Biobased plastics, the good and the bad

Companies haven’t started doing it yet but, technically, there is nothing to stop them from making plastic out of sugar beet or wheat. Good for the environment? Let’s not be too hasty. A study by Sandra Belboom, a research engineer at the University of Liège, shows that there is a considerable gain in terms of CO₂ emissions compared with the use of fossil fuels. But this process also has significant side effects, such as an increase in acidification and eutrophication.

Between 2008 and 2012, oil was on everyone’s lips. Its price soared, sending the finance sector into a spin, and leaving car drivers in despair at the petrol pump. A rapid alternative to the black gold and all its derivatives was needed! Biofuel would save the day. Public authorities released funds, studies were launched… then, as oil prices gradually returned to a more acceptable rate, biofuel was pushed to the wayside. The initial enthusiasm had gone. However, it wasn’t just a new fad, and scientists are continuing their research. Such as Sandra Belboom, a research engineer at the University of Liège’s Chemical Engineering research unit.

When this chemical engineer began her thesis in 2009, her theme couldn’t have been more topical (read Bioethanol: time to stop comparing apples to oranges). What is the best use for bioethanol, a biofuel that can be produced from sugar cane, sugar beet or wheat, and can replace the fuel (partly or completely) in our petrol tanks? “Considering current oil prices, it’s quite clear today that reducing fossil fuels is no longer a priority”, she admits. “But the challenges regarding CO₂ are still considerable. Opting for biobased products is one possibility when it comes to reducing carbon dioxide emissions”.

For this new research, whose results were published last January (1), Sandra Belboom continued to investigate the properties of bioethanol. Not just as a possible fuel source, but also for use in the production of plastic. That’s the magic of chemistry: if you remove a molecule of water from ethanol, the result is ethylene, which lies at the source of many manufactured products, such as polyethylene, the plastic that is part of our daily lives. This time, the researcher put sugar cane aside to focus on sugar beet and wheat, for geographical reasons: the last two grow here! Hence, Belgian specialists are familiar with all the specificities. An important aspect within the framework of the approach favoured by the engineer, i.e. life cycle assessment. “It’s a method that will be able to assess the potential environmental impacts of a product by taking into account its whole life cycle. From the extraction of the raw materials up to incineration”, she explains. “This overview allows you to highlight any possible pollution transfers”.

Take, for instance, the use of bioethanol as a biofuel. At first sight, it appears like a good idea from every angle. Since the carbon it contains comes from plants, the CO₂ released during its combustion is neutral since this CO₂ was originally absorbed by plants. On the other hand, to produce it, it is necessary to put fertiliser on the fields, use tractors, develop transformation techniques, etc. “If we don’t look at all the aspects, biofuel seems wonderful and a great replacement for petrol! But a life cycle assessment shows that the production processes could counterbalance the advantages of using it. The benefit of this method is that it doesn’t only take into account CO₂, therefore the consequences for the climate, but also other types of environmental impacts”.  

So what about bio-ethylene? Does making plastic out of sugar beet and wheat really reduce carbon dioxide emissions? Should other types of pollution be taken into consideration? If the environmental benefit is negligible, would it make any sense to import products from the other side of the world? To answer these questions, Sandra Belboom worked in four stages. First of all, she defined the purpose of the study and the boundaries of the system (in this case, from cultivation to incineration), so she could then gather the necessary data in order to calculate the balances of materials and energy at each stage, ascertain the environmental impacts and interpret the results. The information-gathering phase was the longest. Hence the interest of having detailed knowledge about all the aspects associated with cultivating sugar beet and wheat. Rather than using generic data, Sandra Belboom wanted to stick as closely as possible to Belgian reality. She collaborated with Professor Bernard Bodson (Gembloux Agro-bio Tech). She also approached the BioWanze plant (near Huy), the biggest Belgian producer of bioethanol, at the leading edge of wheat processing, and used the available literature to study sugar beet. Finally, plastic producers were contacted in order to identify the quantities of energy required for polymerisation, yield, etc. “In short, we used different bricks for the construction”, she sums up. Something which, to her knowledge, had never been done before. At least for sugar beet and wheat; sugar cane has already been studied extensively in Brazil. 

In the end, the results were mixed. First, the good news: biobased plastics are good for climate change. During their lifetime, they emit two times fewer greenhouse gases than those made from oil. A tonne of ‘classic’ high-density polyethylene releases four tonnes of CO₂ equivalent, while a tonne from agricultural sources will generate less than two. Bio polyethylene also consumes less fossil resources: 500 kg oil equivalent for sugar beet and 400 kg for wheat, compared with approximately 1,200 kg for oil.

