Alternative Choices

The 2^nd International Workshop on Co-firing Biomass with Coal was organised by the IEA Clean Coal Centre and held in Copenhagen, Denmark, on 27 – 28 March 2012. The heavily-oversubscribed event was a great success, with delegates attending from 17 countries, including much of Europe, as well as Canada, Chile, Malaysia, Russia, South Africa and the US. The presentations were well-received, covered a range of issues and were followed by lively discussion. The workshop was opened by Thomas Dalsgaard, executive vice president of Dong Energy.

The subject of biomass co-firing covers a wide range of technical issues. Most of these were covered in the workshop, with sessions on:

• The status of co-firing.
• Operational experience.
• Biomass fuels and processing.
• Combustion/gasification.
• Emissions and ash.
• The future of co-firing.

There was considerable debate about the potential of biomass and its sustainability, biomass processing to increase its energy density (such as torrefaction), as well as presentations on the gasification of biomass. The final session, looking to the future for co-firing, discussed “solid fuel” power plants, the views of KEMA and also the potential of bio-carbon capture and storage (CCS) to produce “negative emissions”.

Status of co-firing

A wide range of figures exists in the literature for the potential availability of sustainable biomass fuels. Dr Hubert Röder of management consultants Pöyry gave realistic estimates for both biomass demand and supply on a worldwide basis. Röder suggested that, if all the potential available biomass was divided up among the world’s coal-fired power plants, it may be possible to co-fire in the 5 – 10% range in them all – an unlikely scenario, but useful as an indication of how much biomass may be available.

Röder touched on sustainability, but the topic was dealt with in more detail in a presentation by Dr Yves Ryckmans of Laborelec. There is huge debate about the sustainability of biomass: it is the most controversial aspect of biomass co-firing in coal-fired power plants and generates most of the opposition to it. Ryckmans suggested that operators should engage seriously with environmental groups, address their concerns as much as is practicably possible, but accept that it may not be possible to satisfy all of them.

Ryckmans and Dr William Livingston of energy company Doosan Babcock considered the safety issues related to the co-firing of biomass. Chief among them is the greater risk of fire and explosion associated with biomass. Coal-fired power plant operators are also not necessarily used to using biomass. Safety is always an important concern, and has been given impetus by the recent fire at Tilbury power plant in Essex, UK.

Operational experience

Dr Raziyeh Khodayari, from Swedish power company Vattenfall, explained that at Vattenfall R&D, the aim is to profitably increase the level of co-firing in the hard coal-fired power plants to reduce fossil CO₂ emissions by 8 – 10 million tpa through co-firing 4 – 5 million t of biomass. At Vattenfall, refined pellets are the preferred fuel.

Livingston and Dr Wim Willeboer of Dutch energy company Essent covered milling and fuel injection. Livingston described the different methods for biomass injection but favours the direct fuel injection system. Willeboer explained that the very high co-firing ratios at Essent’s Amercentrale plant in the southwest Netherlands had necessitated major modifications to the mills.

Livingston explained that, because in the UK co-firing has until recently taken place at co-firing ratios of less than 10%, there have been no major problems with corrosion or slagging. Willeboer reported serious problems with corrosion at the Amercentrale plant, which could be resolved.

These presentations emphasised the fact that, when a coal-fired unit is converted to co-fire biomass, especially at high co-firing ratios, there will be technical challenges. However, it will be possible to overcome the challenges, although sound technical and research support will be needed.

Biomass fuels and processing

A few years ago, in any co-firing workshop herbaceous fuels would have been mentioned as often as wood-based fuels. However, at this event it became clear that herbaceous fuels have declined in popularity in recent years, especially for full-scale plants and the current focus is very much on wood pellets. The only mention of herbaceous fuels was in pilot plant and modelling studies.

