Comparison of Co2 Abatement by Use of Renewable Energy and Co2 Capture and Storage

Emissions of CO2 and other greenhouse gases to the atmosphere will need to be greatly reduced to avoid the risk of harmful climate change. There are many different ways of reducing emissions of greenhouse gases. It is important to be able to understand how the different options compare. This paper includes a comparison of the costs of three different options for reducing emissions of CO2; wind energy, biomass and capture and storage of CO2. INTRODUCTION The IEA Greenhouse Gas R&D Programme (IEA GHG) has carried out a wide range of studies on capture and storage of CO2 from fossil fuels and is now also carrying out studies to assess the costs and potential of alternative technologies for reducing greenhouse gas emissions, including renewable energy. Based on these studies, this paper compares the costs of three options for reducing emissions of CO2 from electricity generation: wind energy, biomass and capture and storage of CO2. CO2 CAPTURE AND STORAGE CO2 can be captured in power stations and stored underground or in the oceans for hundreds or thousands of years. There are two main options for CO2 capture: pre and post-combustion capture. In post combustion capture, the power station flue gas is usually scrubbed with amine solution. The CO2-laden amine is regenerated by heating with steam and the concentrated CO2 that is released is compressed for transport and storage. In pre-combustion capture, fuel is reacted with oxygen to give a fuel gas consisting mainly of CO and H2. This is reacted with steam in a shift converter to convert the CO to CO2 and more hydrogen. The CO2 is separated by scrubbing with a physical solvent such as Selexol and is then compressed for transport and storage. The hydrogen is used as fuel in a gas turbine combined cycle. A study on leading options for CO2 capture in large power stations has recently been carried out for IEA GHG by Stork Engineering (Audus, 2000). Overall cost and performance data for gas and coal fired power stations with and without CO2 capture are given in table 1. The costs are for notionally 500 MWe plants and are based on a 10% discount rate, a gas cost of $2/GJ and a coal cost of $1.5/GJ. The costs of compressing the CO2 to 110 bar for storage are included. The costs of CO2 capture are calculated by comparing the costs and emissions of plants with and without capture. The costs of avoiding CO2 emissions are between 32 and 39 $/tonne for gas fired plants and 37 and 47 $/tonne for coal fired plants. The quantity of CO2 emissions avoided is less than the quantity captured because the energy consumed during capture results in additional CO2 production. Table 1: Costs and efficiencies of power plants with and without CO2 capture Power generation process CO2 capture Efficiency (% LHV) Specific investment ($/kWe) Generation cost (c/kWh) CO2 emission (g/kWh) Cost of CO2 avoided ($/tCO2) None 56 410 2.2 370 Reference plant Post-combustion 47 790 3.2 61 32 Natural gas combined cycle Pre-combustion 48 910 3.4 65 39 None 46 1020 3.7 722 Reference plant Pulverised coal Post-combustion 33 1860 6.4 148 47 None 46 1470 4.8 710 Reference plant Coal IGCC Pre-combustion 38 2200 6.9 134 37 CO2 can be stored in the deep ocean or underground in disused oil and gas fields, unminable coal seams and deep saline aquifers. The potential capacity for CO2 storage is large, for example it is estimated that 800 Gt of CO2 could be stored in geological reservoirs in the EU alone (Holloway, 1996). This is equivalent to approximately 1000 years of storage, based on current annual EU power sector emissions. The cost of CO2 storage is very site specific and depends on the distance between the power station and the storage site. Studies carried out by IEA GHG indicate that for a CO2 transport distance of 300km, costs of transport and storage should be less than $8/tonne of CO2 stored, equivalent to about $10/t of CO2 emissions avoided. IEA GHG plans to carry out studies to estimate the costs, availabilities and locations of CO2 storage sites, to produce cost supply curves for CO2 capture and storage based on probable CO2 transport distances. Based on the costs given above, the overall cost of CO2 capture and storage is expected to be around $40-60/t CO2 avoided. As most of the cost arises in the capturing of CO2, which is relatively independent of location, the marginal cost of CO2 capture and storage is not expected to increase much as the amount of CO2 emissions avoided increases. BIOMASS ENERGY Biomass fuel for power generation can be obtained from short rotation harvesting of trees such as poplar, eucalyptus and acacia, or certain other fast growing plants. The amount of CO2 emitted from the power station is approximately equal to the amount of CO2 absorbed during growth of the biomass. Short rotation biomass can therefore be regarded as an almost CO2-neutral fuel, although a small amount of CO2 is emitted during harvesting and transport of the biomass. A study on the potential for CO2 abatement by short rotation biomass has been carried out for IEA GHG by CIEMAT (Varela, Sáez and Audus). The study focussed on Spain, which is reasonably well suited to short rotation harvesting of biomass fuel. Specific sites were nominated for the plantations and power stations, thus ensuring realistic assessments of the availability and suitability of the land, and the overall potential for such schemes. The sites were restricted to non-irrigated agricultural land. Irrigated agricultural land is expensive and so is used for high