Wood burning

Wood: the fuel that bridges time

It was the renewable fuel that propelled human evolution, and remains valuable in heating the technological age.

Man's discovery of how to make fire is regarded as one of humanity's most important advances. The fuel of choice for Neanderthal man was wood, and over the ensuing millennia very little has changed.

The burning of wood is currently the largest use of energy derived from a solid fuel biomass. Wood fuel can be used for cooking and heating, and occasionally as feed stock for steam engines and steam turbines that generate electricity.

"Traditionally it's been the primary source of fuel of human history," William Rolls, information officer at the Biomass Energy Centre, a small technical advisory unit within the UK Forest Commission, says. "It lost a lot of popularity in the UK during the industrial revolution, when everyone switched over to coal and then gas when that was found in the North Sea.

"It's now becoming much more important because it has got a lower carbon footprint than a lot of fossil fuels," he continues. "It's an underused resource in the UK. There is a large amount of woodland out there that is unmanaged, which is actually to the detriment of its biodiversity and environmental value. So yes, there is a lot of room for improvement.

"We're not trying to say that this is the whole solution, or that everyone should be using biomass, which just wouldn't work as there aren't enough trees in the UK for us all to use it. It's part of a solution and at the moment in the UK we have a large amount of wood that goes to landfill and we have a huge amount of other biological material, such as straw, that goes to waste. We also have a huge amount of woodland that is not managed in a meaningful way."

Wood fuel comes in a variety of formats - firewood, charcoal, chips, sheets, pellets and sawdust. The particular form used depends upon a number of factors including source, quantity, quality and application. Another advantage is that wood is the most readily available form of fuel, requiring few tools and little technology to harvest.

"Depending on the end use, you can use a variety of different fuels," Rolls says. "Logs stop being practical at a fairly low level just because of the manual handling involved. Then you are looking at wood chips, a low calorific value fuel that requires very little processing. Next are wood pellets that have a much higher calorific value, but require a lot more work to get to a point where they are useful."

Polluting the skies

Wood remains the staple fuel in many emerging nations, but it is not just in the developing world that word burning is on the rise, but in some of the world's most developed economies. The number of households in the US heating their properties with wood rose 34 per cent from 1.8 million in 2000 to 2.4 million in 2010. But despite its impressive renewable credentials, wood has a dirty secret.

"People don't realise burning wood is a source of pollution, indoors and outdoors, especially when you're using an older stove," Janice Nolan of the American Lung Association, says. It can emit tiny particulate matter - soot and ash - that gets lodged in the lungs and toxic substances such as benzene, carbon monoxide and methane.

The United States Environmental Protection Agency has its own concerns about burning wood for fuel. "We are not in the business of telling people how to heat their homes," Alison Davis of the Environmental Protection Agency says. "Boilers are significantly more polluting than wood or pellet stoves because they have short stacks and use 10 times as much wood. The technology has improved for wood stoves."

Boost for woodchips

One wood product with great potential is woodchip. Coming from wood waste from construction, agriculture, landscaping, logging and sawmills, woodchips can also stem from trees planted in less productive land.

"Woodchips have an important value, first of all, to stimulate the care of a resource like the forest that has often been abandoned," Stefano Dal Savio, environmental engineer and manager of the energy and environment area of TIS Innovation Park in Bolzano, Italy, says. "They also enable the production of a local and renewable fuel. Used as a power source, woodchips could help nations ensure that a part of their energy production comes from within the country,"

The EU-funded Bio-Heat project, based at Timisoara's Polytechnic University, promotes the use of a forestry technique based on two- to four-year wood-cutting cycles, called short rotation coppice (SRC), to produce woodchips for district heating systems in Eastern Europe.

As the main solid biomass fuel sources used for combined heat and power production, known as cogeneration, woodchips offer definite advantages as an energy source. "If you produce only heat from woodchips you have an energy efficiency of more than 80'per cent, in cogeneration plants the efficiency is 65 per cent whereas in power-only production plants it is down to 30-40 per cent," says Edita Vagonyte, European Affairs manager from the Brussels-based European Biomass Association.

"Since many power plants in Europe produce electricity only, the aim at EU level is to build only new cogeneration plants so that the efficiency is increased and we use the biomass more efficiently."