In addition, there were two presentations on torrefaction pilot studies by Gregory Dunnu of the University of Stuttgart’s Institute of Combustion and Power Plant Technology (IFK) and Andreas Ohliger from the Institute of Heat and Mass Transfer at RWTH Aachen University. There was also a presentation on refined wood pellets by Carmen Bartolomé from Spanish energy research centre Fundación CIRCE. Robin Post van der Burg gave an update on the current status of the Topell process. Torrefaction provides many advantages for utilities intending to co-fire biomass, but there is still a question on the amount of torrified fuel that can be supplied and at what cost. As yet, torrefied pellets are not being produced on a large scale, unlike wood pellets. There was some mention of the 750,000 t wood pellet facility recently commissioned by RWE in Georgia, US. Dr Adlansyah Abd Rahman (Universiti Tenaga Nasional) described the biomass that is available in Malaysia, which is mainly agricultural residues, and the trials underway for briquetting and torrefying this material.

Combustion and gasification

Martin Möller (Dong Energy) opened the session by describing the Pyroneer process, a low temperature gasifier that the company hopes can solve the challenges of direct co-firing of difficult biomass with coal. The 6 MW demonstration plant in Kalundborg, Denmark, was designed at the end of 2009 and commissioned in spring 2011, with the following features:

• Fuel: straw and manure fibres, i.e. local residues.
• Operating temperature: 650°C.
• Efficiency: ~95% (LHV in the fuel that enters the boiler).
• Capacity: 6 MWth/1.5 tph straw.

It has proved possible to use the ash as an agricultural fertiliser. The demonstration plant has since been scaled up and is now in a process of integration.

The advantages of allothermal biomass gasification for co-firing were explained by Bram van der Drift of Dutch energy research institute ECN.

Emissions and ash

Ronald van Eijk from energy consultancy KEMA gave an overview of the issues and concerns relating to the use of ash from co-firing. As well as being disposed of, ash from co-firing is currently used for:

• Cement replacement.
• Concrete additive.
• Asphalt additive.
• Fertilizer.
• Soil stabilisation.
• Mine stabilisation.

Future uses could include:

• Raw material compost production.
• Soil improvement.
• Road construction.
• Landscaping.
• Raw material in building industry.
• Cement clinker.
• Bricks.
• Synthetic aggregates.
• Industry and energy.
• Trace metal recovery.
• Filler in metals and polymers.
• Phosphor production.

However, there are various bottlenecks that limit the greater use of co-firing ash, including low market volumes, variation in ash quality, limitations by regulations and a lack of knowledge on utilisation options.

The second paper was on models for the prediction of slagging and fouling. This confirmed that there was a less of a problem with wood pellets.

Matshela Koko of South African utility Eskom presented a 1-D simulation tool for biomass co-firing. He also explained Eskom’s co-firing plans for South Africa. Eskom has plans to co-fire up to 10% of biomass in its coal-fired units. Currently, 124.68 million tpa of coal is fired in these units, resulting in 230.3 million tpa CO₂ emissions. The target is to co-fire 16 million tpa of biomass feedstock by 2025. Eskom is investigating pelletised wood-based products and pelletised torrefied biomass in parallel. A commitment to build a 50,000 t torrefaction plant is expected in 2012 after the pilot test. Finally, he said that a request for 10,000 t of torrefied product is currently in the global market.

Several other papers on modelling and pilot plant activities were given, which indicated promising avenues of research for the future.

Future of co-firing

Richard van den Broek described Vattenfall’s expectations for the fuels that are likely to be used in the future in their solid fuel power plants. The general current situation of co-firing and factors that would affect the implementation of further co-firing projects in the future was explained by KEMA’s Jan Middelkamp. Middelkamp concluded that there will be a future for co-firing and repowering, but not in too many countries and only when there is a significant premium on green power from biomass. Tim Dixon concluded the workshop by describing various projects commissioned by the IEA Greenhouse Gas R&D Programme (IEA GHG) to study the potential of bio-CCS to produce negative emissions of CO₂.

Note

The third workshop in the series will be held in March 2013 in the Netherlands. Details of the workshop will be available at http://www.iea-coal.org. The presentations are available online.

This article was published in the July 2012 issue of World Coal. It was written by Deborah Adams, IEA Clean Coal Centre, UK.

Read the article online at: https://www.worldcement.com/europe-cis/12072012/alternative_choices_1165/