Over 2.1 billion tons of Municipal Solid Waste (MSW) is generated globally, every year. That’s enough to fill 822,000 Olympic swimming pools. Sadly, only 16% of this is recycled with the majority of items being sent to landfills having only been used once by consumers. Policymakers face difficulty where to focus efforts within the waste hierarchy. Should they invest in more recycling programs as successfully demonstrated by countries like Sweden? Or, is it time to accept the reality of the present situation and invest in energy recovery methods from the phenomenal amount of landfill waste? Research conducted by the University College of London aimed to focus on innovative waste to energy methods.
Engineering fellows partnered with UK-based company Advanced Plasma Power Ltd to design a chemical plant capable of processing 100,000 tons of MSW every year into energy. The project wanted to move away from energy recovery via waste incineration which is widely seen across countries. Most experts would agree that waste incineration is considered the last and least favorable option within the realm of energy recovery. To improve overall carbon negativity, the research focused on converting waste directly into Bio-SNG (Bio-Synthetic Natural Gas also known as methane). Bio-SNG would then be injected into the Gas Grid to be used by homeowners. Some of the key differences between waste incineration versus waste to Bio-SNG include:
- Ability to produce clean, lucrative fuels as well as electricity.
- Plants produce no emissions to air, there is no stack, no particulate emissions and no dioxins.
- Valuable by-products are produced in the form of captured carbon dioxide and inorganic rock.
Below is a basic schematic of the proposed waste to Bio-SNG plant:
A brief overview of the process:
- Municipal Solid Waste with various types of waste is first passed through a magnetic conductor to separate ferrous metals. The remaining mixture is then shredded and passed through a dryer. Typically 5-10% is rejected at this stage.
- The feed is now classified as Refuse Derived Fuel (RDF) which is then heated at 1380℉to produce crude synthetic gas consisting mainly of hydrogen, carbon monoxide, carbon dioxide and a host of other unwanted outputs such as residual tars, unprocessed chars and ash.
- Using the technology from Advanced Plasma Power, high energetic beams in the form of UV light are passed through the crude synthetic gas. With temperatures operating at 8540℉, those previously unwanted components are broken down into an inorganic plasmarok while organic matter is reformed.
- Purified Syngas passes through two sequential processing units, one to remove Hydrogen Sulphide and the other to remove Hydrogen Chloride.
- Once the gas has been adequately purged of impurities, it enters a water gas shift unit. Carbon monoxide and Water is converted into Carbon Dioxide and Hydrogen to improve the reactant composition for the upcoming methanation step.
- Hydrogen, Carbon Monoxide and Carbon Dioxide pass through a series of 3 adiabatic reactors with the help of a nickel catalyst to produce Methane and Water.
- Finally, water is recycled from the product leaving only Bio-SNG with 87% Methane purity.
Professor Marc-Olivier Coppens, Head of Chemical Engineering at UCL supervised various simulations in the year-long project. Creative freedom was given to assess all the possibilities of a waste to Bio-SNG plant. A few of the considerations included:
- Using a pure biomass feed as opposed to MSW generated from homes.
- Bolting on a steam-electric power station to the methanation process so to exploit the heat generated from the exothermic reaction.
- Recycling heat from the methanation reaction to power the high energy requirements of the plasma power unit.
- Combusting the Bio-SNG yield for electricity generation.
- Optimizing towards Plasmarok, a by-product of the Plasma converter to be used in road paving and pipe bedding.
While evaluating all the possible waste-to-energy innovations improved the overall environmental impact of the plant, favoring the environment directly hampered the economic feasibility. As with all projects, financial profitability is the governing factor. With this in the mind, the simulation delivered the following results:
The total cost of the proposed plant, including all 52 processing units, land, construction and labor was estimated to be $146 million with a Return on Investment after 9 years of operation. The resulting Bio-SNG would be enough to power 15,000 homes every year. It should be mentioned that the figures above represent a somewhat conservative picture as the plant had a maximum design capacity of processing 150,000 tonnes of MSW feed each year.
Results from the simulation were certainly positive. So why isn’t this method of waste to energy recovery commonly seen? Afterall, 90% of the major processing units including the water-gas shift reactor, methanation reactor, hydrogen sulphide absorber and water extractor are already widely used across various industries. The crux lies in the advanced plasma power unit. As the name might suggest, the technology lays on the fringes of commercial viability. So much so, that engineering fellows had to manually insert data from the manufacturer so not to confuse the chemical software. At the core of the unit, temperatures reach over 8540℉, around 4 times hotter than molten lava. At these temperatures, you are pushing the laws of thermodynamics to the absolute maximum. However, after successfully running a pilot plant for over 3,500, the world’s first waste to Bio-SNG plant was given the green light for commissioning in Swindon, U.K. The companies initially involved in the plant were the National Grid, Advanced Plasma Power (APP) and energy development company Progressive Energy. Together, Go Green Fuels was set up to deliver the plant. However, rising construction costs forced the company to go into administration, resulting in the project being suspended for the next 14 months. Since then, the company has raised over $12 million and has resumed construction of the highly innovative plant. Operations are set to begin processing waste at some point in 2020. If and when the facility goes live, it will indeed provide a breath of fresh innovation for the waste to energy sector.