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POLYHYDROXYALKANOATE (PHA) PRODUCTION FROM METHANE & NATURAL GAS - LITERATURE REVIEW PART 2

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POLYHYDROXYALKANOATE (PHA) PRODUCTION FROM METHANE & NATURAL GAS - LITERATURE REVIEW PART 2

Wed, 04/03/2019 - 16:01
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Biodegradable Plastics
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Section 2: Addressing the Plastic Problem

2.1​ The Promise: Biodegradable Plastics

With increasing oil prices, current concerns about depleting oil reserves and the ongoing push for implementing renewable energy systems, there is a sense of urgency for intensive research to address the synthetic plastic problem. Biodegradable polymers present great promise as an alternative to synthetic polymers. Their researchdates several decades back, with the discovery of a variety of options such as cellulose, casein plastics, soy protein plastics, chitosan, etc. [21]. Numerous advancements have already been made over the course of time to improve upon their material properties for application in medicine, agriculture, electronics, etc.

 

Biodegradable polymers can either be naturally occurring, made from non-renewable sources (petroleum products) or derived from renewable sources. The biodegradation of these polymers is initiated either by enzyme activity or chemical disintegration associated with living organisms, and their biodegradability depends on their origin, chemical structure and environmental degradation conditions [22. Biodegradation converts the polymer to CO2, CH4, biomass and other natural substances and can thus be naturally recycled by biological processes [23]. The biodegradability of these biopolymers makes them very attractive as they can eliminate the environmental concerns caused by synthetic polymers.

 

2.1.1​ Synthetic and Natural Biodegradable Polymers

Synthetic biopolymers are derived from petroleum resources and they are designed with hydrolysable functions such as esters, amides, etc. or with carbon backbones in which additives are added to facilitate biodegradation. Table (1) below shows the different categories of synthetic biopolymers, highlighting some of their properties, how they are synthesized and a few examples of each category [22].

 

Table 1: Synthetic Biopolymers

Synthetic Biopolymer Category

Governing Concept

Examples

Aliphatic Polyesters

Very high molecular weight biodegradable compound with hydrolysable ester bonds. Consists of two classes; Polyhydroxyalkanoates (PHAs) and Poly(alkene diocarboxylate)s.

Polygycolide (PGA); Polylactide (PLA); Polycarprolactone (PCL); Poly(butylene succinate) (PBS); Poly(p-dioxanone) (PPDO); Polycarbonate

Aromatic Copolyesters

Consist of a mixture of aliphatic and aromatic monomers and often based on tetraphatalic acid. Their biodegradation decreases at higher concentration of tetraphatalic acid.

Poly(butylene adiapate-co-terephtalate) (PBAT); Biomax by Dupont

Polyamides and Poly(ester-amide)s

Contain the same amide bonds as polypeptides but with stronger chain interactions, leading to a lower rate of degradation. Biodegradation is increased by introduction of side chains such as benzyl, hydroxyl and methyl groups.

Bak 1095 by Bayer

Polyurethanes

Contains a soft segment derived from polyols and a hard segment from di-isocyanate. Its biodegradation depends on the chemical nature of the segments.

Poly(ester urethanes)

Polyanhydrides

Have two hydrolysable sites and degradation rates depend on the polymer backbone. Aromatics will take longer to degrade than aliphatics.

 

Vinyl Polymers

Undergo biodegradation via an oxidation process upon addition of a catalyst.

Polyacrylates, Polyvinyl alcohol

 

Natural biopolymers on the other hand could either be naturally occurring in nature or synthesized by bacteria. Naturally occurring biopolymers are classified into two categories, polysaccharides and protein. Biopolymers synthesized by bacteria can be prepared by a fermentation process or produced by a wide range of microorganism. Tables (2) & (3) summarize these different biopolymer types focusing on their origin and how they are processed [22][24].

