Life cycle assessment of natural gas combined cycle power plant with post-combustion carbon capture,transport and storage

Hybrid life cycle assessment has been used to assess the environmental impacts of natural gas combined cycle (NGCC) electricity generation with carbon dioxide capture and storage (CCS). The CCS chain modeled in this study consists of carbon dioxide (CO₂) capture from flue gas using monoethanolamine (MEA), pipeline transport and storage in a saline aquifer.Results show that the sequestration of 90% CO₂ from the flue gas results in avoiding 70% of CO₂ emissions to the atmosphere per kWh and reduces global warming potential (GWP) by 64%. Calculation of other environmental impacts shows the trade-offs: an increase of 43% in acidification, 35% in eutrophication, and 120–170% in various toxicity impacts. Given the assumptions employed in this analysis, emissions of MEA and formaldehyde during capture process and generation of reclaimer wastes contributes to various toxicity potentials and cause many-fold increase in the on-site direct freshwater ecotoxicity and terrestrial ecotoxicity impacts. NO_x from fuel combustion is still the dominant contributor to most direct impacts, other than toxicity potentials and GWP. It is found that the direct emission of MEA contribute little to human toxicity (HT < 1%), however it makes 16% of terrestrial ecotoxicity impact. Hazardous reclaimer waste causes significant freshwater and marine ecotoxicity impacts. Most increases in impact are due to increased fuel requirements or increased investments and operating inputs.The reductions in GWP range from 58% to 68% for the worst-case to best-case CCS system. Acidification, eutrophication and toxicity potentials show an even large range of variation in the sensitivity analysis. Decreases in energy use and solvent degradation will significantly reduce the impact in all categories.     

Analyzing the carbon footprint from public services provided by counties

In this paper we investigate the Carbon Footprint (CF) resulting from the activities of the county of Sogn og Fjordane (SFK). An Environmentally Expanded Input-Output model is applied to develop a consumption-based Greenhouse Gas (GHG) inventory for this purpose. The model has previously been used for the assessment of municipal CFs, and is in this paper further developed for the purpose of assessing county CFs. Results show that a large fraction of the CF is caused by the purchase of services from private actors, especially the purchase of transportation services, of which a steady increase from the year 2002–2008, is found. In the same period of time, the CF resulting from the operation of county vehicles has been reduced to only 1/6. This outsourcing regarding the provision of services is found in several departments of the county municipality. It indicates a necessary shift in local mitigation strategies, as more focus needs to be on the environmental performance of suppliers. One important finding in the SFK case is the potential of introducing environmental requirements in tenders on providing transportation services. Another is to make environmental requirements to the companies receiving financial support from the county administration.     

Approaches to correct for double counting in tiered hybrid life cycle inventories

This paper presents an approach for inventory compilation and adjustment of double counting in tiered hybrid life cycle inventories (LCIs). The combination of input–output and physical inventory data on coefficient level is a convenient way of constructing a hybrid LCI that has both good detail and completeness. The proposed approach formalizes how to deal with partially overlapping data in inventory compilation. This particular approach requires that the issue of double counting is resolved in a consistent manner. Algorithms for identifying and adjusting for double counting are developed. Identification is performed based on a structural path analysis (SPA). Two algorithms for adjustment are presented. The first method is relatively simple to implement but has limitations to its applicability when performing a detailed assessment. The second method is more complex to implement but provides results that allow for more comprehensive structural inventory analysis. Numerical examples are provided in Appendix.     

The case for consumption-based accounting of greenhouse gas emissions to promote local climate action

One of the challenges faced by local governments in the work with municipal climate action plans concerns accounting for the greenhouse gas (GHG) emissions—what emissions should be targeted, development of emissions over time, and how to effectively measure the success of local climate action. In this paper, we present challenges in developing a GHG emissions inventory related to the provision of municipal services. We argue that a consumption-based perspective, illustrated through the use of the carbon footprint (CF), rather than more conventional production-based inventory, provides a more useful and less misleading indicator. We present an analysis of the CF of municipal services provided by the city of Trondheim. The use of data directly from the city's accounting system ensures a reliable calculation of indirect emissions, and, with some minor modifications, also accurate data on direct emissions. Our analysis shows that approximately 93 percent of the total CF of municipal services is indirect emissions, located in upstream paths, underlining the need of introducing consumption-based indicators that takes into account upstream GHG emissions.     