So that’s good, right? Not so fast! Now for the bad news. There are two points: eutrophication and acidification. The first one is mainly due to the use of fertilisers in the fields. The earth is boosted with nitrates and phosphates, which then percolate into the water. The result isn’t very pretty: algae that proliferate so well and so extensively that they absorb all the oxygen, to the detriment of the fauna and the flora, which are destroyed. The production of biobased plastic leads to terrestrial eutrophication that is six to seven times greater, and even seven to eight times more in aquatic environments.

The results aren’t any better regarding acidification either, with an excess of nitrogen oxides, sulphur oxides, etc. This is caused by significant consumption of fossil fuels and the emission of pollutants owing to the use of fertilisers that generate acid rain, affecting fauna and flora as well as building facades. Acidification caused by bioplastic is two to three times worse than for products based on fossil fuels.

Side effects that are hardly negligible. So if we were to weigh up oil against agriculture, what downsides are we prepared to accept to produce polyethylene? Sandra Belboom offers a careful answer. “Life cycle assessment is a tool to help make decisions, it provides extra information in relation to the economic and technical points of view. But it isn’t a decision-making tool”. At the very least, it allows us to put an end to a popular belief: everything that comes from natural materials isn’t necessarily eco-friendly. “This is a cliché we have to get rid of so that we don’t become the victims of ‘greenwashing’”.

The result is that the decision to choose oil or natural products is a political issue. Is it preferable to reduce greenhouse gas emissions, even if this has other harmful effects on the environment? CO₂ is a global problem. One kilo of carbon dioxide less, wherever it has been captured, will benefit the whole world. As for eutrophication and acidification, they are part of a more local context. It comes down to choosing between the well-being of the planet and the well-being of our own environment, which can also be achieved through reasoned farming which helps to reduce these effects through the optimised use of fertilisers…

Especially as plastic based on sugar beet or wheat would have other repercussions. Space is needed to grow these raw materials. And yet, arable land isn’t extendible and current production almost certainly won’t meet all our needs. We would there for have to ‘evict’ the cattle or abandon other types of cultivation. Where would we put the animals? Which products would we have to import more of? What would the economic consequences be? Of course, modifying the use of a few hectares will go unnoticed. But if this change were to occur countrywide or across the continents… we would experience a domino effect. “If there were to be a wide-scale political decision, we would have to go one step further and envisage a consequential analysis, even if it included quite major uncertainties. No-one has a crystal ball! In any case, integrating an economic approach means that you can try to deduce the type of changes that might occur and avoid causing adverse effects”.

The cultivation of sugar cane in Brazil is a good example. In essence, using this raw material to make plastic seems like a very good idea. The yield is very high (up to 200 tonnes per hectare, according to certain publications!) and nothing is thrown away: the bagasse – the fibrous residue obtained after crushing the sugar cane – can be burnt, therefore the producers are self-sufficient in terms of energy and consume hardly any fossil fuels. As for CO₂, the benefit is even greater than with sugar beet or wheat. The gain is said to be 3 to 4. Since the beginning of this cultivation in the 1970s, the land devoted to sugar cane has been considerably extended. Thankfully, because of the type of soil required, there has been no need for deforestation to free up the space. Instead, it is pastures that have provided this space. The cattle has been moved, in particular towards the Amazon forest. Subsequently, sugar cane is suspected of indirectly causing deforestation. “If we take this parameter into account”, Sandra Belboom emphasises, “the CO₂ impact becomes greater than that caused by fossil fuels. Or you would have to compensate by a payback time (the number of years during which you would have to use bioproducts to counterbalance this deforestation), which varies between 5 and 100 years, depending on the percentage of deforestation taken into account”.

The subject has raised its fair share of controversies, with differing opinions regarding a recognised responsibility in deforestation. In this respect, sugar cane is far from coming out on top. As a result, would it be relevant to import this raw material to make our bioplastic? “All depends on the mode of transport”, the researcher answers. “It could be imported by ship from Rio to Antwerp, but as granulates or the actual plastic, otherwise we would be transporting a lot of water, so the cargo would be heavier and generate more emissions. If all these conditions were present (and providing there is no impact due to deforestation), this could be neutral in terms of CO₂. You would also have to take into account the cost and the social aspect”. It may not be so good for employment at this end, to have it produced at the other end of the world. In addition, in Brazil and other developing countries, it is quite likely that the sugar cane workers don’t receive the best treatment.  “There are sustainable life cycle assessments but the criteria are still under development. We can highlight hotspots, such as: is child labour authorised? Is there a welfare system? How many hours are worked a day? Etc. But where do we put the limit between what is and isn’t acceptable? It’s a subjective point of view”.

Performing a life-cycle assessment is a bit like pulling on the end of a ball of wool. We know where it starts, but we don’t know where it will end. And perhaps knots will appear that complicate things. But in order to knit a greener world, we have no choice but to go through the process!

(1) Does biobased polymer achieve better environmental impacts than fossil polymer? Comparison of fossil HDPE and biobased HDPE produced from sugar beet and wheat, in Biomass & Bioenergy (2016).