Pre-roasting plant matter could enables more of it to be burned in coal-fired power stations: so is torrefaction an answer to some environmental woes?
It is the perennial concern of modern power generators: how do you mitigate the environmental impact of burning fossil fuels? One solution currently under investigation is the co-firing of some sort of sustainable fuel alongside coal. However, this presents its own problems.
The use of biomass for this co-firing makes economic sense, because the proportion of electricity generated by biomass in coal-fired power stations in the UK is eligible for Renewables Obligation Certificates (ROCs), which have an intrinsic market value and also avoid carbon emission allowances costs.
There are two methods of co-firing biomass. It can be fired either by milling and blending with coal, or by direct injection in the furnace. Co-milling consists of blending the biomass component with the coal and pulverising the blend in the existing coal mill.
In the UK, all power stations co-milling biomass are using on-site blending to satisfy Ofgem's audit requirements.
Direct injection involves a completely separate feedline for the biomass fuel with separate handling, drying, milling and burners/injectors. Although this is more expensive to install, it allows for greater flexibility of handling and processing and less potential for adverse affects to the primary coal stream.
However, the environmental benefits of biomass are countered by practical and economic challenges that are forcing power stations to restrict theamount used. Biomass is moist and bulky, making it relatively expensive to transport and difficult to store for long periods without going mouldy. The fibrous plant matter is also extremely difficult to process in the mills where coal is ground into dust before being burned.
The answer could lie in a much neglected and almost forgotten roasting process known as torrefaction. This process, which sees the plant matter heated to around 300°C in an air-free container, transforms bulky biomass into a dry, energy-rich fuel that is cheaper and easier to move around and has a much longer shelf life.
A study of two common energy crops – willow and miscanthus – has also shown that when the plant matter is 'torrefied' it can be ground into a powder as easily as good quality coals. This makes it more practical and cost-effective to replace existing containers of coal with biomass.
'The simplest way to think about torrefaction is putting your bread in the toaster and it goes brown, except in our case it will be brown all the way through,' Professor Jenny M Jones of the school of Process, Environmental and Materials Engineering at the University of Leeds and an EPSRC advanced research fellow, explains. 'It would be more of a gentle toasting, and as there is no air around there is no ignition nor will it combust.
'The other way of thinking about it is if you were to have paper or wood in a fire, then once the fire goes out, you pick it up and it crumbles. The paper will crumble before it has been burnt. With our process the material will go in looking like a wood chip and come out looking a nice brown, burnt ember colour.
'From our perspective we have been looking at what the energy content will be and how it has improved in terms of the space it occupies. Then we obviously want to look at how well it grinds since it must be pulverised before it can be co-fired.'
Despite its huge promise, it is still early days for the technology, and there are only a couple of plans for large-scale torrefaction plants. However, the idea has been around since the 1930s, when it was considered as a gasification tool for lighting. The idea returned as a possible fuel for the iron and steel industry to replace coke, but did not find favour there either. In the past decade there has been a renewed interest because of the growth in biomass.
As with all fuels there is a delicate calculation into the cost of production. In the case of torrefying biofuels, it is pointless to expend more than the fuel will be enhanced. 'Some work published on the energy requirements in 2005 [by ECN in the Netherlands], shows torrefied pellets would have a similar energy requirement to pelletising woody biomass, and there would be savings on transporting and delivery of the fuel to the power station,' Jones adds. 'With the wood you have to dry your material and then press it into pellets, and it would be doing that, but at a slightly higher temperature during the last stage. The main energy penalty in torrefaction is the drying, not the actual torrefaction process itself.'
Research is focused on several different avenues. The first thing is predicting the characteristics of torrefied biomass. Things such as what will be coming off in the gas phase, how much is going to stay as a solid and what the heating value of that solid is and how long this process is going to take. 'This allows you to calculate the size of your reactor from a knowledge of the residence needed,' says Jones.
'We are going to develop a model that you can run on your laptop – you put in your material and temperatures and it will be able to give you an output with information on what gases and vapours are going to be evolved, what your solid product will be, and the energy content of that solid product.
'We are also hoping to be able to say how long you would need to dry it for and how long you would need to torrefy it to the point you want it torrefied to. Obviously, having a model will mean you can play around with that to optimise the end result.'
The second thing they are investigating is what is lost in the gas phase. 'We want to know if you can recover the heat from the gases and vapours, how much you can recover, and what are your potential emissions from the process, either in water or to air,' Jones adds. 'We would look at the environmental impact of the whole process.'
Given its explosive risk, dust is another concern, especially if the torrefied material will be milled and pelletised. The research will catalogue the dust averages and the ignition limits, allowing developers to design safe torrefiers with the appropriate safety systems to minimise the risk.
'At the moment we have a small torrefier,' Jones explains. 'It is a batch system and we can only load in maybe a quarter of a kilogram at a time. We have been optimising torrefaction and we will use that system to help develop the model and make sure the model is right by comparing it against the experimental data.
'During the new 42-month project, we are also planning to initiate and assist collaboration. We are involved in EC bids concerning scale up and technology demonstration.'
The study is centring on willow and miscanthus as well as forestry and agricultural residues. When torrefied, the group of materials suitable for use as the original fuel does grow. This is because the main problem with co-firing is that you have to mill everything to such a small size. Some of the types of biomass, particularly the straws and even some of the woods, do not mill properly in the existing mill equipment. This can lead to problems in the feeding and combustion of those fuels.
The availability of torrefied biomass in large enough quantities would likely convince generators to consider converting one of their boilers to co-fire or even burn-only biomass. 'At the moment what some power generators have been doing is putting a certain fraction of biomass into all the burners in one of the boilers, or in a few cases, taking a lot of biomass through one burner in a boiler,' Jones explains. 'What they are considering now is converting a whole boiler to 100 per cent biomass.'
The problem with biomass is that it burns with two-thirds of the heat of coal; so you need more biomass going in. But torrefied biomass could contain an extra 20 per cent and greater efficiency from that boiler.