3. The environmental and technical characteristics of compressed natural gas (CNG)/compressed biogas (CBG) applications

Among the hydrocarbons known as energy carriers, methane is the simplest formulation molecule, with the highest hydrogen-to-carbon ratio. This feature allows it to achieve uniquely clean combustion among internal combustion engines (Table 2).

If the source material of the CNG is biogas, it can save a further 13 kg CO2e/t when supplied with organic fertilisers, by replacing artificial fertilisers [23].

The CO2 emissions of biogas plants throughout their life cycle also depend to a large extent on the composition of organic matter. Fuchsz and Kohlhéb [27] calculate that cogeneration-based animal fertiliser-based colonies in electricity production and in all environmental categories (GHG, eutrophication potential, acidification potential) are less effective than biogas plants which (also) use energy crops. This is equally true for both the period of setting up the plant and the operational period. The CO2 emissions of all three examined types were below the average emissions of electricity currently produced in a natural gas-fuelled power plant. At the same time, Bordelanne et al. [26] explicitly analysed the life cycle of sewage waste, finding that with the fermentation of municipal wastes, energy plants also produce 7–10% less greenhouse gas emissions; thus, the biomethane production of fuel in our test is the most environmentally friendly compared to other biogas raw materials.

Examining the various pollutants, CNG CO and NOx emissions were 50–80% of those for EURO 4 petrol vehicles [26], while CH values were close to the same values.

The technology of diesel buses also significantly influences the environmental impact of CNG/CBM. According to Ryan and Caulfield's [28] estimates, the use of CNG in the case of all diesel particulates used in EURO 2–4 buses was reduced to a minimum of 70% for all pollutants and 100% for SO2 and heavy metal emissions. The GHG emissions of the CNG tested were also 7% better than the CBM. Compared to EURO 5 buses, the emissions of CNG buses were at least 50% more


#### Table 2. GHG savings with CNG.

### The Possible Role of Large-Scale Sewage Plants in Local Transport DOI: http://dx.doi.org/10.5772/intechopen.86699

We need to grow food on even less land, with less water, using less energy, fertilizer and pesticide than we use today for feeding more and more people [21], so the energy use of wastes (e.g. sewage) instead of plants can be taken as one of the most important reserves of land management; with the use of them, the net use of

3. The environmental and technical characteristics of compressed natural gas (CNG)/compressed biogas (CBG) applications

Among the hydrocarbons known as energy carriers, methane is the simplest formulation molecule, with the highest hydrogen-to-carbon ratio. This feature allows it to achieve uniquely clean combustion among internal combustion engines

If the source material of the CNG is biogas, it can save a further 13 kg CO2e/t

The CO2 emissions of biogas plants throughout their life cycle also depend to a large extent on the composition of organic matter. Fuchsz and Kohlhéb [27] calculate that cogeneration-based animal fertiliser-based colonies in electricity production and in all environmental categories (GHG, eutrophication potential, acidification potential) are less effective than biogas plants which (also) use energy crops. This is equally true for both the period of setting up the plant and the operational period. The CO2 emissions of all three examined types were below the average emissions of electricity currently produced in a natural gas-fuelled power plant. At the same time, Bordelanne et al. [26] explicitly analysed the life cycle of sewage waste, finding that with the fermentation of municipal wastes, energy plants also produce 7–10% less greenhouse gas emissions; thus, the biomethane production of fuel in our test is the most environmentally friendly compared to

Examining the various pollutants, CNG CO and NOx emissions were 50–80% of

The technology of diesel buses also significantly influences the environmental impact of CNG/CBM. According to Ryan and Caulfield's [28] estimates, the use of CNG in the case of all diesel particulates used in EURO 2–4 buses was reduced to a minimum of 70% for all pollutants and 100% for SO2 and heavy metal emissions. The GHG emissions of the CNG tested were also 7% better than the CBM. Compared to EURO 5 buses, the emissions of CNG buses were at least 50% more

those for EURO 4 petrol vehicles [26], while CH values were close to the same

Biomethane raw material Maize silage Fertiliser Municipal solid waste

CNG/CBG Petrol Diesel Biodiesel Bioethanol 56\*\*/65\*\*\*/74\*\*\*\* 74\*\*/93\*\*\*\* 73\*\*/93\*\*\*\* 76\*\*\* 71\*\*\*

Compared to diesel 66 96 95 Compared to petrol 70 97 96

Sources: Own construction based on \* [23] \*\* [24], \*\*\* [25], \*\*\*\* [26] (figures refer to life cycles).

when supplied with organic fertilisers, by replacing artificial fertilisers [23].

land declines [22].

Transportation Systems Analysis and Assessment

(Table 2).

other biogas raw materials.

GHG savings over the entire life cycle (%)\*

GHG emissions produced during engine use (kg/GJ)

values.

