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Environmental Impacts of RNG



evnext-logo-v-smallThis post is a part of BioBiz’s Bio-CNG Perspectives.

BioBiza division of EAI, is a leading market intelligence & strategic consulting firm for the Indian bio-based sectors.


This blog post uses the terms bio-CNG and renewable natural gas (RNG) interchangeably.

Bio-CNG or bio-compressed natural gas, also known as sustainable natural gas or biomethane, is a biogas which has been upgraded to a quality similar to fossil natural gas and having a methane concentration of 90% or greater. As the gas is derived from natural and renewable sources, it is also termed renewable natural gas (RNG).

Introduction

Renewable natural gas is an emerging fuel globally. While it is well known that it is obtained from various waste sources and thus is a renewable, sustainable fuel, it is also important to understand its life cycle emissions i.e. the overall impact RNG has on the environment. A thorough understanding of the advantage of RNG in all aspects would be an avenue for various industry stakeholders to promote and encourage the use of RNG.

This blog post highlights the net carbon emissions of RNG when compared to other conventional fossil fuels and emerging sustainable fuels including electric vehicles. It is to be noted that net emission potential represents a normalized metric for amount of CO2 emitted by the fuel along the entire product lifecycle and hence for normalization, the net emission potential for gasoline is taken as 1.

Environmental benefits of RNG

Climate change has always been a threat globally for more than a decade. According to Eurostat (2016), the transport sector, responsible for 51% of the total oil consumption, contributes significantly to GHG emissions. As per the UN World Meteorological Organization, global temperatures will rise by 3.5 degrees celsius in this century if urgent measures are not adopted. 

Nations have been focusing on exploring renewable energy sources to reduce the transport sectors’s environmental impact. 

One mid-term solution to reduce the carbon emissions from the transport sector could be renewable natural gas.

A recent study by IFPEN (the French Institute of Petroleum) has revealed that the biogas sector can contribute to decarbonising transportation. The study collated data about the complete life cycle of compressed natural gas (CNG), bio-methane, gasoline, diesel and electric vehicles, and it concluded that bio-methane is the best option for maintaining air quality. The study recommends using a mix of bio-methane and CNG till 2030 to power vehicles, so that they have a climate impact equivalent to that of an electric car running on sustainable energy, as a mid-term solution.

Biogas as vehicle fuel is typically assumed to have >50% GHG reduction compared to fossil fuels. Its use reduces the fossil fuel consumption. But its use can reduce GHG emissions not only by substituting fossil fuels, but also by avoiding methane emissions, for example from manure storage and waste disposal.

Estimates of fuel emissions reduction refer to a life cycle assessment. The assessment takes into account the fuel production (from feedstock supply to upgrading, including intermediate transportation for biomethane) and distribution through the gas grid. To identify the biomethane potential with regard to GHG emissions reduction, GHG emissions are compared between biomethane and conventional fossil fuels. Overall, the main greenhouse gases are methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O). Global GHG emissions results are given in grams of carbon dioxide equivalent.

The following graphical representation compares the net carbon emission potential of RNG with other conventional fossil fuels and emerging sustainable solutions like electric vehicles.Net emission reduction by RNGResults show that biomethane offers significant reduction of GHG emissions. Current GHG emissions with gasoline and diesel are estimated at 164-156 gCO2-eq./km, respectively. Natural gas used as an alternative fuel emits 124 gCO2-eq./km, equivalent to a 24% reduction compared to gasoline. Biomethane can improve the results significantly. GHG emissions of biomethane use depend on the feedstock type. With biomethane produced from crops such as maize, GHG emissions are equal to 66 gCO2-eq./km, over half of which is represented by cultivation and harvesting (use of fertiliser) of raw materials and 28% due to upgrading. GHG emissions can be reduced by roughly 60% compared to gasoline.

Manure reduces GHG emissions by about 80% compared to gasoline. Biomethane production from municipal waste and its use as vehicle fuel give GHG savings of 73% compared to a standard comparator for fossil fuels of 83.8 gCO2-eq./MJ, following the Renewable Energy Directive (RED).

This is equal to a reduction of GHG emissions per driven km of roughly 70% in comparison to a gasoline driven passenger car. To reduce even more GHG emissions from biomethane production, several solutions can be adopted:

  • Reuse digestate as fertiliser
  • Use a nitrogen inhibitor to slow down the nitrification producing N2O as GHG, which is typically released during digestate storage and spreading as fertiliser
  • Reduce the transportation distance of feedstock as much as possible
  • Install measuring equipment for methane losses in the anaerobic digestion system

Aside from GHG emission savings, additional factors must be considered when evaluating the sustainability of biomethane production and its use as vehicle fuel. Environmental, social and economic factors are all relevant. 

It could thus be inferred that RNG has the potential to be a replacement for all conventional fossil fuels and significantly helps in climate change mitigation through its low net carbon emission potential. 


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Categories: Bio-CNG /renewable natural gas, Environmental impact of RNG

Author - Narasimhan Santhanam


Thank you for reading this post – hope it was helpful. You can connect with me at narsi@eai.in. My detailed profile here and my LinkedIn profile here

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