February 28, 2007.