Effects of boreal forest management practices on the climate impact of CO2 emissions from bioenergy

In Life Cycle Assessment (LCA), carbon dioxide (CO₂) emissions from biomass combustion are traditionally assumed climate neutral if the bioenergy system is CO₂ flux neutral, i.e. the quantity of CO₂ released approximately equals the amount of CO₂ sequestered in biomass. This convention is a plausible assumption for fast growing biomass species, but is inappropriate for slower growing biomass, like forests. In this case, the climate impact from biomass combustion can be potentially underestimated if CO₂ emissions are ignored, or overestimated, if biogenic CO₂ is considered equal to anthropogenic CO₂. The estimation of the effective climate impact should take into account how the CO₂ fluxes are distributed over time: the emission of CO₂ from bioenergy approximately occurs at a single point in time, while the absorption by the new trees is spread over several decades. Our research target is to include this dynamic time dimension in unit-based impact analysis, using a boreal forest stand as case study. The boreal forest growth is modelled with an appropriate function, and is investigated under different forestry regimes (affecting the growth rate and the year of harvest). Specific atmospheric decay functions for biomass-derived CO₂ are then elaborated for selected combinations of forest management options. The contribution to global warming is finally quantified using the GWP_bio index as climate metric. Results estimates the effects of these practices on the characterization factor used for the global warming potential of CO₂ from bioenergy, and point out the key role played by the selected time horizon.     

Evaluation of different CHP options for refinery integration in the context of a low carbon future

This study presents a comparison of different concepts for delivering combined heat and power (CHP) to a refinery in Norway. A reference case of producing high pressure steam from natural gas in boilers and electricity in a combined cycle power plant, is compared to a: (1) natural gas fueled CHP without any CO₂ capture; (2) hydrogen fueled CHP with hydrogen produced from steam methane reforming (SMR); (3) hydrogen fueled CHP with hydrogen produced from autothermal reforming (ATR); and finally (4) natural gas fueled CHP with postcombustion CO₂ removal. The options are compared on the basis of first law efficiency, emissions of CO₂ and a simplified cash flow evaluation. Results show that in terms of efficiency the standard natural gas fueled CHP performs better than the reference case as well as the options with carbon capture. The low carbon options in turn offer lower emissions of greenhouse gases while maintaining the same efficiency as the reference case. The cash flow analysis shows that for any option, a certain mix of prices is required to produce a positive cash flow. As expected, the relationship between natural gas price and electricity price affects all options. Also the value of heat and CO₂ emissions plays an important role.     

Fugacity superposition: a new approach to dynamic multimedia fate modeling

The fugacities, concentrations, or inventories of pollutants in environmental compartments as determined by multimedia environmental fate models of the Mackay type can be superimposed on each other. This is true for both steady-state (level III) and dynamic (level IV) models. Any problem in multimedia fate models with linear, time-invariant transfer and transformation coefficients can be solved through a superposition of a set of n independent solutions to a set of coupled, homogeneous first-order differential equations, where n is the number of compartments in the model. For initial condition problems in dynamic models, the initial inventories can be separated, e.g. by a compartment. The solution is obtained by adding the single-compartment solutions. For time-varying emissions, a convolution integral is used to superimpose solutions. The advantage of this approach is that the differential equations have to be solved only once. No numeric integration is required. Alternatively, the dynamic model can be simplified to algebraic equations using the Laplace transform. For time-varying emissions, the Laplace transform of the model equations is simply multiplied with the Laplace transform of the emission profile. It is also shown that the time-integrated inventories of the initial conditions problems are the same as the inventories in the steady-state problem. This implies that important properties of pollutants such as potential dose, persistence, and characteristic travel distance can be derived from the steady state.