Massimiliano Materazzi and Richard Taylor discuss the promise of a bio-substitute for natural gas
WHAT you do now will come back to you tomorrow. Imagine if this applied to the garbage you produce every day, wouldn’t that be… great? OK, let’s rephrase this. Imagine it will return to your home in the form of greener gas, or even fuel your local bus. Doesn’t that sound much better?
That’s what advanced conversion technologies (ACTs) do; they promise to transform the entire carbon content of household waste (including contaminated and unrecoverable plastics) into bio-substitute natural gas (BioSNG), a green and sustainable copy of natural gas. Once it has been produced close to the source of the waste feedstock, it can be readily transported, using the existing gas network, directly to consumers.
When teaching thermochemical processes to engineering students at UCL, the questions that are commonly asked are: “Why produce gas from waste? Why not incinerate it to generate electricity? Isn’t electricity better than gas”? While these questions are legitimate, the answer in the UK context is a no brainer – producing gas is preferable.
There are a number of reasons why incineration is one of the least preferred measures of waste treatment, not least because it is inefficient. Typical waste incinerators are 5–10% less efficient than fossil fuel power stations, so need to be very big to be competitive. This means hundreds of trucks moving across the country every day to feed the hungry giant (which, by the way also produces its own problematic waste, eg ashes, etc).
Also, gas currently dominates UK heat supply, with 83% of the UK’s buildings heated by gas1. Completely replacing this with electric alternatives would require a huge investment in infrastructure and in consumer appliances. In transport, batteries will play a key role, but there are specific areas, such as heavy goods transport, for which the use of compressed natural gas (CNG) would offer significant carbon and emissions benefits.
Our local waste (which happens to be the UK’s largest source of biomass) is an abundant, low-cost, native feedstock, which we could use to partially satisfy demand (for gas) while decarbonising the energy sector at the same time
However, we are becoming increasingly reliant on imports for gas, raising concerns over environmental impact, security of supply, and exposure to the price volatility of world markets. Our local waste (which happens to be the UK’s largest source of biomass, https://bit.ly/2u1EnoE) is an abundant, low-cost, native feedstock, which we could use to partially satisfy this demand while decarbonising the energy sector at the same time. Furthermore, the advantage of dealing effectively with waste disposal, which in this country still relies on landfill and export to Europe (Brexit permitting), is far from being modest! Renewable gas in the form of biomethane from anaerobic digestion (AD) is playing its part, with around 50 plants injecting into the UK gas network. However, the types of waste that can be treated in this way are limited to food, agricultural, and sewage. Recent analysis has indicated a maximum potential contribution of 40 TWh/y of renewable gas from AD using current technologies. If renewable gas was made from a wider variety of organic waste materials (such as residual ‘black bag’ household and commercial waste), availability would increase by around 100 TWh/y, ie 30% of current domestic gas demand1. The BioSNG project and partnership launched in 2012 between UCL, Advanced Plasma Power (APP), Progressive Energy, and Cadent Gas (collectively known as the GoGreenGas consortium) is demonstrating and commercialising the technology to make this possible.
The BioSNG process is similar to the well-established coal-to-gas process for production of synthetic methane (or other fuels and chemicals) from fossil sources, with the main difference being that the feedstock in this case is half renewable (the other half is made of low-grade plastic which cannot be recycled).
The first step in the process is the gasification of the solid feedstock to produce a synthetic gas (or syngas) that is rich in carbon monoxide and hydrogen, the building blocks of any synthetic fuel or chemical. The syngas is cooled and cleaned through a number of processes to remove impurities such as heavy metals, sulfur, ammonia and chlorides. The chemical reaction to produce methane requires a relatively high H2:CO ratio, so the syngas is mixed with steam and passed through an iron-based catalyst to increase the hydrogen content utilising the water gas shift reaction:
H2O + CO ↔ CO2 + H2 ∆H0 (289 K)= -41.1 kJ/mol
The product gas then enters a succession of methanation reactors with nickel-based catalysts, where the syngas is transformed to valuable methane:
3H2 + CO ↔ CH4 + H2O ∆H0 (289 K)= -206.1 kJ/mol
The methanation reaction is highly exothermic, so the reactor design, quantity of catalyst and the gas flowrates have to be carefully selected to ensure a controlled reaction. The gas exiting the reactors contains significant quantities of water vapour and carbon dioxide, which have to be removed to produce a product gas which is acceptable to current gas grid regulations. Pressure swing adsorption (PSA) or membranes can be used to produce high purity CO2, suitable for industrial and food applications or for carbon sequestration, thereby further reducing GHG emissions.