Key words

biorefinery; process design; process synthesis; optimisation; process up-scaling; integration

4.1 Introduction

In the last 2–3 decades, biomass materials, i.e. agricultural crops, forestry products, organic fractions of household and industrial wastes and aquatic biomass (i.e., algae), have attracted increased research and commercial interest as renewable sources of fuels and high value chemicals. Increasing global energy demands and the need to reduce dependence on fossil fuel-based production systems, given the negative environmental impacts associated with their use, have been the main drivers for the use of these biologically-sourced process feedstocks. Although the advantages of using bio-derived products are well known, their production and use has been hindered by the availability and lower relative cost of fossil fuels. Advances in modern organic chemistry techniques have resulted in the increased production of high-volume low-value transportation fuels (accounting for more than 90% of the total global transportation consumption) and chemicals (Bozell, 2008). Improving the design, utilisation, energy efficiency and economics of heat, power, fuels and chemicals from biomass sources are therefore key requirements if biomass feedstocks are to become competitive with conventional fossil-based feedstocks. This would subsequently lead to the establishment of bio-materials as a potentially viable and sustainable alternative industrial raw material.

The use of the biorefinery concept has been proposed as a way to optimise the overall technical, economic and energetic efficiencies of the production processes of bio-products resulting in an array of marketable products, energy and process heating/cooling. This concept uses an extensive range of technologies to convert bio-materials into their component monomeric units which are then further reconstituted to produce high-value industrial precursors, chemicals and energy (including fuels and combined heat and power streams). An overview of factors influencing biomass inputs, process intermediates, conversion technologies and possible products and energy streams obtainable using a biorefinery set-up is shown in Fig. 4.1.

The main incentive encouraging the implementation of a biorefinery platform is the replacement of fossil-based industrial and energy feedstocks with ‘green’ biomass sources. The production of energy from these biomass materials is especially favoured over fossil-based feedstocks (i.e., coal combustion) since an almost neutral net carbon dioxide (CO₂) emission is achievable when biomass and energy production processes are managed sustainably. However, the attainment of such ‘green’ energy goals requires economic motivations to support the conversion of bio-materials to renewable fuels. As the fuel products are usually low-valued irrespective of the biomass feedstock, this financial incentive can be met using biorefinery systems producing high-value products at the same time. Consequently, biorefinery platforms have the potential to perform better economically and energetically than stand-alone biomass-to-fuel facilities which are currently overwhelmed by a low return on investment (Bozell, 2008).

With biorefinery research still only at a developmental stage, different definitions and categorisations for biorefinery platforms have been advanced (IEA, 2007; Kamm and Kamm, 2004; Kamm et al., 2006). These categorisations are quite limited in scope and do not properly illustrate the full potential of biorefinery concepts for biomass processing. However, for the sake of discussion in this chapter, four main categories of potential biorefinery systems are used to demonstrate important process parameters which should be considered for integrated biomass-to-products conversion. These are:

The emphasis of this chapter will be to illustrate various principles which should be considered in designing and implementing potential biorefinery systems. Although used to support the discussions in this chapter, an in-depth analysis of the potential economic, energetic and environmental advantages of biorefinery platforms will not be carried out since these topics are elaborated in Chapters 1–3 of this book. Furthermore, though Part II of this book specifically covers various biofuels and high-value products which could be obtained from biorefinery schemes, this chapter will draw on evidence from existing established, test and conceptual biorefinery platforms to help explain important plant design and conversion processes.

4.2 Microalgae biomass for biorefinery systems

Before the actual installation, implementation and operation of a proposed biorefinery system can be considered, careful planning, synthesis and design of the systems must be carried out. Section 4.3.1 looks at underlying principles and techniques which could guide biorefinery installation design and implementation. In particular, it covers the potential use of aquatic biomass inputs (specifically microalgae) in biorefinery systems since this feedstock is attracting interest as a useful ‘non-food impinging, non-arable land requiring’ biomass. Other factors promoting the use of this feedstock for chemicals and fuel production are its higher biomass productivity and photosynthetic efficiency compared to conventional terrestrial energy crops (Pirt, 1986; Goldman, 1980), and the ability to control the microalgae cultivation process for the production of specific macromolecular components (Illman et al., 2000). This potential to optimise the production of specific macromolecular components from microalgae biomass provides a unique platform for the production of a wide variety of products, chemicals and fuels confirming the use of this feedstock as an excellent basis for the application of a biorefinery concept.

