The main benefit of using biofuels is that energy derived from recently harvested plants does not release carbon stored underground for millions of years, unlike fossil diesel. Biofuels are in effect ‘neutral’ and cannot add to the total CO2 present in the atmosphere because chemically they only contain carbon already around as the plant grew.
Not only do biofuels offer a benefit because they do not release fossil carbon, but they generally produce lower amounts of regulated emissions from the exhaust. The exception would be mono-nitrogen oxides or NOx emissions. Nitrogen monoxide (NO) and nitrogen dioxide (NO2) are produced in greater quantities because biofuels tend to combust at higher temperatures than fossil diesel. Although biofuels would tend to have a lower total energy value than fossil diesel, they do contain proteins and oxygen which increase combustion temperatures.
The NOx emissions from an engine are not ‘greenhouse gases’ but they will combine with other pollutants in the atmosphere and react with sunlight to form ozone and what is called ‘photochemical smog’. NOx in this context should not be confused with nitrous oxide (N2O), which is a potent greenhouse gas.
Emissions of NOx from a Cantium CHP plant are controlled by a combination of modifications to the combustion process and exhaust gas treatments.
A very effective way to reduce NOx is to operate the engine on a mixture of liquid biofuel and biogagses, such as the output from an Anaerobic Digestion plant.
Where required by the customer Cantium can also supply CHP plant with the following adaptions:
Exhaust Gas Recirculation (EGR)
Biofuels produce elevated levels of NOx partly because the fuel contains oxygen atoms. Fossil diesel is usually a blend of complex hydrocarbons with virtually no oxygen. NOx is formed by combining oxygen and nitrogen at high temperatures and biofuels simply have more oxygen available.
The simplest way to control NOx therefore is to reduce the oxygen available for combustion by recirculating some of the exhaust gases back to the air intake on the engine. As the exhaust gas has already passed through the engine its oxygen content is depleted. By careful control of the amount returned it is possible to manage the oxygen available to create NOx. Recirculating some of the exhaust gas also reduces soot particles and unburnt fuel emissions by 're-burning' the exhaust.
Atomised Water Injection (AWI)
We use our own purpose built heat exchanger system to capture energy from the hot exhaust gases. Incorporated into the design is a system we call Atomised Water Injection. As the name suggests, this uses water to extract more heat from the exhaust (a bit like a condensing boiler) with the additional benefit that some NOx is reacted and removed from the gas stream. This system is also very helpful in reducing particle emissions, especially on start-up.
Selective Catalytic Reduction (SGR)
Although not fitted as standard to a Cantium CHP plant an SGR system is available to reduce NOx emissions even further. SGR works by injecting an aqueous solution of ammonia (a product called AdBlue®) into the exhaust stream prior to a purpose designed catalyst. The catalyst converts NOx into water and nitrogen. This type of system is now becoming common on trucks and buses, especially where these are required to operate in urban ‘low emission zones’. There is an additional cost for the installation and ongoing maintenance of this system but this is not excessive. | 