 

Table 2: Naturally Occurring Biopolymers

Naturally Occurring Biopolymer Category

Origin

Examples

Polysaccharides

Marine Sources

Chitin, Chitosan,

 

Vegetal Sources

Starch, Cellulose, Alginic acid (Alginate),

 

Human Sources

Hyaluronic acid, Chondroitin sulphate

Protein

Animal Sources

Collagen, Gelatine, Elastin, Albumine, Fibrin

 

Vegetal Sources

Wheat gluten, Soy protein,

 

Table 3: Bacterial Synthesized Polymers

Bacterial Polymer Category

Governing Concept

Examples

Semi-synthetic Polymers

Starch is fermented to lactic acid by lactic bacteria and used to synthetically produce a biodegradable polymer (PLA)

Polylactic acid (PLA)

Microbial Polymers

A diverse range of bacteria can accumulate a variety of polyhdroxyalkanoates (PHAs) as intercellular reserve material in the presence of a carbon source and under limiting nutrient conditions. These polymers are completely biodegradable and have excellent mechanical properties.

Polyhydroxybutyrate (PHB), Polyhydroxyvalerate (PHV), Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV)

 

All these biodegradable polymers have great potential to lower the carbon footprint of the final product, reduce CO2 emissions, and they can be used for countless applications. However, there are still several challenges associated with the manufacturing of these biopolymers and their commercialization remains an issue of concern.

2.2​ Current Challenges with Biodegradable Polymer Production

Even though biodegradable polymers have numerous advantages, especially in terms of greenhouse gas mitigation and biodegradability, they still face a wide variety of challenges. Some of the main challenges include the unsustainable feedstock used to produce the polymers, the high cost of production, and their waste management.

 

Many of the biodegradable polymers listed above and in the market today are still produced from petroleum sources, making the solution unsustainable with regard to current rising oil prices and depleting natural reserves. Moreover, the additives added to most synthetic biodegradable polymer have a chance of washing off from the landfill sites into the groundwater and hence contaminating these water sources.

 

Those biodegradable polymers produced from renewable sources use food crops such as corn, sugar cane etc. for raw materials. The use of food crops has detrimental implications on food security in many developing and even some developed nations. This approach will significantly increase the prices of staple foods since the supply will decrease and become diverted for polymer production. Furthermore, huge amounts of land will be taken up to grow these crops in-order to match the feedstock demand. This would require a substantial amount of water on a regular basis to irrigate these crops. With a fast growing global population and millions of people living in abject poverty and hunger conditions, this direction is not only unfavorable but also unrealistic.

 

There is still a significant cost associated with the production of biopolymers compared to traditional synthetic polymers, which is limiting the wide-scale commercialization of biopolymers. A major part of this cost is allocated to the raw materials, followed by the manufacturing cost which includes the process energy input and the final product transportation costs [26, 27]. Raw materials such as glucose cost about $1.30/kg and the market price for PHAs from companies like Metabolix, USA is set at about $12/kg which is over 15 times more than the market price of most synthetic polymers. This price disparity already places the biopolymer at a major disadvantage, as consumers may not be prepared to purchase a high priced material with biodegradable properties. If these expensive carbon sources continue to be used, even the most effective processes will not permit biopolymers to compete with conventional petroleum-based polymers. In order to reduce the cost of production, it is important to focus on developing processes that utilize inexpensive carbon sources for the production of biopolymers [28]

 

Finally, most biodegradable polymers are not designed for material recycling where recycled plastics are reheated and processed further. Therefore mixing these polymers with the plastic recycling feedstock will damage the process and quality of the recycling products. Composting has been identified as the most efficient method of disposing biodegradable polymers. However, very few countries, homes and establishments around the world possess the required infrastructures and only a fraction of them are capable of properly operating these systems.Moreover, mixed waste composting has detrimental effects on the quality, usability and marketability of the compost. It is therefore important for the users of the composting facilities to be educated about composting process and on ways to appropriately separate their waste materials for composting purposes [29].

 

2.3​ Possible Solutions for Biodegradable Polymer Production

An effective way to address some of the issues related to the production of biodegradable polymers is to utilizeinexpensive or waste carbon substrates. Since the cost of raw materials alone is known to contribute to about 50% of the production cost of the biopolymer, this approach would potentially solve both the feedstock and cost concerns. A number of options have been investigated especially for PHA production such as methane gas [30, 31, 8]. This paper will focus on the solutions of utilizing methane gas as feedstock and natural gas as an alternative feedstock option.