Microalgae biomass production and conversion to fuels and chemicals is not novel. The use of microalgae biomass obtained from natural water bodies and used as human food has been well documented over time. However, most scientific work carried out on mass cultivation and use of this feedstock for fuel and chemical production is relatively recent (within the last 60 years) with the first pilot scale Chlorella cultivation plant reported in the 1950s (Arthur D. Little Inc., 1953) and fuel production via the production of methane (CH₄) using this biomass demonstrated by Meier (1955) and Golueke et al. (1957). Since then, research on algal-derived chemicals and fuels has been pursued aggressively by various applied phycology research groups worldwide leading to the present knowledge base. It is the findings of these research groups which have formed the basis of current operations of commercial large-scale microalgae plants aimed at single nutritional, mariculture and pharmaceutical applications. Coupling advances in biotechnology and conversion technologies with the vast information currently available on industrial microalgae cultivation could present an excellent foundation on which to base potential microalgae biorefinery processes. The co-production of a range of ‘high- and low-valued’ products and energy from microalgae biomass potentially optimises the utilisation of the various algal macromolecular components, and maximises the value which could be derived from this non-food feedstock.

Section 4.3 concentrates solely on the potential engineering and process optimisation schemes to be considered when using microalgae biomass in biorefinery schemes.

Although proposed microalgae biorefinery systems are included in the overall discussions on planning and synthesis due to their increasing interest as a model example of biomass feedstocks, which are expected to be used more and more for chemical and fuels production, other biomass inputs will also be considered. Having looked at the biorefinery planning and design aspects requirements, the technical and engineering requirements of a case study biorefinery plant is then presented in Section 4.4.

4.3 Planning, design and development of biorefinery systems

4.3.2 Design and synthesis of biorefinery systems

Basis for biorefinery design

A widespread approach used for the design of potential biorefinery systems is to begin with primary conversion technology, usually deemed the ‘mainstay method’ of the biorefinery, followed by the addition of side processing routes for the processing and upgrading of the biomass feedstock to other useful product streams. Such an approach is particularly appropriate in already established production systems, e.g., the pulp and paper industry, which could benefit from a gradual conversion of the primary milling operations to a biorefinery, reducing the overall process energy inputs, producing secondary products, and improving the overall economics of such a plant. The conversion and integration of a biorefinery scheme in existing plants could be spurred by the availability of useful waste streams. For example, the facilities at Cognis Australia Pty Ltd, Australia (the largest producer of algal beta-carotenes and carotenoids) (Ben-Amotz, 2006) could be converted to take advantage of the post-extracted residues after the carotenoid extraction and production process. The technical knowhow already accumulated on the cultivation, biochemical characteristics and conversion of the Dunaliella salina biomass (grown in two on-site operated saltwater lagoon farms in western and southern Australia) would help in the selection of suitable secondary co-products and conversion streams which could be integrated into the already established primary product line.

Another route for designing biorefineries is to combine and upscale laboratory-scale investigations with a revision of the process configurations based on the perceived feasibility of intended large-scale chemicals and fuel production. This route, although more time consuming, could be beneficial particularly for the implementation of novel conversion technologies avoiding any retro-fitting problems.

Selecting best conversion biorefinery pathways

Even having established the potential multiple conversion routes or utilisable streams in existing plant processes which could be adapted for biorefinery platforms, one of the key challenges in the design and synthesis of biorefinery systems is selecting and allocating the optimal conversion pathway for the production and separation of the product streams from the biomass input.

