Alternative fuels reduce harmful emissions during combustion

• Harmful emissions from combustion engines are an ongoing problem associated with fossil fuels.
• Alternative fuels from renewable sources such as biomass can be part of the solution.
• Dr Ruoyang Yuan and Dr Abdallah Abu Saleh at the University of Sheffield, UK, have been assessing emissions by alternative fuels.
• The team measured the production of oxides and soot from dimethyl ether and isopentanol, with promising results.

Transportation is a key part of our life, either directly (travelling for various purposes) or indirectly (buying products transported from elsewhere). Fossil fuels are still the most widely applied fuel for transportation purposes, given that the vast majority of engines are traditional combustion engines. The harmful emissions from these engines are well researched, and their direct or indirect contributions to climate change are well documented.

Besides greenhouse gas (GHG) emissions, particulate matter emitted from combustion engines can also have harmful effects on our health. Soot and gas emissions are associated with lung diseases, asthma, cancer, and infant morbidity. According to the UK’s Department for Environment, Food & Rural Affairs, the biggest environmental danger to public health is poor air quality. This has intensified the need for alternative, renewable fuels that can be used for transportation purposes. In August 2022, the UK government approved a £37 million grant to support research in this area.

The quest for alternative fuels

Bioethanol (produced by fermentation of starchy corps like soybeans, corn, or other biomass) and biodiesel (produced by vegetable oils or cooking oils) are currently available on the fuel market in blends. Other alternatives to gasoline and diesel can be derived from fossil fuels themselves.

This can be achieved from petroleum fractions previously unexploited for this purpose or via processes that have shown potential to generate products suitable for fuel use in combustion engines. Alternative fuels produced by fossil fuel streams are liquefied petroleum gas, naphtha, and dimethyl ether. Finally, alternative fuels can also be produced through electrolysis (use of electricity to break down chemicals). The electrolysis of water, for example, results in hydrogen, which is an emerging fuel with high potential.

According to the UK’s Department for Environment, Food & Rural Affairs, the biggest environmental danger to public health is poor air quality.

An important point of discussion and research is the need to characterise these alternative fuels and identify their suitability in traditional combustion engines. Furthermore, it is imperative to assess the effect of such fuels on the environment and public health, and compare it to traditional fuels. Towards these goals, Dr Ruoyang Yuan and Dr Abdallah Abu Saleh at the University of Sheffield in the UK have been trying to understand the potential of alternative fuels and their suitability as replacements for fossil fuels.

Alternative fuel candidates

When deciding on suitable or promising alternative fuels, a crucial factor is the performance of the fuel during combustion and how close it is to the ‘golden standard’ set by traditional fuels. The performance is dictated by the energy produced per gallon of fuel, which in turn is affected by the composition of the fuel. Yuan and Abu Saleh have been researching three specific alternative fuels, dimethyl ether, ethanol, and isopentanol, to assess their harmful emissions and soot production compared to that of diesel for compression ignition engines, gasoline for spark ignition engines, and kerosene in a modelled aero-engine combustor. This article will discuss the potential of dimethyl ether and isopentanol.

Dimethyl ether is an organic compound that can be produced directly from fossil fuels but also from renewable sources such as biomass. One of its main advantages compared to other alternative fuels is its relatively high cetane number, which indicates its ability to ignite – a property important for combustion, plus it can be produced at scale. Isopentanol also has a higher cetane number compared to other alternative fuels and is considered a second-generation biofuel in the sense that the crops used for its production, such as grasses, municipal waste, and other biomass rich in cellulose, are not suitable for human consumption. There have also been indications of lower risk for cancer generation from combustion emissions of current fuels blended with isopentanol.

Measuring harmful emissions

Yuan and Abu Saleh aimed to optimise the set-up of existing methods to assess the harmful emissions of dimethyl ether and isopentanol in pure form or when blended with other fuels. They focused on the emission of nitric oxides, harmful acidic emissions that can lead to acid rain, and soot. For the measurement of soot production, the chosen methods were two-colour laser-induced incandescence (LII) and planar two-colour soot pyrometry, whereas for nitric oxide emissions, planar laser-induced fluorescence was used.

Yuan and Saleh aimed to optimise the set-up of existing methods to assess the harmful emissions of dimethyl ether and isopentanol in pure form or when blended with other fuels.

During LII, a laser beam hits the combustion gas that contains soot and the soot particles are heated to the temperature at which carbon evaporates. Upon evaporation, soot emits electromagnetic radiation (incandescence) which is picked up by a suitable camera. Through complex mathematics, incandescence can be translated to soot particle density and size. During soot pyrometry, the aim is to identify the temperature of emitted soot particles, which is done via quantification of the light emitted by the particles against certain wavelengths. This light is then ‘translated’ to temperature. In the case of planar laser-induced fluorescence, the flame was hit by an energy source (laser) to excite nitric oxides, and images were captured by a suitable camera, which were then processed and analysed to derive the intensity of nitric oxide emissions.

Yuan and Abu Saleh’s work applying these techniques on different fuel emissions showed potential for the more robust characterisation of the emissions, either gaseous or black carbon. This is in line with the newly introduced requirements for emission certification testing. Their results could be used towards the improvement of calibration protocols, in combination with additional emission characterisation methods.

Are dimethyl ether and isopentanol better than fossil fuels?

Are alternative fuels better than fossil fuels? This depends entirely on which aspect we focus on. In terms of renewability, dimethyl ether and isopentanol are better options than fossil fuels, but the question of economic feasibility for production at scale remains. In terms of combustion performance, the answer depends on the conditions used, the optimisation of the combustion process, and the blend ratio with other fuels. Currently, alternative fuels show potential with performances close to fossil fuels, but there is hope that the performance will increase if engines are optimised towards the use of alternative fuel.

Yuan and Abu Saleh’s work showed that using alternative fuels could lead to reduced emissions. Blending n-heptane (one of gasoline’s main components) with isopentanol can lead to reduced emissions and reduced soot production. Mixing dimethyl ether with other fuels showed reduced emissions at higher blend ratios. Overall, there is certainly potential in the development and use of alternative fuels. However, more research is necessary towards the optimisation of blends, but also towards the optimisation of methods able to provide exact information on the harmful emissions.