Notes and comments
Examining the benefits of using bio-CNG in urban bus operations

https://doi.org/10.1016/j.trd.2010.04.002Get rights and content

Abstract

Public service fleets offer an attractive option for introducing new renewable fuels on a large scale, which allow for the reduction of both greenhouse gas emissions and exhaust air pollutants. This study examines the use of biomethane (bio-CNG) and compressed natural gas for part of the bus fleet in Dublin, Ireland. The emissions produced from the 2008 fleet based at one of the city’s seven bus depots are compared to use of new diesel and bio-CNG buses. The optimum feedstock for bio-CNG production in Ireland was then investigated, as well as the quantity of feedstock needed to produce the required bio-CNG to fuel the bus fleet examined. As expected the results showed a substantial decrease in all exhaust emissions from the use of bio-CNG buses compared the 2008 fleet. Grass silage was chosen as the optimum feedstock for production of bio-CNG in Ireland.

Introduction

In 2004, the Irish government launched a pilot scheme for excise relief on biofuels (Sustainable Energy Ireland, 2005). The aim was to stimulate the initial development of a biofuel market and concerned the production of pure plant oil, biodiesel and bioethanol in approved pilot projects. The scheme was subject to a maximum production capacity of 8 million litres per annum of biofuel, and was valued at €3 million. More recently, the Department of Transport has instructed public transport operators to move to a 5% biodiesel blend in the current fleet, and this is expected to be implemented in 2009 (Department of Communications, Energy and Natural Resources, 2008). The Department has also instructed public transport operators to ensure that all new buses, as part of future fleet replacement, can operate on a 30% biodiesel blend. This research examined the potential benefits of switching 81 buses in the Dublin bus fleet to alternative fuels. Currently, Dublin Bus operates a fleet of 1008 buses (Dublin Bus, 2009). The emission model COPERT 4 was used to estimate the reductions in green house gas emissions and air pollutants from introducing alternative fuels to the Dublin Bus fleet. The Euro standards referred to relate to a rating given to buses to measure how efficient buses are in terms of emissions. These standards are set by the EU and classify vehicles in accordance with their emissions.

Several studies have explored the benefits of bio-CNG. Karlström (2005) completed a work that expressed the local air pollution benefits in monetary terms from hydrogen fuel cell buses, CNG buses and Euro V diesel buses. Euro V standard is the current mandatory limit for new buses purchased in the European Union. This study presented a quantitative assessment of the local environmental benefits of using each type of bus along a central bus route in Götenburg. Euro II diesel buses were used as a reference scenario. The results show that the present local environmental benefits for a hydrogen fuel cell bus are much smaller than the annualised purchase cost, although the local monetary benefits would be meaningful to consider if compared with the incremental costs of a mass-produced fuel cell bus. Schimek (1998) examined the bus fleets in New York City, Los Angeles, Chicago and Boston to ascertain if moving to alternative fuels could reduce PM and NOx emissions. The results of this study suggest that increasing the turnover of diesel fleets could produce more rapid emissions reductions. The author suggests that the little difference in the cost of CNG relative to diesel, and the shorter range of CNG vehicles, explains why CNG fuelled buses have not been adopted, on a larger scale. Clark et al. (2006) examined the use of CNG and hybrid electric buses in Mexico City. The results of this study suggest that while hybrid electric buses produced significant fuel economy, while CNG buses had the lowest PM emissions.

Section snippets

Methodology

To calculate the quantity of emissions produced by the fleet examined, it was necessary to obtain the necessary input data for the COPERT model. Table 1 details the bus fleet modelled, including Euro level, the number of kilometres travelled, and the number of kilometres travelled per-bus. An average speed of 13 km/h was assumed. Four different models are estimated. The first measures the status quo; the second model assumes that the current fleet is replaced with Euro V buses. The third model

Emissions modelling

Four emissions models were estimated in this study. Table 2 presents the results of the estimated emissions from each of the four models estimated over a 1 year period. The results show that each of the alternatives examined would realise a significant reduction in air pollutants. The findings show that the bus operator could realise a 64% reduction in CO2 emissions from changing the fleet to EEV buses fuelled with bio-CNG. Models 3 and 4 show a 71% reduction in CO emissions compared to the

Conclusions

The results show a major decrease in all pollutants from the use of CNG EEV buses compared to the 2008 fleet for Euro II, III and IV buses. There was a minimum reduction of 70% in emissions of all air pollutants, and a 100% reduction in SO2 and heavy metal emissions due to the fuel used. There was a decrease of 63% in the emission of greenhouse gases when bio-CNG was used instead of CNG. CNG showed a 7% reduction in CO2e emissions. When the use of CNG and bio-CNG was compared to a new fleet of

Acknowledgements

The authors would like to thank Dublin Bus for providing the vehicle activity data. The authors would like to thank the referees sincerely for their helpful suggestions to improve the original manuscript.

References (13)

There are more references available in the full text version of this article.

Cited by (0)

View full text