Various techniques have been put forward in publications to guide the design and synthesis of potential biorefinery systems (also applicable for microalgae biomass). Process synthesis approaches such as the matrix synthesis, symbol triangle, retro-synthesis and Gibbs free energy approaches were introduced in the 1970s (and still enjoy use in chemical engineering process design today). An increasing environmental awareness has led to the integration of environmental considerations in refining process designs as proposed, for example, in Crabtree and El-Halwagi (1994) and Pistikopoulos et al. (1994).

The use of a systematic approach to the design of optimal biorefinery pathways was reported by Bao et al. (2009). This is based on advancing a ‘superstructure’ of conversion technologies and products, then applying a tree-branching and searching technique to select the best candidate pathways and products for the intended biorefinery system. With the number of synthesised pathways being potentially extensive, such a technique provides a quick screening method to reduce the number of conversion technology alternatives to obtain the preferred pathways for the production of desired chemicals and fuels in the biorefinery setup. Other optimisation synthesis methods, such as the two-stage approach proposed by Pham and El-Halwagi (2012) for the design and synthesis of biorefinery systems, could also be easily implemented when considering biorefinery conversion.

As previously highlighted, the design considerations for microalgae biorefineries (and other biomass feedstocks) are generally complex, requiring a thorough knowledge of the component system inputs and intended conversion processes as well as an ability to develop innovative solutions to address potential problems which might be encountered. The application and integration of system tools for the initial design and synthesis of biorefinery systems is important, as it helps identify the proposed conversion routes with the minimum economic implications. Kokossis (1993) describes how such system tools could be applied in biorefinery synthesis by presenting a hierarchical cascade of information flows which should be considered. This cascade encompasses the use of forward-and-backward information flows aiding decisions on the system design, process synthesis and integration and the intended biorefinery system technologies (Fig. 4.2) (Kokossis and Yang, 2010).

Such an analysis could also provide the basis for the support of an integrated system where existing facilities already exist, with the existing processes (right of the figure) modelled and upgraded with the introduction of new process paths and products (Kokossis and Yang, 2010). This cascade analysis is thus repeated several times before selected processes are demonstrated and implemented (Kokossis and Yang, 2010).

With one of the credentials supporting the use of biomass feedstocks for chemicals and fuels production being the potential of the products to be CO₂ neutral (or optimistically negative), the optimisation design method for synthesis of biorefinery systems should establish the conversion routes requiring minimum process energy inputs. This is especially important with the potential use of microalgae biomass as the principal biorefinery raw material as is evident when its large-scale cultivation is factored into the overall framework of the production of fuels and chemicals from this feedstock. As opposed to the collection and supply of other first and second (i.e., terrestrial-based lignocellulosic biomass and waste) generation feedstocks, the industrial cultivation of single-celled microalgae biomass is a comparatively energy-intensive process mainly due to its harvesting method (i.e., centrifugation). Results of the energetic assessments carried out in Ehimen (2010) showed that the energy requirements for the biomass harvesting step could potentially account for more than 85% of the total energy requirement of the microalgae biomass production process, potentially making it the limiting step in a microalgae biorefinery. Therefore, although most biorefinery synthesis approaches concentrate on optimal conversion schemes, the biomass production (or acquisition) and harvesting stages must also be taken into account, especially when feedstocks like microalgae biomass are utilised. Also when designing the framework of potential biorefinery systems, any system analysis should include biomass production and transportation issues as well as the conversion routes as shown in Fig. 4.3.

