Fuelling a change - wood for iron and steel
The Australian steel industry produces approximately eight million tonnes of steel per year (2007-08). This production is associated with the generation of 14Mt of CO2, mainly from the use of fossil carbon in the form of coal and coke as fuels and reductants. Although this level of emission is small on world scales, it represents between two and three per cent of Australia’s total annual greenhouse gas emissions.
With the likely introduction of an emission trading scheme, the cost of non-renewable carbon will increase. Therefore, the steel industries may find it advantageous to reduce the use of such fuels, both to limit net CO2 emissions and to alleviate business costs. One option is to substitute various types of charcoal (made from biomass) for coal in the production process.
The Australian Co-operating Research Centre for Sustainable Resource Processing, a collaborative project between Melbourne-headquartered BlueScope Steel, Sydney’s OneSteel and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) was initiated in 2006, aiming to identify, evaluate and demonstrate applications of renewable carbon in ironmaking and steelmaking. This is part of Australia’s contribution to the WorldSteel Association’s CO2 Breakthrough Programme. The scheme has been structured with three main activities:
1. Identifying and quantifying available sustainable biomass resources.
2. Processing biomass to form various types of charcoal through a pyrolysis process.
3. The use of charcoals in ironmaking and steelmaking, such as in sintering, coke making, pulverised fuel injection into the blast furnace and steel recarburisation and slag foaming.
Tree of choice
The establishment of a Mallee eucalypt industry in the wheat belt of Western Australia has been advocated as a way of diversifying farm income, and as a means of ameliorating the growing threat of dry land salinity, which is affecting the productivity of agricultural land.
Agricultural development over the last 120 years has seen the replacement of 50Mt of native deep rooted bushland with shallow rooted annual winter growing crops and pastures in the southern half of Australia. This change in vegetation has caused the water table to rise and bring previously stable salt deposits to the surface. Approximately, five million hectares of land has already been affected and the problem will worsen over time.
One method of restricting the rise in the water table is to complement agricultural crops with long rooted native woody crops. These trees are grown in contoured rows (alley cropping) to capture the subsurface water. An important benefit of using Mallees is their coppicing ability where harvesting can occur on a three to five-year cycle indefinitely. Mallees can also be used for on-going eucalyptus oil production.
Up to 2006, more than 12,000ha of Mallee had been planted. It is therefore hoped that up to 10Mt of dry biomass may be available annually from Western Australia and up to 39Mt from the Australian Southern states. This potential biomass resource gives the opportunity to develop a sustainable annual carbon supply for the metallurgical industry of three million tonnes from WA and 10Mt from the southern states. A large industrial application for this would help underpin the economics of the Mallee project. The value added industry chain is shown on p18.
Sustainable biomass collection
Residues from forestry operations represent another major source of biomass, which could be available to the steel industry. These materials are attractive because of their low value, issues with disposal and relative proximity to steelmaking centres in New South Wales (NSW) and South Australia (SA). A survey has recently been conducted by CSIRO to estimate the amount of biomass available from the main forestry and agricultural regions of southeast Australia. Data were collected from state Primary Industries departments and wood processing industries. These were reported under the categories of woody residues from forestry and wood processing operations, and residues from agriculture and biomass from the harvesting of woody weeds. The image right, shows a map of southeast Australia and highlights the regions where forestry residue surveys have been conducted.
The biomass survey work focused on six main areas, including the so-called green triangle of southeast SA and southwest Victoria, the Gippsland region of southwest Victoria, southern NSW, the Macquarie forestry region of central NSW, the southern highlands of southeast NSW and the NSW northern forests. The aggregated amount of dry biomass residues from all regions has been found to be 7.5Mt/year. The aggregated biomass consisted of forest residues, including reject logs, bark, stumps, branches, and tree crowns (45%); wood processing residues such as saw dust, off-cuts, chips and shavings (30%); non-forestry residues such as grain stubble, grape marc; wastes from almond and macadamia processing and woody weeds such as camphor laurel (25%). If charcoal pyrolysis yields 30%, as assumed, then this biomass resource could provide 2.25Mt annually of charcoal to industry.