2.3.1​ Methane as a Feedstock

Methane is a very potent greenhouse gas and a major contributor to the global climate change. It is 25 times more potent than carbon dioxide making it a significant contributor to the earth’s anthropogenic warming. Methane emissions are obtained from several routes with the majority of these emissions from the production and transportation of coal, oil and gas, and a number livestock and agricultural practices. Methane is also emitted fromthe decay of organic matter at landfill sites, and even through wastewater treatment systems. These methane emissions are projected to increase by 20% in 2030 if no efforts are put in place to reduce them [32, 33]]. Therefore, mitigating methane emissions is crucial in-terms of the benefits it would have on our climate at large and its reduction could help prevent potential explosion hazards at methane emission sites. Additionally, reducing methane emission would have significant benefits to global human health since methane is a precursor of the tropospheric ozone, which is an air pollutant known to be associated with premature mortality [34].

 

However, due to the abundance of methane in the environment, it remains a very cheap gas and mitigating methane emissions hereby presents a plethora of opportunities with its most common use as a low-cost fuel for electricity generation or cooking gas. More recently, methane has been used as a cheap feedstock for biopolymer production. By using methane as a feedstock for polymer production, carbon sequestration is encouraged, as the gas is stored in a valuable environmentally friendly product and only released at the end-of-life of the product. The methane can then be recaptured for further production of more valuable product, hence closing the loop of the carbon cycle.

 

2.3.2​ Natural Gas as an Alternative Feedstock

The EPA describes natural gas as a fossil fuel formed when layers of buried plants and animals are exposed to intense heat and pressure over thousands of years. Natural gas is comprised of methane gas (85-95%) as its primary component alongside other hydrocarbons such as ethane (2-5%), propane (0.1-1.5%), butane (0.01-0.3%), and pentane (0-0.14%). Natural gas also includes trace amounts of impurities such as hydrogen sulfide, nitrogen, helium and carbon dioxide. It is important to note that the composition of natural gas varies depending on the source and processing procedure of the gas [35]. The current market for natural gas continues to grow at a very fast pace and as its extraction technologies improve, the price of natural gas has plummeted from about $5/MMBtu in 2014 to $3/MMBtu in 2015 [36]. This has made natural gas an attractive alternative fuel since it is also know as the cleanest-burning fuel.

 

Given the high methane content of natural gas, it is another potential alternative feedstock in the production of biopolymers like PHAs, in addition to just methane alone. However, with the diverse nature of the natural gas mixture, the higher-level alkanes alongside the other impurities present could be toxic and likely result in growthinhibition if fed directly to pure cultures of methane-utilizing organisms (or methanotrophs in general). To address this concern, mixed culture enrichments, which contain majority methanotrophs together with other higher alkane oxidizers, will be capable of oxidizing natural gas concurrently. Other members in the consortium capable of growth on their metabolites and impurities, which would otherwise be toxic to methanotrophs alone, will consume these metabolites and hence hinder the possibility of inhibition. Furthermore, the diverse nature of natural gas presents an opportunity to synthesize a variety of PHA copolymers with methane as primary feedstock. The nature and the percentage of the co-polymer composition would reflect the percentage of higher alkanes present in the natural gas. Adjusting the alkane fraction could lead to a modification or diversity in the resulting co-polymer conferring numerous useful properties such as flexibility, toughness and impact resistance. A recent invention has been able to capture a similar concept [37]. This study used mixed culture enrichments dominated by a methanotrophic population. The researchers demonstrated that the addition of soluble volatile fatty acid (VFA) co-substrates (like valerate) during polymer synthesis results in the production of a copolymer, poly(3-hydroxybutrate-co-3-hydroxyvalerate) (PHBV). Methane gas was the primary growth substrate [37].

 

With the recent drastic drop in natural gas prices compared to other food crops like corn (Fig. 2), using natural gasas a feedstock will address the expensive production of PHAs by providing a cost advantage through large-scale centralized production especially with current delivery infrastructure being ubiquitous. Additionally, natural gas as a feedstock will create avenues to construct decentralized industrialization systems for PHA production. Therefore, natural gas could become a bridge to the production of renewable PHAs and other high value products.

 

Figure 2: Trend Comparing Corn Prices in ¢/Bushel (units on the left) and Natural Gas Prices in $/MMBTUs (units on the right) Over a 1-year Period. (Retracted from tradingeconomic.com)

Biodegradable Plastics Figure 2

 

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