4.3.3 Engineering considerations on biorefinery up-scaling and implementation

Biomass production

The inclusion of the biomass cultivation (or acquisition) has been included in the assessment boundaries for potential biorefinery schemes (Fig. 4.3). This factor is a particularly important engineering consideration when using microalgae biomass since the optimal choice for microalgae production and harvesting routes (as well as their energetic requirements) are still contentious and depend mainly on the specific microalgae species cultivated, type of bioreactor used, cultivation requirements and the climatic conditions available. Specific engineering solutions must be tailored to fit the particular microalgae biomass being produced. The outdoor large-scale cultivation and harvesting of Chlorella biomass, for example, will require technical considerations for culture stirring and harvesting techniques such as centrifugation. This is unlike the engineering requirements for microalgae species such as Arthrospira (Spirulina), where biomass harvesting could be accomplished using filters (Hu, 2006). A trade-off between the economics and the improved efficiency of the biomass production (and recovery) is thus essential. Although integration with waste treatment plants could potentially improve the process techno-economics of microalgae biorefineries, most high-valued products are aimed at the pharmaceutical market, where product purity and source are important parameters leading them to be sceptical about, or entirely reject, products obtained via such systems.

Biomass conversion

A range of conversion technologies can be applied and integrated for the production of energy carriers from microalgae biomass, depending on the intended energy application. These processes can be broadly classified into four main groups: mechanical (e.g., pressured extraction), biological (e.g., fermentation), chemical (e.g., solvent assisted extraction, transesterification), and thermal (e.g., drying, pyrolysis) (Fig. 4.4). A selected conversion route for the production of a chemical or fuel would generally involve a combination of two or more processing technologies, e.g. the combination of a mechanical oil extraction from oleaginous biomass and a thermochemical transesterification process for the production of the automotive fuel biodiesel from the extracted biomass oil.

Consequently, the engineering specifications of the conversion aspects of a proposed biorefinery systems are based on the intended product streams. Biotechnology and microbial advances have been especially important when considering the production of specialty chemicals and products. Important factors such as the sizing and control of the bioreactors, the required optimal biological conditions such as the process temperature and pH, the presence of inhibitory toxic components in the biomass and the biomass loading must be considered for such processes. However, for the production of lesser valued refining products, the application of rapid thermo-chemical conversion processes is still expected to be essential for proposed biorefinery systems, on the basis of economies of scale (Klass, 1998).

The engineering and integration of most of the constituent unit operations of the proposed biorefinery, i.e. extraction, distillation, filtration and heat utilisation, will be similar to existing systems using modern chemical engineering principles as the basis for the chemical and mechanical conversion processes and is expected to be the same for most process streams.

Potential biorefinery integration and upscaling

For the potential up-scaling of laboratory research into biorefinery systems, process modelling tools like Aspen Plus® could be used for the initial design of the reaction and separation units of the intended biorefinery processes. Such an approach would include a description of the reaction pathways and chemical components, and selection of a suitable thermodynamic model for estimating the component’s physical and thermodynamic properties. To ensure that the modelled processes reflect the observed practical results, empirical data on product yields obtained from laboratory experiments should be used in the process simulation. The user could further define the plant capacity, process input and operational conditions (e.g., flow rates, temperature and pressure) and use that as a basis to assess the technical feasibility of the biorefinery.

When considering the integration of different process streams in biorefineries, care should be taken in directly comparing refining schemes using biomass feedstocks with conventional petroleum refining systems. Unlike crude oil, which is a concentrated high-energy feedstock, most biomass raw material (especially microalgae, even after a dewatering step) usually has a high moisture content. A biorefinery scheme using such feedstocks must therefore be treated as a novel paradigm with innovative products and process streams (Subhadra, 2010).

Conceptually, the biorefinery scheme should also be treated differently from conventional refineries since in the latter, material and energy integration is usually applied to minimise the external energy inputs, with the by-products (including waste heat) of one oil refining process feeding into another complementary process stream for conversion into another product. This has led to the evolution of giant oil refineries with the associated benefit of maximum economic returns from minimum economic inputs. With the biomass inputs (especially in the case of microalgae) expected to be produced locally, it is speculated that small to medium-sized processing plants spread regionally around the biomass cultivation areas are most likely to develop as opposed to one or two giant refining plants (Willems, 2009). The scale of the proposed biorefinery system is important from an engineering perspective since smaller units could potentially limit the extent of energy recovery and even possibly hinder the re-utilisation of major stream by-products for onward conversion to other chemicals or fuels.