The capability of the Australian steel industry to use transformed biomass or charcoal in different applications has been estimated for the short term (less than five years), medium term (five to 10 years) and long term (>10 years). These reflect the amount of research work, carbon required and the technical risk associated with charcoal use. For each application (blast furnace injection, sintering, coke making and recarburisation), the degree of potential charcoal substitution was estimated for each time period. The results are shown on p18.
In the medium term, the Australian steel industry could use up to 550,000t of charcoal annually. This amount could be generated through transforming 25% of the available biomass.
Each of the different potential applications of biomass in steelmaking will be maximised using charcoal of a specific quality. Recarburising requires a charcoal low in volatile matter, ash and moisture, while charcoal for blast furnace injection requires higher volatile matter, and sintering requires a low reactivity fuel. Research is focused on controlling the pyrolysis conditions to produce charcoal with the required properties. This concept has been called ‘designer charcoal’.
The use of charcoal in ironmaking is not new. Prior to the industrial revolution, the world steel industry depended on this material. This is evident in Brazil where 30-35% of the pig iron production (33Mt/year) is produced using charcoal, derived from Eucalypts. The barriers to increased use of renewable carbon in modern ironmaking processes can be summarised by technical, financial and resource competition issues.
The technical challenges relate to the use of low density, highly reactive charcoal in processes, which were designed for dense, low reactivity coal and coke. Recent pilot-scale work conducted at the CSIRO has found that charcoal has many advantages when used in iron ore sintering, including lower emissions of CO2, sulphur oxides and nitrous oxides. However, charcoal added to iron ore blends tends to gasify ahead of the approaching flame front and hence reduces the amount of heat available for sintering and the quality of sinter produced. Work is focusing on preparing and using more compact and less reactive charcoal.
Coke substitution with charcoal in the blast furnace is restricted because of the strength requirements to support the furnace burden. This problem has been overcome in Brazil where small, less productive furnaces are used. Introducing charcoal or biomass to coke making activities is an ongoing area of research.
Spreading their wings
Other industrial operations such as power generation and cement production also face the need to reduce net CO2 emissions. In addition, there is also a growing movement to add bio-char or agri-char to agricultural land for long-term carbon sequestration, improvements in soil structure and decreased fertiliser usage. And increasing amounts of Australian wood wastes are being pelletised and transported to European power plants to help meet their legislated renewable energy requirements. Hence, this growing competition for a finite resource needs to be understood in terms of likely future wood waste pricing and availability.
Charcoal properties such as ash content and composition may render it unsuitable for some applications. The design of some power plants may prevent high levels of substitution of biomass for coal. The ultimate use of biomass residues by industry is likely to be determined by what users are prepared to pay and the largest benefit in terms of net CO2 reduction. The steel industry may have an advantage in this regard due to the rising cost of metallurgical coal and the net reduction in CO2 emissions.
The business case for using biomass as a coal substitute in ironmaking and steelmaking is dependant on the cost of collecting and processing the dispersed biomass resources and processing the biomass into charcoal through pyrolysis. Work conducted by CSIRO has shown that charcoal can be competitive with coal depending on the cost of the raw biomass, the market price of carbon established through an emission trading scheme and the value received from pyrolysis by-products such as bio-oil.
In lifecycle analysis studies, CSIRO has shown that the reductions in non-renewable greenhouse gas emissions of up to 4.5kgCO2/kg steel are possible if pyrolysis by-products are fully used in the production of electricity.
Charcoal also has processing benefits over coal due to lower sulphur and ash contents, plus higher fixed carbon. Work conducted by BlueScope Steel shows that optimised charcoal may have value-inuse benefits of over 60%, compared with a coal used blast furnace injection and could replace around 30% of the total fuel.
Thanks to co-authors Sharif Jahanshahi, Nawshad Haque, J. G. Mathieson and P. Ridgeway. The project was carried out under the auspice of the Centre for Sustainable Resource Processing, established and supported under the Australian Government’s Cooperative Research Centres Program. Financial support from CSIRO Minerals Down Under Flagship, CSRP, OneSteel and BlueScope Steel towards this project is acknowledged.