When it comes to energy production, woodchips offer an alternative to other energy sources. "From one hectare of land planted with poplar, between 15 and 30t per hectare per year of biomass can be obtained," Dal Savio explains. "The shorter the duration of the cycle, the greater the amount of biomass obtained, though of lower quality since it will have a higher quantity of bark."

He estimates that at an average value of 22t per hectare per year, the amount of energy produced by woodchips would be 77,000kW/h per year. This is equivalent to 45 oil barrels or to 23 photovoltaic roof systems each producing 1.8kw from solar power.

Biomass research

In the UK, the new Supergen Bioenergy Hub began work in August 2012, directed by Dr Patricia Thornley of the Tyndall Centre for Climate Change Research at the University of Manchester. Initially the hub will address 10 research projects ranging from turning biomass into transport fuels to capturing carbon dioxide from burning biomass feedstocks.

There is significant interest in substituting natural gas in the national network with bio-derived gas. This is already being trialled via anaerobic digestion routes, which can produce a close match to the natural gas composition, but generally uses feedstocks such as slurry.

One of the projects will look at alternative routes to producing a natural gas substitute from other feedstocks, including wood, and establish if the environmental and economic balances are worth pursuing when the whole lifecycle is taken into account.

Another project looks at the lifecycle of biomass. Many biomass supply chains are long and complex, with multiple processing stages and the extent to which material is lost (for example, in drying or storage) is poorly understood. Work will focus on identifying the most significant losses along key supply chains, quantifying their impact and proposing measures to mitigate these.

Lignocellulose materials

One promising technology is producing ethanol from lignocellulose-rich materials such as wood residues, waste paper, used cardboard and straw. However, although the technology is developing rapidly it cannot yet be achieved at the same efficiency and cost as from corn starch, although it has other advantages.

A cost comparison has concluded that using lignocellulose materials is unlikely to be competitive with starch until 2020 at the earliest. The last 15 years has seen a massive growth of so-called first-generation processes that use enzymes and bacteria to turn the starch and sugars in corn and sugarcane into ethanol.

But corn and sugarcane are also important components of the human food chain and their use for ethanol production has the potential to affect the price and availability of these basic commodities.

On the other hand, lignocellulose materials are often hard to dispose of, but they are rich in sugars that can be fermented into ethanol following appropriate processing. "Not only is cellulose the most abundant polymer on Earth, it cannot be digested by humans, so using it for fuel production does not compete directly with food supplies," Jamie Stephen, the study's lead author who works in the Department of Wood Science at the University of British Columbia (UBC) in Vancouver, Canada, says. "The race is on to commercialize this second generation ethanol."

Focus on costs

The work at UBC focuses on the fact that the cost of building large-scale, ethanol-producing facilities will likely be higher for second generation ethanol compared to first generation technologies. Sources of lignocellulose may require significant and costly pre-treatment. "Researchers and companies are going to have to concentrate on reducing the cost of pre-treatment and increasing the output of the digester in order to reduce the costs of the lignocellulose-to-ethanol process," Stephen adds.

Another reason costs are higher is that lignocellulose is made of multiple kinds of sugar, while corn starch consists of pure glucose. Corn starch can be reduced to glucose with low-cost amylase enzymes, while pre-treated lignocellulose requires a cocktail of cellulase enzymes. Providing these enzymes is one of the major costs, but you need 12 times more cellulase than amylase protein to generate the same amount of ethanol from woody biomass.

"Despite much effort and progress over the last few years, the cost of cellulase enzymes is still significantly higher than amylase-based processes, and needs to be reduced substantially before lignocellulose starts to become competitive with corn and sugarcane as a feedstock," says Stephen.

Finally, while the input to sugarcane- and corn starch-based systems is fairly constant, the feedstocks that go into lignocellulose systems are much more variable. Different species of tree produce wood that has different properties, and waste paper and agricultural wastes will have many different types of material in them. To get maximum efficiency, each type of biomass needs to be processed under different conditions, which introduces another challenge for anyone wanting to make ethanol from these materials.

Overall Stephen believes we have a considerable way to go before second-generation ethanol production will be ready for commercialisation. "Production requires significant cost reductions and at least the same level of financial support that was given to the first-generation systems if second-generation ethanol is going to be fully competitive by 2020," Stephen says.

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