The integration and/or retrofitting of some stages of existing petrochemical refineries to utilise biomass feedstocks could still provide an attractive route for implementing proposed biorefinery processes. Such integration has the added advantage of potentially upgrading the conventional refinery systems to a ‘greener’ level. The ability to utilise waste heat streams from power generation plants could be an important criterion for the siting of a biorefinery plant, particularly when thermo-chemical processes such as biomass gasification and pyrolysis are proposed, as these processes require high operating temperatures (more than 300 °C) for the production of syngas (a mixture of CO, CO₂, CH₄ and H₂) and bio-oil from a wide variety of biomass feedstocks. The syngas produced can be used directly as a fuel, starting block or intermediate for the production of chemicals (e.g., ammonia, organic acids and alcohols) and fuels (e.g., Fischer–Tropsch fuels, ethanol and di-methyl ether). Apart from the bio-oils obtained from the pyrolysis process, charcoal and light gases are also produced (Bridgwater and Peacocke, 2000). The integrated biorefinery system could also use existing platform units such as the gasification, hydro-treating and reformation refining stages of the petro-refinery to facilitate the biomass conversion or product formation processes. The availability of by-products and energy from the conventional refinery or power plant could help reduce the net material and energy inputs in the biorefinery process, improving the system’s environmental impact while providing a foundation for the gradual shift to using biomass feedstocks.

To properly illustrate the operations and technological outlay of biorefinery systems and research and development considerations supporting such schemes, a case study plant using lignocellulosic biomass as the process feedstock is presented in the next section, which will also be used to highlight some of the design and integration considerations described in the earlier sections.

4.4 Case study: a second generation lignocellulosic biorefinery (Inbicon® Biorefinery)

Unlike the technologies associated with the production of energy and chemicals from first generation feedstocks (e.g., sugar and starch crops) which are already well established, the development of processes for the conversion of second generation biomass is still considered to be in the early developmental stages. Of the second generation feedstocks, the use of lignocellulosic biomass (e.g., residues from agriculture and forestry, as well as specially grown lignocellulosic crops) has been highlighted due to the potential availability of this raw material as a process feedstock.

The Inbicon biomass refinery (operated by Dong Energy, Denmark), selected as the case study in this chapter, uses lignocellulosic biomass for its operations. Although the conversion processes were developed and initial large-scale production carried out using wheat straw as the principal raw material, the conversion processes have been developed over time to use a variety of lignocellulosic inputs including corn stover, barley and rice straw, bagasse from sugar production, as well as garden and household waste. The biorefinery produces ethanol as its primary product, clean lignin powder and C5 molasses as a side stream, with electricity and heat coproduced via lignin combustion in a co-generation plant.

4.5 Upgrading biorefinery operations

Any potential process upgrading or optimisation of integrated biomass refining systems depends on a thorough understanding of the operations of the existing plant to be improved. Before considering the addition of new process and product streams which could potentially increase the useability of the biomass inputs of the refining or processing plant, it is useful first to optimise the existing biomass acquisition, pre-treatment, conversion and product purification technologies.

4.6 Optimising biorefinery processes using process analysis

With process analytical technologies (PAT) expected to reach maturity and become commercially viable in the pharmaceutical sector within this decade, the next step will be to introduce these advanced process control tools into the biorefinery sector (Holm-Nielsen, 2008). With such large investments being made in the biorefinery sector worldwide, there is as great a need to be pro-active in process monitoring and control regarding PAT instrumentation as in the leading food processing and pharmaceutical sectors.

In the biorefinery sectors, the competition for high quality grade fuels and products is fierce, and each percentage reduction in production prices is critical. Control tools like PAT will help monitor the large volumes of forecasted production/waste products in the coming decade and ensure the high quality of advanced biofuels as early in the process streams as possible. The ability to control and optimise the products obtainable from the various conversion schemes and to develop standardised products for the chemical and biofuels sectors in relation to conventional oil refinery products is highly relevant and presents us with extremely complex and fascinating scientific and technological challenges.

