Black Liquor Gasification Summary and Conclusions from the IEA Bioenergy ExCo54 Workshop This publication provides Wood and Wastes the record of a workshop organised by IEA Bioenergy. CO2 Pool Black liquor gasification is an interesting option for production of synthesis gas that can subsequently be converted to a variety of motor fuels. The technology can be integrated into modern, Carbon ecocyclic, kraft pulp mill Dioxide Pulp and Paper biorefineries. USA and Sweden lead developments in this field. It is of interest primarily among countries with strong pulp and paper industries and national policies which promote substitution of petrol and diesel by biofuels. IEA Bioenergy IEA BIOENERGY: ExCo:2007:03 INTRODUCTION a large pulp and paper industry. It is thus of great interest to convert the primary energy in the black liquor to an This publication provides a summary and the conclusions energy carrier of high value. from a workshop organised by IEA Bioenergy. It was held in conjunction with the 54th meeting of the Executive Worldwide, the pulp and paper industry currently processes Committee in Ottawa on 6 October 2004. The purpose of the about 170 million tonnes of black liquor (measured as dry workshop was to present the developments of black liquor solids) per year, with a total energy content of about 2EJ, gasification for the production of energy and/or biofuels making black liquor a very significant biomass source (see for transport and discuss the remaining barriers, either Figure 1). In comparison with other potential biomass technical or strategic, that need to be overcome in order sources for chemicals production, black liquor has the to accelerate the successful demonstration of black liquor great advantage that it is already partially processed and gasification technologies and subsequently their penetration exists in a pumpable, liquid form. Using black liquor as a in the market. raw material for liquid or gaseous biofuel production in a biorefinery would have the following advantages: There is a growing interest in finding cheap and efficient ● Biomass logistics are extremely simplified as the raw ways to produce CO2-neutral automotive fuels by using material for fuel production is handled within the biomass as the raw material, as CO2 is the main gas boundaries of the pulp and paper plant. responsible for climate change. However, the consumption ● The process is easily pressurised, which enhances fuel of fossil fuels by all economic sectors is decreasing with production efficiency. the exception of road transport; that continues to increase ● Due to the processing of wood to pulp, the produced and there is a need to cut down on CO2 emissions. There is syngas has a low methane content, which optimises fuel therefore an urgent need to develop not only alternative but yield. also additional fuels. ● Pulp mill economics become less sensitive to pulp prices when diversified with another product. Emissions from the transport sector are growing at an ● Gasification capital cost is shared between recovery alarming rate. Road transport in particular generates of inorganic chemicals, steam production, and syngas 85% of the European Union transport sector’s emissions. production. Furthermore, 98% of the European transport market is dependent upon oil. The external energy dependence has Overall, if the global production of black liquor were to passed 50% and will increase to more than 70% and 90%, be used for transport biofuel production, then this would for oil in particular, in 20-30 years if nothing is done. This correspond to about 48 million tonnes of methanol, is viewed as economically and strategically unacceptable. compared with current world production from fossil fuels of about 32 million tonnes, a significant impact. Black Liquor Gasification (BLG) is an interesting alternative to produce synthesis gas that can subsequently be converted Pulp and Paper Making to a variety of motor fuels such as Fisher Tropsch, DME, Most of the pulp and paper produced today (90%) originates methanol, and hydrogen. BLG can be integrated in a future from wood. The major components of softwoods (e.g., pine, modern, ecocyclic, pulp mill biorefinery for the production spruce) as well as hardwoods (e.g., birch, aspen, eucalypts) of renewable energy sources in the form of CO2-neutral fuels are cellulose (40-50%), hemicellulose (25-30%), and lignin for automotive uses. (25-30%). Extractives constitute a minor part. Pulp for paper production is obtained via two classes of BACKGROUND processes that differ greatly in principle: The Pulp and Paper Industry ● Mechanical pulping, in which the fibres are separated The pulp and paper industry is a vital part of the global mainly through mechanical treatment in refiners. Most of economic cluster – the paper and forest cluster – that in the wood thus becomes pulp, including the lignin. Europe alone generates an annual turnover of more than ● Chemical pulping, in which the fibres are separated €400 billion. In 2002, more than 1260 pulp and paper mills mainly through chemical treatment in either acidic or produced a total of some 91 million tonnes of paper and caustic solutions. These processes aim to separate the board. The industry provides direct employment for about lignin from the cellulose fibres. 