Green biorefinery technologies based on waste biomass

Of the many factors threatening a more sustainable society for future generations, those of increasing waste and decreasing primary resources are among the most important. Our 20th century linear economic model of mine-process-consume-dispose was based on rapid growth in consumer demand fed by a rapidly growing industry, itself fed by cheap and abundant mineral resources. The model became auto-catalytic with more consumer demand encouraging more production and as that led to lower costs and to a lower perceived value of articles, so the practice of replacement before redundancy became widely adopted. This in turn led to a “throwaway society”, whereby an article that had once been seen as useful and valuable quite quickly became something to be disposed of. However, this model is not and cannot be sustainable. Our traditional resources, at least in the foreseeable future, are from one planet and they can only become more expensive and more scarce, at least in the medium term, especially as we have already extracted most of the “easy” resources. At the same time, our throwaway items, which were not designed for end-of-life, have been accumulating in landfill sites and more disturbingly, all over the environment including seas and rivers.

While there seems to be a real need for people to review their attitudes about resources and the value of articles, a successful transition from a linear to a more circular economic model must be driven by industry. This must be based on the use of renewable resources, green manufacturing and the production of environmentally compatible, recyclable products. Since the most widely useful element in society is carbon and the most widely used resource is fossil carbon, it seems appropriate that we place a lot of attention on more sustainable organic products and on their production from biomass. The choice of biomass is critical – being renewable does not guarantee being green and we must learn from our experience of biofuels that the sourcing of the biomass is critical. The use of non-food biomass is vital and even better if we focus on bio(mass)-waste. These can include agricultural waste, food processing waste, municipal solid waste and sewage sludge, although the organic components in these waste streams may not be entirely bio-based, with (petroleum derived) plastic being an increasingly common component of waste. By using bio-waste feedstocks to make bio-based products, we can help solve both resource and waste problems and be “double green”.

Bio-waste is rich in biodegradable organic matter and is available in large quantities worldwide. The utilization of bio-waste helps reduce pollution but can also provide renewable energy and bio-based chemicals for the future. Therefore, biomass waste resource utilization has attracted increasing attention in scientific, industrial and government communities. Anaerobic digestion is probably the most established technology (other than composting) that is widely applied to the utilization of bio-waste, but the (bio) gas produced from this process is of low value and the costs of pre-treatment reduce the cost-effectiveness of the process. The chemical potential of the bio-resource is also lost. An increasing amount of new research is focused on the application of different technologies (including fermentation, hydrothermal conversion, pyrolysis and microwave treatment) to make chemicals from bio-waste and thus improve resource utilization. However, these technologies can suffer from high cost and secondary pollution. There is also a concern about separation steps, since these can represent up to 80% of the total cost of many common chemical processes and this is likely to increase the cost for biomass based processes.¹ This should be a driver for industry to consider more the use of mixtures and target properties and effects, rather than always aiming for pure compounds (which are then inevitably mixed into formulations). The combination and ultimately the integration of green technologies is crucial for realising the full potential of bio-waste as a truly sustainable chemical and energy resource.

In this themed issue of the world’s leading journal on green and sustainable chemistry, we sought to address the challenges and opportunities of bio-waste utilization and in particular its use for making chemicals. This follows on from, and is informed by, a successful workshop in China in December 2018 that brought together experts from universities and companies all over the world to share their recent results and progress on applying green technologies in biomass utilization. The workshop “Green Biorefineries for Biomass Waste and the Environment” was organized by Professors Buxing Han (a member of the Green Chemistry editorial board) and Shicheng Zhang from Fudan University. It was held in Shanghai and was run as part of the ISEH (International Symposium on Environment and Health). The workshop featured many talks from speakers from China, Europe and the USA. Topics included those focused on common types of bio-waste, including bio-energy production from swine wastewater, bio-chemicals from food waste, sludge processing, and the valorization of cereal straws and forestry residues. There were also more technology-focused presentations on solvo-thermal processing, microwave activation, solid acid catalysis, pyrolysis and other bio-processing methods including anaerobic digestion (AD) as well as membrane bio-reactors. Additionally, other papers addressed the important issues of biomass availability, integrated technologies and bio-refineries. Overall it was clear that the resource opportunity is very large and multinational, and that we can learn from traditional (petroleum) refineries in aiming for a range of products including (lower value) fuels and (higher value) chemicals. We are still exploring a very diverse range of valorization technologies. Only AD is currently widely used and here the resource efficiency is poor and the product range very limited. More advanced technologies offer a much more interesting range of products and potentially higher resource utilization, but separation will be a major obstacle to their widespread application.