As described in Section 4.3.3, the biorefinery concept includes several mechanical, physical, chemical, and biological pre-treatment and processing steps. Integrated PAT tools are critically necessary to control and manage these very different processing steps to optimise the biorefinery plants.

4.7 Conclusion and future trends

The biorefinery concept (and research) has developed rapidly in the last decade. Biomass sources will increasingly become all-round competitive alternatives to crude oil as feedstocks for fuels, chemicals and materials, matching their role as raw materials for food and feed production as presented in Holm-Nielsen et al. (2006, 2007) and Holm-Nielsen and Kirchovas (2011). Many of these obligations will be fulfilled by integrated biorefinery processing facilities for optimal yield and energy efficiency. Future biomass conversion and use will be very different from biomass utilisation before the industrial revolution. It will also be different compared to the mono-processing facilities in the twentieth century (e.g., sugar factories and grain or potato starch processing plants). New synergistic multi-process flows will be developed far beyond yesterday’s single-line processing plants. The resulting product portfolio for society will have the potential to replace petro-chemically synthesised products and allow new sustainable bio-products to find their way to the market, for example new biomass-based building and insulation materials made directly from processed fibres and plant materials.

Another important lesson learned from the initial stages of bio-refining is a trend towards decentralised production in strong contrast to the prevailing twentieth-century strategy of centralisation. The initial phases of these innovative steps have already begun in the design of such modern biotechnological processes and process equipment where the units will be simple and robust and economy of scale will be replaced by economy of numbers (Born, 2005). Micro fuel-cell implementation programmes running on biofuels for combined production of electricity and heat in domestic housing illustrate this well, fulfilling the idea of a bottom-up approach as a virtual power plant, compared to the top-down coal and nuclear centralised power plant concept from the last century. Biomass-based resources integrated with fuel-cells – in which the fuel itself is bio-ethanol, bio-methanol, bio-methane, or bio-hydrogen – is a fascinating concept because all the raw materials can be produced at a rural biorefinery scale. The end product, exemplified by biogas, can then be distributed through the existing natural gas grid system to houses in urban areas.

The ultimate goal for biorefineries is to be processing facilities that include centralised innovative technologies fuelled by renewable energy sources generated from wind, solar, and biomass resources. Such biorefineries will process a wide range of high- and medium-value products needed by society to replace fossil fuel-based products. The technologies involved are all known today, and are all based on flexible, more or less non-sterile, fermentation processes (Born, 2005). Biorefineries can be considered a ‘bridge between agriculture and chemistry’ (Braun, 2005) designed to produce basic and intermediate organic chemicals and fuels for direct or further product developments.

One of the most valuable by-product lines in the biorefinery could be the anaerobic digestion (AD) step. It is a fully integrated part of biorefinery processing due to its efficient use of by-products and waste streams from various other biological and biotechnological production processes. It is an important step producing biogas aimed for integration in the overall process energy needs but with the added advantage of producing bio-fertilisers to replace conventional chemical fertilisers in the agricultural sector. The AD process could be considered as the initial conversion step producing methane (CH₄) and CO₂ which would subsequently be used as the starting material for a variety of chemical and fuel synthesis. As highlighted in Section 4.5.1, where such gaseous intermediates are produced on decentralised sites close to the biomass source, the techno-economics and logistics of the conversion processes could potentially be improved with the use of pipelines to transport the products from these on-site facilities to the biorefinery systems. Alternatively, this step could be considered as the final conversion process applied to the biomass residues reclaiming any remaining carbon components following preceding conversion processes. The biorefinery concept provides endless new possibilities for making highly significant contributions towards the ultimate closed-system eco-solutions needed by society in the near future. Optimising the AD process is an important step towards this ecological goal. These solutions are ideologically invariant, functioning in both capitalist as well as socialist economies all over the world.

Although being a major incentive for technological advances, the use of profit to decide improving research and the implementation of biorefinery systems should be considered of secondary importance, compared to environmental concerns, pollution pressure and global warming.