Table 2.

140

GHG savings with CNG.

favourable for most air pollutants, but the CO and non-methane volatile organic compound (NMVOC) values for diesel were slightly more favourable.

Regarding the emission factors of petrol- and diesel-fuelled cars and buses, it can be stated with great certainty that bus transport is the best option, in almost any of harmful gases with respect to g/pkm (passenger km), assuming that the car is not full of passengers. The emission factors strongly depend on size and utility of the given vehicles. CO and CH4 emission is higher in petrol-fuelled cars, while NOx emission is stronger in diesel-fuelled vehicles including buses (Figure 1). It should be noted that CH4 and CO emission can reach outstanding values regarding motorcycles.

Methane content also affects the emission of pollutants. Lim et al. [31] examined gases composed of 82–98% methane and found that total hydrocarbons (THC), CO, NOx and CO2 emissions increased, while volatile organic compound (VOC) emissions decreased with an increased CH4 content in the fuel.

Because the activation energies of high carbon-numbered hydrocarbons (HCs) (e.g. ethane or propane) are lower than those of low carbon-numbered HCs (methane), the combustion efficiencies of ethane and propane are greater than that of methane. Thus, a fuel with higher methane content has more incomplete combustion and/or poor oxidation of unburned hydrocarbons, resulting in higher emissions of HC [31]. The level of this, according to measurements taken by Subramanian et al. [32], was 12.5%.

Considering biogas and natural gas, NOx emissions were lower for biogas than for the natural gases.

Biomethane used as a biofuel produces emission savings which are 73–82% of the base values (GVA) used for the EU's sustainability legislation. Compared to biofuels

produced from other raw materials, this is very favourable because beet ethanol produces 52%, wheat ethanol 16–69%, maize ethanol 49% and rapeseed biodiesel 38% (Directive 2009/30 EC) [33].

(LPG) is the most widespread, while in the United States, it is compressed natural gas (CNG), and the infrastructure for the latter is perfectly suited to biomethane utilisation. CNG is, in theory, subjected to 200 bar pressure when put into steel or composite tanks. With LNG technology, a higher energy density (55 MJ/kg,

pressure. It can then be filled into well-insulated containers and stored under low pressure. Because of the higher calorific value, LNG is more suitable for longdistance traffic than CNG [36]. If biogas is the raw material, the CBM operation will have a combined efficiency of 15–18%, and LMS (liquified biomethane) an efficiency of 14–17%, but both significantly outperform the overall efficiency of the

Large-scale sewage plants in large cities are suitable for the production of large quantities of biogas, using economically viable biogas upgrading technologies and generally available public transport fleets of a sufficient number of local buses, as well as municipal vehicles. The conditions for the sale of locally produced CNGs do not depend on gas suppliers, they can be very well integrated with local waste management and the local emission reductions occur in the inner city where air

At the same time, it is not clear from the point of view of sewage plants whether it is the production of biomethane or the cogeneration solution—which is currently more important—which is more viable economically and in terms of harmful emissions. Here, it must be taken into account that the consumption of heat and electricity by these plants is significant, and this must be purchased when producing

The economic and environmental approach should take into account not only the substituted energy source but also the cost and emissions of fossil fuels bought because there is no cogeneration (including the electricity and heat needed for

In this section we present the investment and operating costs of CBM produc-

The expected level of the biomethane plant's investment costs is greatly influenced by technology and size. Goulding and Power [34] provide the following equation for the average of the investment costs of biomethane plants operating

<sup>C</sup> <sup>¼</sup> <sup>1066</sup>:2x<sup>0</sup>:<sup>8455</sup>

where C is the investment cost (€/t yearly raw material) and x is the processed

The equation—with a 91% degree of confidence (C = 21080x0.5367)—also shows that, in the case of larger dimensions, from an economies of scale perspective, it is advisable to use biomethane instead of cogeneration technologies. At the same time,

at bigger dimensions cogeneration technology also has economies of scale. According to Patrizio et al. [37], the proportion of available heat and electricity between an operating capacity of 300 and 200 kWe increases from 58 to 70%,

) can be obtained by liquefaction at �161.6°C at atmospheric

24–26 MJ/Nm<sup>3</sup>

biomethane.

liquid fuel systems (4–13%) [7].

pollution is the most serious problem.

self-consumption at sewage plants).

4.1 Investment and operating costs

raw material quantity (t/year).

143

tion and the specific aspects of economic evaluation.

with agricultural raw materials, with a 95.6% probability:

4. Economic characteristics of production and use

The Possible Role of Large-Scale Sewage Plants in Local Transport

DOI: http://dx.doi.org/10.5772/intechopen.86699

According to data from Goulding and Power [34], the biomethane energy yield from grass silage is 2.5–3.3, which corresponds to 67–78 GJ of annual energy surplus per hectare. The figures for wastewater with the same biogas yield are between 3.5–4 and 85–90 GJ/year. These are also well above those of most biofuels.