The articles in this themed issue largely reflect the topics and the key messages from the workshop. Residues and by-products from biorefinery operations are of particular interest. Biofuel production always leaves unused biomass and this can be a rich source of chemicals, as demonstrated by a group of Chinese researchers working on algae-based biodiesel production, who also used the CO₂ by-product (DOI: http://dx.doi.org/10.1039/C8GC03645D). In a related article, Chinese and US research groups led by the China Agricultural University studied the organic content of the often neglected aqueous phase from the hydrothermal liquefaction of wet biomass (DOI: http://dx.doi.org/10.1039/C8GC02907E). In a joint paper from groups in Belgium and Singapore, microbial chain elongation is described as a method for valorizing solid-free, thin carbohydrate-containing wastes or side-streams from food processing (DOI: http://dx.doi.org/10.1039/C8GC03648A).

Lignin is the biggest bio-waste challenge and opportunity, being produced in multi-million tonne quantities in many existing processes, including bioethanol and paper and pulp. In a paper from the USA led by Princeton, microbial electrochemical treatment is used to treat biorefinery black liquors and to make chemicals (DOI: http://dx.doi.org/10.1039/C8GC02909A). Speed of reaction can be essential to maximise the conversion of lignin to small molecules (minimizing repolymerisation) and this is demonstrated in a paper from Spain on ultra-fast processing using supercritical water (DOI: http://dx.doi.org/10.1039/C8GC03989E). Once the lignin is depolymerized and even if we successfully prevent re-polymerisation, we still have to deal with mixtures of products. In an article from several groups at the University of Wisconsin-Madison, microbes are used to metabolise such mixtures, sometimes leading to excellent selectivity to just one aromatic product (DOI: http://dx.doi.org/10.1039/C8GC03504K). Downstream from such processes, we will need clever green chemistry to convert lignin decomposition products into more valuable chemicals. Hydrodeoxygenation is a particularly important process in this context and another American group, in collaboration with Palacky University in the Czech Republic, report new green methods for achieving this (DOI: http://dx.doi.org/10.1039/C8GC03951H). Lignin itself may have a much wider range of applications as a material than those presently used: groups from Hong Kong report a new functional material based on a lignin-porphyrin polymer (DOI: http://dx.doi.org/10.1039/C8GC02904K). There are also important papers on agro-food residues such as corncobs from a group of Chinese universities led by the Guangzhou Institute of Energy Conversion (DOI: http://dx.doi.org/10.1039/C8GC02854K). A review on the chemical potential of cashew nut shells from a team of African chemists at Universities including Dar es Salaam is also featured in this special issue (DOI: http://dx.doi.org/10.1039/C8GC02972E).

The application of green chemical technologies to the valorization of bio-waste is the main topic of a significant number of the articles published here. Catalysis is the most important of all of these and in an article from a multinational collaboration between Hong Kong, Thailand and Korea, aluminium biochar is reported as a composite catalyst for glucose isomerization (DOI: http://dx.doi.org/10.1039/C8GC02466A). Catalysis is also predominant in the article on corncobs, and in another article from Poland, zinc oxide is used for oxidations under ultrasonic activation (DOI: http://dx.doi.org/10.1039/C8GC03131B).

Microwave activation of biomass is becoming a very popular approach to making bio-based chemicals, both in terms of efficient heating but also for more selective processes that minimize run-away reactions to gases and small molecules. Two papers from the York Green Chemistry Centre of Excellence describe different examples of how microwaves can be used to help direct biomass pyrolysis processes towards particular chemical products (DOI: http://dx.doi.org/10.1039/C8GC03015D and http://dx.doi.org/10.1039/C8GC02994F). This special issue also contains a major review on using microwaves to convert biomass into chemicals and energy, which includes a feature on its combined use with alternative solvents that interact strongly with microwave radiation (DOI: http://dx.doi.org/10.1039/C8GC03908A).

Ethanol remains the most developed biomass-derived chemical product. In a review from groups in China, Spain, Brazil and Azerbaijan, emerging techniques in bioethanol production are considered (DOI: http://dx.doi.org/10.1039/C8GC02698J). The potential of adding value to the main production by utilizing biomass side-products like pectin is also considered.

The future is very uncertain and we are seeing almost daily examples of the impact of climate change and political turmoil. We are also being made increasingly aware of the harm to our planet and to ourselves of waste and pollution. Scarcity of resources is also a major threat to a stable and sustainable society. As Green Chemists we have the privilege to be able to work on some of these problems and with that comes an opportunity to “make a difference”. But we also have the responsibility to use our skills and knowledge effectively and wisely. Turning waste streams from being environmental threats into valuable resources, while also reducing our dependency on virgin resources, has multiple appeals and could make a real difference to our ability to provide a sustainable future for our planet and those that live on it.

James Clark

Green Chemistry Centre of Excellence

University of York, UK

References

1. V. G. Zuin, Pure Appl. Chem., 2016, 88, 29.

Footnote

† This paper is dedicated to the memory of Egid Mubofu, a former student of mine who died recently. Egid was a successful scientist in academic and government circles, ending up as the Vice Chancellor of the University of Dodoma. He was a real champion for Green Chemistry, especially in Africa, and a gentle and kind person. He will be evergreen.

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