250,000 people, and indirect employment – through the paper and forest sectors – for a total of 3.5 million people. Due to their high lignin content, mechanical pulps quickly become yellow. They are therefore used mostly for products A pulp mill that produces bleached kraft pulp generates with a short life span, such as newsprint and magazine 1.7-1.8 tonnes of black liquor (measured as dry content) paper. Another reason why mechanical pulps are used in per tonne of pulp. Black liquor thus represents a potential these products is because they contain large fractions of energy source of 250-500 MW per mill. As modern kraft relatively short fibres and fibre fragments; therefore they pulp mills have a surplus of energy, they could become key make dense and opaque sheets that are suitable for printing suppliers of renewable fuels in the future energy system. paper. Approximately one-third of the pulp produced in the Today, black liquor is the most important source of energy European Union is mechanical pulp. from biomass in countries such as Sweden and Finland with 2 300 3.6 -ÕÌ ÊiÀV> "Vi>> 250 3.0 ÀÌ ÊiÀV> ÕÀ«i -ÕÌ ÊiÀV> 7À` 200 Ã> HÈääÊ/7 {{Ê/7 2.4 vÀV> ÀÌ ÊiÀV> 150 ÎxäÊ/7 1.8 -Üi`i 100 ÎÊ/7 1.2 ÕÀ«i £ÓäÊ/7 >«> 50 0.6 !PPROX%NERGY#ONTENT ,(6%* >VʵÕÀÊ>ÃÊ ÀÞÊ-`ÃÊ­ÌiÃ®Ê xäÊ/7 Ã> näÊ/7 0 0.0 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 2025 Figure 1: Estimated Black Liquor World production (FAOSTAT, 2001). Chemical pulps constitute the other two-thirds of the pulp Kraft Pulping Process production. These pulps are characterised by high strength Nearly all of today’s paper is manufactured by a century-old and, if bleached, by high brightness and long-term brightness sulphur-based chemical process known as the ‘kraft pulping’ stability. Typical products made from bleached chemical process. In Figure 2 a schematic is shown of a modern kraft pulp include fine paper, tissue, and a number of board mill that produces bleached market pulp. Many mills have grades. Unbleached chemical pulp is mostly used to produce integrated pulp and paper production where only a part of corrugated board and sack paper. the pulp production, or none at all, is sold on the market. In principle, in an integrated mill, the pulp dryer in Figure 2 is Out of the total fibre furnish used for papermaking in replaced by a paper machine. Europe, about half the pulp is supplied directly from the processes above as ‘virgin fibre’. Recovered paper makes up The process starts in the wood yard where logs are debarked the remaining fraction, which increased from 40% in 1990 and cut into wood chips a few centimetres in length. It is to 54% in 2002 (FAOSTAT, 2003). also common for mills to use a fraction of purchased sawmill chips. The wood chips are impregnated with cooking liquor From an energy point-of-view the two processes differ and then fed to the digester, commonly of the continuous greatly. Mechanical pulping consumes electrical energy, type, although there are many mills that use batch digesters which to some extent is recovered as steam and usable low- as well. The residence time in the digester is several hours, grade heat. Only a small fraction of the wood is dissolved in during which the chips are cooked at a temperature of 150- the process. In contrast, about 50% of the wood is dissolved 170°C under strongly alkaline conditions and in the presence in chemical pulping. The processing of this dissolved organic of sulphide. The main objective is to dissolve as much of substance can make a mill self-sufficient in energy and, the lignin as possible while minimising the simultaneous depending on the type of product, even generate a surplus of dissolution of the carbohydrates. steam or electricity. It is this dissolution of the energy-rich lignin fraction from the pulp fibres that makes chemical Sulphide has two positive effects; it both reduces the pulping an interesting candidate for the production of liquid reaction rate for carbohydrate dissolution and increases the fuels from biomass. delignification rate. The drawback is that small amounts of sulphide react to produce organic sulphur compounds The dominant process for chemical pulping (at about 90%) is such as methyl mercaptan and dimethyl sulphide. The odour the kraft process, sometimes called the sulphate process. threshold for these compounds is very low, and despite the efficiency of modern collection systems for odorous gases there is always a characteristic smell from a kraft pulp 3 Figure 2: Schematic of a modern kraft pulp mill with its process units.