Performance and emission characteristics of biogas used in diesel engine operation

Highlights

• Biogas is an environmentally friendly biofuel for diesel engines.

• Results of diesel engine tests when fuelling with biogas are presented.

• Engine and environmental characteristics depends on carbon dioxide content in biogas.

• Using biogas in a diesel engine requires certain operational modifications.

Abstract

The objective of this study it to evaluate the impact of the carbon dioxide concentration in biogas on the operating characteristics and exhaust gas emissions of a diesel engine running on a mixture of biogas and mineral diesel fuel. The tests were carried out in two stages. In the first stage, the impact of different biogas compositions and the exhaust gas recirculation system (EGR) on the engine parameters was determined.

Lower pollutant levels were measured in the studies without the EGR system, except for the nitrogen oxides NO_x levels. The NO_x concentration decrease was directly proportional to the concentration of methane in the common fuel mixture.

In the second stage, the gas with the highest methane content was used to determine the impact of the start of injection timing on the engine operating parameters. As the methane content in the common fuel mixture increased, the start of injection timing had to be progressively advanced to increase the thermal efficiency and to lower the fuel consumption, the CO and HC concentrations and the smokiness of the exhaust; however, advancing the start of injection timing increased NO_x pollution.

Introduction

Mitigating the rapid increase in atmospheric pollution by greenhouse gases is an important problem. Pollution from transport accounts for one fifth of all of the emitted hazardous compounds that contribute to the greenhouse effect. The new European Commission Directive 2009/28/EC was approved, aiming to replace 10% of all fuels consumed in transport with biofuels by 2020. Biogas is a potential renewable energy source if properly prepared and adapted for use in transport vehicles [1]. Raw materials (e.g., agricultural, urban infrastructure and industrial waste; green biomass) exhibit a sufficiently high potential for the production of biogas but are underutilised [2], [3], [4], [5], [6]. The main components of biogas are methane (CH₄) and carbon dioxide (CO₂). The biogas composition depends on the parameters of the production process and the type and composition of the raw materials [7].

A typical biogas composition is as follows: methane – CH₄ (50–75%), carbon dioxide – CO₂ (25–45%), nitrogen – N₂ (0–10%), hydrogen – H₂ (1–2%), hydrogen sulphide – H₂S (0–0.5%) and oxygen – O₂ (0–2%) [8]. The combustion value of a biogas is directly related to its methane concentration. A biogas containing 60% methane has a calorific value of 21.5 MJ/m³, whereas a biogas with a 96% methane concentration has a calorific value of 35 MJ/m³ [9].

The use of biogas in diesel engines (i.e., for compression ignition) is environmentally beneficial not only because it increases renewable fuel consumption in transport but also because methane is considerably lower in carbon than ordinary diesel fuel; therefore, using biogas can reduce pollutant levels in exhaust gases, including emissions of solid particles and nitrogen oxides [10]. However, biogas cannot be used separately from diesel, a dual-fuel supply system when biogas–biodiesel and biogas–mineral diesel are used as fuel is required [11].

In some countries, biogas is used in urban transport. In Linkoping (Sweden), biogas with methane concentrations of at least 97% is used in 64 buses and heavy and light motor vehicles. In Sweden, the use of biogas since 2002 in urban transport alone has reduced carbon dioxide emissions by 9000 t/y [12].

There is no universal international standard for the quality of the biogas used in the transport sector. Some countries (Germany, France, the Netherlands, Sweden, Austria and Switzerland) have approved national standards for biogas used in transport (see Table 1) [10].

In most countries, the main biogas quality indicator is the methane concentration, which should be at least 96%, and carbon dioxide, which should not exceed 4%. The Wobbe index is another indicator for fuel that depends on higher heating value and can be used as a basis for comparison between different gases. The concentrations of sulphur hydrogen and water vapour in gases are also restricted because reactions of these substances can corrode engines [13]. Gases that comply with the requirements of the standards can be used in diesel engines; however, biogas obtained by anaerobic processing contains excessively high concentrations of sulphur hydrogen and water vapour, and thus does not comply with the requirements of the standards. Sorption technologies can be used to remove biogas from these compounds [14], [15]. Sorption processes require additional consumption of energy and materials. The carbon dioxide concentration in biogas is higher than the corresponding sulphur hydrogen and water vapour concentrations, such that removing carbon dioxide from the biogas accounts for most of the energy and materials consumption. Although the negative impact of sulphur hydrogen and water vapour on corrosiveness are clearly understood, much less is known about the impact of high carbon dioxide concentrations on engine operation and the pollutant concentrations in the exhaust gas. It is absolutely clear that increasing the carbon dioxide concentration decreases the fuel combustion value, thereby increasing fuel consumption; however, the impact of the carbon dioxide concentration on the engine operation remains unclear. If this impact is low, it may be reasonable and economically efficient to only partially remove carbon dioxide or not remove carbon dioxide at all from biogas for use in diesel engines.

The objective of our study was to evaluate the impact of the carbon dioxide concentration in biogas on the operating characteristics and exhaust gas emissions of a diesel engine running on a mixture of biogas and mineral diesel fuel.