DECENTRALIZED ENERGY SYSTEMS: ENVIRONMENTAL EFFECTS OF COMBINED HEAT AND POWER

SO₂ concentrations, which are likely to increase the removal efficiency of SO₂ in FGDs. Also, the NO_x formation is expected to be lower as NO_x formation is suppressed when combustion occurs in an atmosphere with reduced nitrogen quantities. The energy penalty, however, will result in increased emissions of SO₂ and NO_x from coal production and transport. Of the three studies that are examined this technology, two report a relative decrease of 38 per cent and 80 per cent and one an increase of 40 per cent in AP values.

For IGCCs without CO₂ capture, AP values are reported in the range 0.25-1.5 g SO₂-eqeq/kWh and for IGCCs with pre-combustion CO₂ capture using Selexol or Rectisol, in the range 0.33-1.5 g SO₂-eqeq/kWh. For IGCC with MEA, reported emission are 2 g SO₂-eq/kWh. In all cases, this corresponds to a 32 per cent relative increase in AP. Finally, compared to PCs, the range of NGCCs without CCS is lower, at 0.06-0.56 g SO₂-eqeq/kWh. The sulfur content of natural gas is very low and thus SO₂ emissions are lower for natural gas-fired power plants without CCS.

The implementation of post-combustion CO₂ capture using MEA results in a relative increase of 23-26 per cent.

3.6.3.1.4 Toxicity

In LCA, four different kind of toxicity categories are reported: human toxicity potential (HTP), which refers to the impact of toxic substances on human health in the air, water and soil; freshwater aquatic ecotoxicity potential (FAETP) refers to the impact of toxic substances on aquatic ecosystems; terrestrial ecotoxicity potential (TETP), which is the impact of toxic substances on terrestrial ecosystems and marine aquatic ecotoxicity potential (MAETP), which refers to the impact of toxic substances on marine ecosystems.

Figure 3.22 to Figure 3.25 show the values reported in the literature. Note that for most of the technologies, there are too few studies examining toxicity to allow the drawing of robust conclusions. We thus limit ourselves in this report to discussing PCs with and without CCS. In the case of HTP, the values reported in the literature go from a 30 per cent decrease to 260 per cent increase compared to similar PC plants without CCS. MEA production is reported as the main contributor to human toxicity, primarily because of the ethylene oxide emissions from MEA production. For FAETP, relative increases ranging from 8 to 256 per cent are reported. In this case, the energy penalty and the steel consumed for the production and operation of the CCS system are indicated as the main causes. Additionally, the ethylene emissions from MEA production contribute to the FAETP.

The TETP values shown both decreases (-34 per cent) and increases (42-57 per cent) compared to plants without CCS. As in the case of FAETP, the energy penalty and increased steel consumption are considered the main drivers. The study reporting a relative decrease in FAETP assumes significant removal of trace metals in the CO₂ capture unit. The validity of this assumption is currently under discussion (see 3.6.2.1).

3.6.4 DECENTRALIZED ENERGY SYSTEMS: ENVIRONMENTAL EFFECTS OF COMBINED HEAT AND POWER

There is a rather limited number of LCA studies on fossil fuel-powered CHP systems performed to date (Bauer and Heck, 2009; Fischer et al., 2008; Lund et al., 2010; Pehnt, 2008). The relative CO₂ performance of CHP technologies compared to separate generation of electricity and heat depends on a number of factors such as choice of prime mover technology, heat-to-power ratio (HPR), allocation of emissions to electricity and heat, and the source of grid electricity that will be replaced by CHP plants.

CHP saves direct fuel consumption compared to separate production of heat and power. However, the degree to which GHG emissions are reduced is largely case-specific. A “cradle-to-grave” LCA analysis on

FIGURE 3.22

Human toxicity potential of fossil fuel power plants with and without CCS

HTP (g 1,4-DBeq/kWh)

0 20 40 60 80 100 120 140 160 180

PC

PC + CCS MEAOxyfuel + CCS NGCC

NGCC + CCS MEA IGCC

IGCC + CCS IGCC + CCS + Amine

FIGURE 3.23

Freshwater aquatic ecotoxicity potential of fossil fuel power plants with and without CCS

0 2 4 6 8 10 12 14 16

FAETP (g 1,4-DBeq/kWh)

PC

PC + CCS MEA Oxyfuel + CCS

NGCC

NGCC + CCS MEA

IGCC

IGCC + CCS

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various residential and district heating CHP technologies under German conditions has been performed by Pehnt (2008). Results of this study indicate that both micro-CHP and district heating CHP are superior not only to a reference case using a German average electricity CO₂ emission factor, but also to a reference case with electricity from a state-of-the-art NGCC²³ facility. In another study, Fischer et al. (2008) investigate the performance of fuel cell CHPs. Their results indicate that fuel cell CHPs are not advantageous regarding GHG emissions compared to conventional prime movers. This is partly due to the assumed high heat-to-power ratio (HPR) of 1.7:1, which does not maximize the high electrical conversion potential of fuel cells. Strachan and Farrell (2006) stated in their study of an American case that at high HPR of about 2:1, the combustion-based CHP technologies have an emissions profile comparable to that of fuel cells because low electricity demand relative to the heat demand reduces the importance of the inherent efficiency of the prime mover (Strachan and Farrell 2006). Note, however, that in real-life applications, the GWP reduction potential of CHP may be smaller than the values published in the literature because an economically optimized configuration does not necessarily maximize its technical, and consequently environmental, performance.

