Modeling the Benefits of CCS Technology Development and Adoption

Some groups like the Stanford Energy Modeling Forum (EMF) specify a “linear” reduction in CO₂ emissions to the year 2050. According to our simulations, a linear reduction would be very hard to meet without an accelerated and well-funded Department of Energy R&D program for CCS. We present simulation results for CO₂ emissions projections for coal, gas, and oil power generation under a cumulative emissions target. After some early post-2010 reductions (due largely to slower economic growth), CO₂ emissions stay persistently flat until 2030 after which rapid progress is made. The largest reduction is due to CCS. Most capture is from retrofitting existing pulverized coal (PC) plants. Coal-biomass circulating fluidized bed (CFB) plants with CCS have an industrial market. Later in the horizon, advanced base-load coal with CCS (e.g., IGCC with pre-combustion capture) and some co-production plants with CCS penetrate significantly. The later can optimize electricity vs. liquid fuel production.

There would also be significant CO₂ reduction benefits from developing capture technology well-suited for the less-concentrated CO₂ streams from natural gas combined cycle units. By 2050 most of the remaining power sector CO₂ is from natural gas.

In addition to accelerating CO₂ reductions, we show in our paper that further R&D in CO₂ capture that lowers cost and improves efficiency will result in economic benefits to electricity consumers.

The main model used in our scenario study is the Electricity Supply and Investment Model (ESIM) developed at Argonne National Laboratory. It includes representations for the operation of existing generation units and for capacity investment in new technologies. The shape of the load facing generating units plays an important role in modeling the electric system. The Load Duration Curve (LDC) is adjusted (i.e. elongated at peak load) to account for probabilities of unit forced outages. The resulting demand curve facing generating units is called the equivalent LDC in the power literature. We calibrate the equivalent LDC to existing data in year 2010. Non-dispatchable intermittent generation (e.g., wind and solar) must be subtracted from the equivalent LDC. Wind and solar generate a significant portion of power off-peak, leading to a reduction in capacity credit. Available dispatchable units, such as existing PC power plants are then stacked on the remaining load curve. Under scenarios with high intermittent renewable penetration, capacity factors for existing PC plants are reduced and their cycling is increased.

Units are loaded in order of least to highest variable costs, including any emission charges. Must-run units include nuclear power, opportunity fuels (wastes and biomass) co-fired with coal (CFBC), and CHP units. Then existing PC units retrofitted with CCS are dispatched before non-retrofitted units because the former incurs lower variable costs associated with CO₂ emissions when there is a price on carbon emissions. In later years, with substantial intermittent capacity on-line, non-retrofitted PC units are pushed far down the loading order resulting in low capacity factors (i.e., utilization rates). Finally, intermediate load natural gas combined cycle NGCC units and peak load combustion turbines complete the required capacity demand.

After PC units are retrofitted with CCS, their utilization increases. Units retrofitted with CCS will obtain a partial generation offset due to movement up the LDC loading order. For 162 existing PC units that were retrofitted by 2035 in our model run, on average, their generation after the retrofit was 8.2 percentage points higher than the year before retrofit. This increased utilization of retrofitted units offsets about 1/3 of the derated capacity (parasitic plant electric load due to CO₂ capture and compression). For example, if a unit were capacity derated by 28.2%, its net generation on average would be reduced by 20%, the difference being the increased utilization of the retrofitted unit. The 8.2 percentage point increase in utilization rate significantly increases the economic attractiveness of CCS.