Description

1. Overview

The global energy exhaustion has been a serious issue all over the world. The traditional energy resources are running out of supply. Until now, the world has made many efforts on developing new energy sources and technologies. But many of them are facing serious problems, including expensive raw materials and inability of mass-production. So, what we are trying to do is to find a new way to produce an efficient biofuel by transforming common waste like shrimp shells into butyl butyrate. That will not only lessen the energy exhaustion but also help reduce pollution from kitchen waste.

2. Current challenges in energy

The World Energy Outlook (WEO) 2007 claims that energy generated from fossil fuels will remain the major source and is still expected to meet about 84% of energy demand in 2030(IEA, 2007). Fossil fuels, also known as ore fuels, are hydrocarbons or derivatives thereof, including natural resources such as coal, oil, and natural gas. At present, about 80% of the world’s primary energy is derived from fossil fuels with oil accounting for 32.8%, coal for 27.2% and natural gas for 20.9% (IEA, 2011). However, the reserves of fossil fuels are not unlimited and the exploiting of fossil fuels may cause serious consequences like climate change and air pollution. According to a report from IEA, “A failure to accelerate clean energy transitions would continue to leave people exposed to air pollution. Today, 90% of the world’s population breathe polluted air, leading to over 5 million premature deaths every year.”(IEA, 2021)

The potential damage of fossil fuels includes:

• 1)Energy depletion
• 2)Climate change and global warming
• 3)Human health problems
• 4)Environmental damage
• 5)Energy crisis caused by the dependence on few countries with huge energy resources

Figure 1 Gas emission from factories

3. Biofuel

3.1. Situation of biofuel

Many attempts have been made towards new energy resources. Biofuel is produced from plants or from agricultural, domestic or industrial biowaste. It is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. In countries like United States, Brazil and so on, biofuel is already being used as vehicle engine fuel and these trends may continue in coming years mainly because of sufficient availability of edible oils (soya oil, sunflower oil, rapeseed oil, starch and maize) (Joshi,2017). Biofuel is a kind of new and clean energy, and its application will help solve the problem of energy depletion and environmental pollution.

Figure 2 Types and generation of biofuels (Muhammad Rizwan Javed, Muhammad Junaid Bilal, Muhammad Umer Farooq Ashraf, Aamir Waqar, Muhammad Aamer Mehmood, Maida Saeed and Naima Nashat, via Wikimedia Commons)

3.2. Butyl butyrate as a biofuel

Butyl butyrate, or butyl butanoate, is an ester formed by the condensation of butyric acid and n-butanol. It is a clear, colorless liquid that is insoluble in water, but miscible with ethanol and diethyl ether.

Figure 3 Butyl butyrate 3D ball

Butyl butyrate can work as a promising biofuel and can be used in blend with other fuels such as kerosene. It has a combustion heat of -4 839.6 kJ/mol, similar to traditional gasoline. It also has an octane number of 97.3, exceeding the minimum value of 95 in EN228. Its melting point is -91℃, far lower than kerosene (-47℃). Moreover, the flash point, viscosity and low-temperature features of butyl butyrate meet aviation standard and are comparable to Jet A which is a fuel used globally in civil aviation (Kumar, 2020)

However, the production method at present is a Fischer esterification reaction which requires high temperature of 200-250℃ and concentrated sulfuric acid as catalyst. This is highly energy-consuming, and produces dangerous chemical waste. As a result, butyl butyrate has a market price of $21.50/kg.