The specific pollutant emissions per capita of public transport are much lower than those of car transport, and this is especially true if the fuel itself is environmentally friendly. Since many trips are local, the analysis by the Department for Transport (UK) shows that 44% of all CO2 emissions from cars come from journeys of between 5 and 25 miles [35]. Although the pollutant emissions of buses per passenger kilometre are higher than those for trains and trams (Figure 2), their energy consumption is nearly identical and much better than for individual transport [29].

The use of raw biogas in CNG vehicles has been investigated. These tests have shown that raw biogas (not upgraded) can be used as a fuel, if blended with natural gas. In fact, the use of raw biogas can be envisaged in dedicated CNG engines, if new engine technologies (lean CNG combustion) are developed. In such a case, natural gas can be blended with up to 70% volume of non-upgraded biogas.

Tests by Bordelanne et al. have shown that raw (not upgraded) biogas can be used as a fuel, only if blended with natural gas, mainly in CNG engine types (lean CNG combustion). The biogas proportion of natural gas can be a maximum of 70% [26].

When comparing two identical brands and types of waste collection vehicles, Domanovszki [36] concluded that the average noise level measured by microphones located at a distance of 7 m from vehicles is 71 dB in the diesel engine and 66 dB in the gas engine version, which is about three times the noise load. Gas fuels do not contain anti-knock additives due to their high-octane number.

There are two technologies for compressing methane gas: CNG and LNG. In Hungarian and EU practice, among gas-fired propellants, propane-butane gas

The Possible Role of Large-Scale Sewage Plants in Local Transport DOI: http://dx.doi.org/10.5772/intechopen.86699

produced from other raw materials, this is very favourable because beet ethanol produces 52%, wheat ethanol 16–69%, maize ethanol 49% and rapeseed biodiesel

According to data from Goulding and Power [34], the biomethane energy yield from grass silage is 2.5–3.3, which corresponds to 67–78 GJ of annual energy surplus per hectare. The figures for wastewater with the same biogas yield are between 3.5–4 and 85–90 GJ/year. These are also well above those of most biofuels.

The specific pollutant emissions per capita of public transport are much lower than those of car transport, and this is especially true if the fuel itself is environmentally friendly. Since many trips are local, the analysis by the Department for Transport (UK) shows that 44% of all CO2 emissions from cars come from journeys of between 5 and 25 miles [35]. Although the pollutant emissions of buses per passenger kilometre are higher than those for trains and trams (Figure 2), their energy consumption is nearly identical and much better than for individual

The use of raw biogas in CNG vehicles has been investigated. These tests have shown that raw biogas (not upgraded) can be used as a fuel, if blended with natural gas. In fact, the use of raw biogas can be envisaged in dedicated CNG engines, if new engine technologies (lean CNG combustion) are developed. In such a case, natural gas can be blended with up to 70% volume of non-upgraded biogas.

Tests by Bordelanne et al. have shown that raw (not upgraded) biogas can be

When comparing two identical brands and types of waste collection vehicles, Domanovszki [36] concluded that the average noise level measured by microphones located at a distance of 7 m from vehicles is 71 dB in the diesel engine and 66 dB in the gas engine version, which is about three times the noise load. Gas fuels do not

There are two technologies for compressing methane gas: CNG and LNG. In Hungarian and EU practice, among gas-fired propellants, propane-butane gas

Heating value and CO2 emission of some transport modes per pkm. Source: Own construction based on [30].

used as a fuel, only if blended with natural gas, mainly in CNG engine types (lean CNG combustion). The biogas proportion of natural gas can be a maximum of

contain anti-knock additives due to their high-octane number.

38% (Directive 2009/30 EC) [33].

Transportation Systems Analysis and Assessment

transport [29].

70% [26].

Figure 2.

142

(LPG) is the most widespread, while in the United States, it is compressed natural gas (CNG), and the infrastructure for the latter is perfectly suited to biomethane utilisation. CNG is, in theory, subjected to 200 bar pressure when put into steel or composite tanks. With LNG technology, a higher energy density (55 MJ/kg, 24–26 MJ/Nm<sup>3</sup> ) can be obtained by liquefaction at �161.6°C at atmospheric pressure. It can then be filled into well-insulated containers and stored under low pressure. Because of the higher calorific value, LNG is more suitable for longdistance traffic than CNG [36]. If biogas is the raw material, the CBM operation will have a combined efficiency of 15–18%, and LMS (liquified biomethane) an efficiency of 14–17%, but both significantly outperform the overall efficiency of the liquid fuel systems (4–13%) [7].
