Biomass are collected and transported to the power plant, generating electricity by direct combustion. The biomass transport distances are presented in Table 6.5. GHG emissions of BC1 and BC2 were calculated and the results are shown in Table 6.15. Feedstock cultivation exhibits the largest GHG emissions and minimal impacts from transportation. In all cases, biomass combustion emits relatively few GHG emissions. BC2 case, being of higher efficiency, shows lower GHG emissions than the BC1 case. Both the BC1 and BC2 cases show more than 90% GHG savings compared to coal electricity emissions.
0 20 40 60 80 100 120 140 PyOil transported
50km PyOil transported 100km PyOil transported 150km PyOil transported 200km
g CO2 e q/ kW h
PyOil substitution Coal (waste) PyOil substitution NG (waste) PyOil substitution Oil (waste)
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Table 6.15: Life cycle GHG emissions of electricity from biomass direct combustion g CO2 eq/kWh BC1 (residue) BC1 (poplar) BC1 (willow) BC2 (residue) BC2 (poplar) BC2 (willow) U.S coal electricity Feedstock 30.56 48 62.44 22 34.56 44.96 Feedstock transport 4.02 0.86 0.91 2.45 0.52 0.56 Biomass combustion 4.13 4.13 4.13 2.97 2.97 2.97 Total 38.70 52.99 67.49 27.42 38.05 48.49 1087 GHG savings 96.4% 95.1% 93.8% 97.5% 96.5% 95.5% 6.4. Discussion
Power generation by biomass direct combustion, by co-firing in fossil fuels plant, by GTCC and stationary diesel generator are compared together to determine GHG emissions and savings compared to conventional fossil fuel-generated electricity. The results are shown in Figure 6.5. Pyrolysis oil co-fired in fossil fuels plants (coal, natural gas or fuel oil) has the highest emissions, although the emissions are significantly lower than fossil fuels electricity baseline. The major contributor to life cycle GHG emissions is electricity used in the pyrolysis plant. Pyrolysis oil produced in a parasitic configuration and combusted in either a GTCC or stationary diesel generator release the lowest GHG emission; even lower than the direct biomass combustion cases presented above, because the GHG emission caused by grid electricity used for production of pyrolysis bio-oil is avoided. Biomass direct combustion in steam turbines reside in between, but the leading contributor to the life cycle GHG releases in this case are biomass cultivation/harvesting and transportation. The low energy density of raw biomass relative to pyrolysis oil becomes increasingly important as transportation distance increases.
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Figure 6.5: Life cycle GHG emissions of power generation.
As shown in other studies, power generation from direct combustion of biomass produced by urban sources, normal land filling and mulching operations has great advantage with regard to GHG emissions compared to the fossil fuel plants174. However,
problems and limitations of direct biomass combustion remain. High nitrogen oxides (NOx) emission is one of the top air quality concerns. Biomass power plants show a
relatively high NOx emission rate per kWh generated compared to other combustion
technologies. Carbon monoxide (CO) is also emitted, sometimes at levels higher than those for coal plants. Another air quality concern is PM emission, as burning biomass will release relatively large amounts of particulates. Furthermore, ash produced from biomass co-combustion is not yet certified for reuse in cement manufacturing, and hence, ash from co-processing may become a solid waste rather than a useful co-product. In addition, volatile alkali salts produced during biomass co-processing have been shown to hurt the effectiveness of catalysts used in selective catalytic reduction (SCR)175, and as a
resykt, NOx emission from the power plant may further increase. Solid and wet biomass fuels are of relatively low energy density, compared with fossil alternatives, and
0 20 40 60 80 100 120 140 160 180 BC1 BC2 Coal co-
fire Natual gas co-fire Oil co-fire GTCC generator Diesel
g
CO2
eq/
kW
h
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consequently large volumes are typically required to be stored and transported, hence the emission and cost due to biomass transportation are much higher than other feedstocks, for example pyrolysis bio-oil. In addition, biomass will usually absorb moisture if exposed, thus it may naturally biodegrade in storage176. Using pyrolysis oil instead of biomass as feedstock to generate electricity can eliminate many of the problems
associated with direct biomass combustion. Because of low ash content in pyrolysis oil (often well below 1%), it can also minimize ash related issues caused by biomass
combustion177. In addition, pyrolysis oil is more flexible in power plant application, as it
can be combusted in coal, oil, and natural gas-fired plants133, as compared to wood chips
that can most easily displace solid fossil fuels (coal).
6.5. Conclusion
Pyrolysis bio-oil can be produced through fast pyrolysis step from solid biomass, and then combusted to generate power, replacing fossil fuels as feedstock. Combusting pyrolysis oil as a liquid biofuel to generate power can reduce the climate changing greenhouse emissions significantly because the CO2 emission at the pyrolysis oil
combustion stage is considered carbon neutral as CO2 is sequestered during feedstock
growth. In this LCA study, life cycle GHG savings of 80% to 99% were estimated for power generation from pyrolysis oil combustion relative to fossil fuels combustion, depending on the biomass feedstocks and combustion technologies used. A parasitic system scenario in which electricity is provided from an integrated pyrolysis oil production-electricity generation facility was also considered, and it shows more GHG savings because use of imported electricity from the U.S. grid is avoided. With expected improvement of pyrolysis technology, and more efficient power generation technology, the life cycle GHG emissions of power generation using pyrolysis oil can be further reduced. Pyrolysis oil has the potential to replace fossil fuels as an alternative energy source to generate power, reducing GHG emissions caused at power plant, as well as the dependence on fossil fuels.
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