3.6.4.1 Air pollution

It has often been argued that CHP facilities contribute to environmental relief due to their decentralized nature and high overall efficiency (Pehnt, 2008). However, the introduction of CHP technologies does not necessarily lead to improved air quality. The emission performance of CHP plants on air pollutant emissions compared to separate generation of electricity and heat depends, in addition to the factors already discussed, on the emission control performance and the location of CHP plants.

Regarding acidifying emissions such as NO_x, SO₂ and NH₃, the performance of natural gas-based CHP plants depends on the prime mover technology used and the application of emission control measures. Figure 3.26 shows a comparison of these emissions between large CHP and conventional electricity production plants without CHP in a German case study. The figure shows that fuel cells and Stirling engines with the innovative burner reduce acidification impact, while gas engines may increase acidifying emissions due to less efficient emission control systems compared to that for large-scale centralized power plants. In the same paper, Pehnt (2008) also shows that the use of oil-fired reciprocating engine CHP plants lead to a seven-fold increase in acidifying emissions compared to gas-fired reciprocating CHP plants.

The fact that CHP plants can emit more acidifying pollutants than the separate generation of electricity from NGCC without CHP and heat from a gas-fired condensing boiler is also suggested by (Allison and Lents, 2002).

Furthermore, the authors report a breakdown of acidification potential by contribution substance. The results show that NO_x is the largest contributor, accounting for about 60 per cent of the total acidification potential, followed by SO₂. The majority of life cycle NO_x emissions is attributable to the fuel combustion from CHP plant operation, while SO₂ emissions are almost entirely from fuel supply chain since natural gas combustion itself emits little SO₂.

Some authors have indicated that the prevalence of district, residential and commercial CHP plants may aggravate the urban air pollution and health problem because these CHP plants are located near the consumers whereas large scale power plants are often further from populated areas (Canova et al., 2008; Pehnt, 2008). The literature seems to agree that gas engine CHP plants increase NO_x emissions both locally and globally, but the local environmental impacts depend largely on specific local characteristics such as orography, meteorological conditions and design of the plant such as stack height. Pehnt (2008) reports that in the case of Germany, the increase in local NO_x concentration caused by gas engine CHP plants is not significant according to the local legislation. The author performed a dispersion calculation for a hypothetical residential area with relatively critical weather conditions (e.g., large share of stable weather situations, low wind speed) flat topography and urban housing structure. The results showed that the annual average NO₂ concentration in the residential area

23 These results are based on a functional unit of 1 kWh electricity and on the avoided burden approach, in which the cogenerated heat is credited with an alternative generation route. This approach allocates all the benefits of cogeneration to electricity generation

FIGURE 3.24

Terrestrial aquatic ecotoxicity potential of fossil fuel power plants with and without CCS

TEETP (g 1,4-DBeq/kWh)

0,0 0,2 0,4 0,6 0,8 1,0

PC

PC + CCS MEA Oxyfuel + CCS

NGCC

NGCC + CCS MEA

IGCC

IGCC + CCS

FIGURE 3.25

Marine ecotoxicity potential of fossil fuel power plants with and without CCS

0 100 200 300 400 500 600

MAETP (g 1,4-DBeq/kWh)

PC

PC + CCS MEA Oxyfuel + CCS

NGCC

NGCC + CCS MEA

IGCC

IGCC + CCS

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increases by 0.6 µg/m³ while the national limit is 40 µg/m³. The results also show that the maximum short-term NO₂ concentration (by Pehnt defined as concentration that should not be exceeded in more than 18 hr) is found to be generally below 7 µg/m³, where the national short-term limit is 200 µg/m³. The author concludes that such an increase in NO_x concentration does not create serious additional environmental impacts. A study by Canova et al. (2008) draws similar conclusions. The authors investigated the changes in both global and local emissions of NO_x for microturbine and gas engine CHP plants in the Italian context. The results (Figure 3.27) show that there is a significant difference between the changes in global and local emissions (scenario 1 versus scenario 2, and scenario 3 versus scenario 4), but the overall conclusion remains the same: microturbine CHP plants reduce emissions and gas engine CHP plants increase emissions.