Figure 4 True colorflame images of fuel samples (Kumar, 2018)

Figure 5 Fischer Esterification mechanism （V8rik at English Wikipedia, CC BY-SA 3.0, via Wikimedia Commons）

4. Current problems of kitchen waste

4.1. Massive quantities

Kitchen waste is rich in water and organic matter, which makes it easy to rot and produce a stench. With the growth of urban population and the rapid development of economy, the living consumption level of urban and rural residents has been greatly improved, which directly leads to a sharp increase of the output of household garbage at an annual rate of 8%-9%. According to garbage statistics released by the Ministry of Housing and Urban-Rural Development, China produces more than 400 million tons of domestic waste every year (Gao,2021). As an important part of urban solid waste, the proportion of kitchen waste is also increasing year by year. At present, the proportion of kitchen waste in solid waste in China is about 40%~60%, mainly composed of family daily leftovers, melon skins and fruit shells (Gao,2021)

Figure 6 Kitchen waste, shrimp and crab shells (Taz, EHRENBERG Kommunikation, Traci L. Taylor, via Wikimedia Commons)

4.2. Disposal challenges

At present, most of China's kitchen waste is treated by incineration for power generation and sanitary landfill. However, there can be safety and health concerns. For the burning solution, if the kitchen waste is not adequately burned, or contains too many impurities, all harmful gases such as dioxins will be emitted in the incineration process, causing a huge impact on the environment.

Figure 7 Incineration of kitchen waste (Charles Steinhacker, Public domain, via Wikimedia Commons)

If the treatment mode of sanitary landfill is adopted on a large scale, there are high requirements for the landfill process and treatment level. In recent years, due to the annual increase of kitchen waste, the odor concentration of landfills across the country has exceeded the standard. Due to the diversified eating habits in China, kitchen waste composition is complex. Transport, paving, compaction and other landfill pretreatment process will produce pollution; the process of landfill also easily produces various chemical reactions and microbial reactions, resulting in a large amount of flammable and odorous gas and toxic leachate and secondary pollution to the atmosphere, soil and water. In addition, many malodorous substances can also harm people's health.

Figure 8 Sanitary landfill of garbage (Eric Guinther, CC BY-SA 3.0, via Wikimedia Commons)

For example, ammonia is a colorless gas with a strong pungent smell. Low concentration of ammonia can cause stimulation and corrosion of human upper respiratory tract, while high concentration of ammonia damages the human alveolar capillaries, leading to pulmonary edema. According to statistics, ammonia level in most urban solid waste landfills in China exceeds legal standard by 10.8%, and causes huge harm to civilians’ health (Gao,2021).

5. Clostridium tyrobutyricum as a candidate for manufacturing butyl butyrate

Clostridium tyrobutyricum (C. tyrobutyricum) is a Gram-positive, strictly anaerobic bacteria. C. tyrobutyricum can use glucose, xylose, arabinose and other chemical materials to produce organic acid through a process of fermentation. Because its metabolic flux toward butyryl-CoA and butyric acid tolerance are very high, this strain has the most potential in manufacturing butyric acid as a cell factory. It produces butyrate with higher yield and purity as compared with other bacteria (Bao, 2020). C. tyrobutyricum has the inner machinery for butyryl-CoA synthesis, and with some engineering, it has the potential to produce butanol efficiently as well (Suo, 2018). Butyl butyrate can be synthesized by lipase using butyric acid and butanol as substrates. Therefore, C. tyrobutyricum is a good candidate as a cell factory for the biofuel, butyl butyrate.

Figure 9 Metabolic pathway of Clostridium tyrobutyricum (Suo, 2018)

The metabolic pathway of C. tyrobutyricum in producing these product use nitrogen and carbon sources which are commonly found in kitchen waste. Hence, C. tyrobutyricum can be used as a manufacturing media for kitchen waste recycle into biofuel production.

6. Our solution

What we are doing is developing a novel method to produce a biofuel. We aim to convert trash into energy by an efficient and environmentally friendly approach. Here comes our project: produce butyl butyrate, a biofuel, from shrimp shells and lignocellulose from kitchen waste using engineered C. tyrobutyricum.

In our project, protein in shrimp shells is used as nitrogen source, and lignocellulose is used as carbon source. By editing genes for corresponding enzymes, our engineered C. tyrobutyricum produces butyrate and butanol efficiently. Finally, the butyrate and butanol binds into butyl butyrate in an esterification reaction catalyzed by lipase. Common kitchen trash becomes clean biofuel. Turning waste into wealth, we are pioneers.

Figure 10 Schematic illustration of our project to convert trash into energy by engineered C. tyrobutyricum