Editor: Professor Emeritus Raimo Alén, University of Jyväskylä
Pulping and biorefining
Everything you need to know about the processing of wood into fibres and chemical products
The utilisation of wood as well as other renewable lignin- and cellulose-containing materials (lignocellulosic biomass) has a long history; our living was almost exclusively based on these resources from its early times and this situation did not drastically change until during the first part of the 19th century. The prominent coal-based production of energy started about 200 years ago and the manufacture of organic chemicals and other products from fossil resources began as coal-based thermochemistry − coal carbonisation − about 150 years ago. Furthermore, petroleum-based industrial activities followed some 60 years ago, leading to an enormous increase in the number of varying products.
For many reasons, including, for example, environmental aspects, the chemical industry is now gradually entering a new era and is shifting together with new technologies to more efficient utilisation of various CO2-neutral lignocellulosic feedstocks with the simultaneous decrease in the traditional use of fossil resources. Hence, much interest has also been directed to the versatile possibilities of using wood and forestry residues for manufacturing of a great variety of products. In spite of these straightforward considerations and trends, the strategic decisions are even in the near future still rather complicated in view of the present challenges in society and the world.
The term bioeconomy comprises those parts of the economy that use renewable carbon resources to produce energy, chemicals and other bioproducts. On the other hand, the biorefinery concept can be defined as a process of fractionating and/or converting biomass in an eco-friendly way through advanced technologies into solid, liquid and gaseous bioproducts. Thus, the main objective is to maximise the value of the product while minimising the production of waste. The basic principle is analogous to that of petrorefineries utilising fossil resources, but biorefineries use a wider range of feedstocks and process technologies. As we move towards a world of greater diversity and balance with the natural cycles of various materials, it is still important to learn the illustrative lessons from oil refining and petroleum industries.
In the chemical pulp industry, large amounts of the fibrous starting biomaterials are dissolved during the delignification process into the cooking liquor, thus corresponding to a full-scale biorefinery. Recovery of the soluble degradation products in the pretreatment stage of chips prior to delignification or from the spent liquors is an interesting alternative to using them for versatile purposes. In general, this integrated biorefinery approach can be considered to represent a processing facility that leads, besides to the principal product pulp (mainly cellulose) and extractives (turpentine and tall oil), also to carbohydrates and their degradation products (mainly from hemicelluloses) and various lignin-derived fractions. Against this background, a historical fact actually is that the first significant industrial biorefineries were started to operate in the pulp and paper industry about 150 years ago, whereas new types of biorefineries have only been developed more recently by the agricultural industry.
In principle, lignocellulosics can be processed in a number of ways using mechanical, chemical and thermochemical conversion methods. Additionally, biotechnology plays an important role when, for example, convert lignocellulosics-derived sugars from the carbohydrates (cellulose and hemicelluloses) into a great variety of platform chemicals. Within a framework of the present theme of Pulping and Biorefining, the main objective was to write a comprehensive description covering independently all the important areas related to modern pulping and biorefinery chemistry. Although it is a fact that much of the content is based on the earlier book series Papermaking Science and Technology published by Forest Products Engineers (formerly known as Paper Engineers’ Association), the material included in this novel approach has been considerably updated and improved and discussed more clearly from the standpoint of biorefining. In practice, it simply means that new data on changes that have been taken place in the key areas during the last decade are taken into account.
In outlining the material to be included, a simple structure with nine parts was selected. It was hoped that these would help readers to understand versatile possibilities to utilise and fractionate lignocellulosic materials, especially wood and wood wastes and to realise the huge industrial potential of various renewable resources. The main purpose of the second part (Extraction-based methods) after the general introduction is to give basic information on extraction-based methods of wood materials including some aspects on extractives-based products and the storage of extractives. The next two parts (Chemical and biochemical conversion and Thermochemical conversion) provide a general approach to chemical and thermochemical conversion methods of lignocellulosic biomass, respectively. Next the important area of kraft pulping is described, whereas next parts describes oxygen-alkali delignification. Lignin-removing bleaching is briefly discussed and next part outlines generally other minor delignification methods. Finally, relevant topics related to mechanical pulping are given.
This comprehensive material can be primarily used as a “conventional advanced textbook” for a wide range of readers in many disciplines. Its utility not only extends to students and teachers, but also researchers who work on production and technical planning, or everyone who needs a reliable and useful data on this area. Furthermore, it should be pointed out that many specialised publications are available that cover each separate topic in more detail, and these references are suggested at the end of each chapter.