value crops such as oranges, tomatoes and vegetables. Rural land that is not designated as ‘agricultural land’ is likely to be relatively unproductive and its use would result in more expensive electricity. Each site would contain a small (25-60 MWe) power station located centrally with respect to its biomass plantation. Potential economies of scale, achievable by having centralised processing facilities on a large scale, were outweighed by the increased cost of transport and storage. The state of the art biomass power generation technology is currently fluidised bed combustion (FBC). The thermal efficiency of the FBC plants in IEA GHG’s study is 25% and the capital cost is $1590-1950/kWe. The efficiency is lower than that of the reference coal fired plant, mainly because of the smaller plant sizes, which necessitate the use of lower efficiency steam cycles. The cost of electricity at the best site was predicted to be 9.2 c/kWh. Installing 22 such plant on the best sites found in the study would produce an installed capacity of 0.8 GWe. The cost of the most expensive of these plant was predicted to be 11.1 c/kWh. The study predicted that short-rotationbiomass-fired FBCs could provide about 5% (2.3 GWe) of the present Spanish electricity generating capacity. Conceivably, this could be raised to about 10% if less suitable land was used and short rotation biomass began to compete with agricultural land use. The use of biomass could therefore be only a partial solution to a need for a major reduction in emissions of CO2 to the atmosphere. Integrated gasification combined cycles are being developed to increase the efficiency of power generation from biomass (Pitcher, 2000). Costs are currently high but they may decrease in future when the technology is fully proven. The use of biomass gasification was also assessed in a range of the sites studied. The cost of greenhouse gas abatement depends on the costs and emissions of the fossil fuel fired power stations that would be displaced. For the range of biomass electricity prices given above, the cost of avoiding CO2 emissions is $75-100/t compared to the reference pulverised coal fired power station shown in table 1 and $190-240/t compared to the natural gas fired combined cycle reference plant. WIND ENERGY In recent years wind energy capacity has been growing at 30% or more per year. At the end of 1999, there was about 14 GWe of installed wind energy capacity worldwide (Wind Directions, 2000). A study on the global potential for greenhouse gas abatement by wind energy has recently been carried out for IEA GHG by Garrad Hassan and ECON (IEA GHG, 2000). Wind energy availability and costs were estimated in detail for four regions; the EU, China, India and the USA. The predictions for these regions were extrapolated to give a less detailed analysis for the rest of the world. Costs and wind energy potentials were analysed separately for onshore and offshore wind farms in each region. This paper concentrates on the results for the EU, as these can be compared most easily with IEA GHG’s costs of biomass and CO2 capture and storage. Methodology Wind flow modelling was carried out to determine the distributions of annual wind energy availability in each of the study regions, broken down into 1x1km squares. Environmental and technical constraints on wind farm siting were applied to exclude areas such as those with steep gradients and those in nature conservation areas or urban areas. In addition it was assumed that turbines could not be sited close to inhabited dwellings, on grounds of public acceptability. The minimum distance was assumed to be 300m in the EU. The density of onshore wind farms was also limited to 150 kW/km, based on Danish experience. At this density, about 2.5% of the total land area would be covered with wind farms, although the land occupied by turbines and infrastructure would be only about 0.05% of the total land area. It was considered that onshore wind energy could develop along two rather different patterns: those currently typical of Northern Europe – small wind farms widely scattered, termed the “Small Onshore Scenario” – and those currently typical of the USA – large wind farms concentrated in windy areas, termed the “Large Onshore Scenario”. It is not clear which pattern of development will predominate in the future, or where, as this will depend on public acceptability, so it was decided to model both scenarios in all regions. The two scenarios were modelled by applying the maximum wind farm density criterion (150 kW/km) over different areas 20x20km blocks in the small wind farms scenario and nationally in the large wind farms scenario. In the offshore scenario, it was assumed that turbines could not be sited closer than 5km to the shore and that 75% of the area would be excluded for reasons such as shipping, marine conservation and unsuitable seabed conditions. Typical capital costs of large onshore, small onshore and offshore wind farms in the year 2000 were assumed to be 1000, 1217 and 1676 $/kW respectively. Electrical grid connection, reinforcement and transmission costs were adjusted on a site specific basis. It was assumed in the base case analysis that the average costs of wind farms (excluding transmission) installed between now and 2020, would be 82% of current costs. This will depend on when during this time period the wind farms are built, the rate of technological development, economies resulting from larger scale manufacture and the degree of competition in the market. Electricity generating costs Predicted costs of wind energy in Europe in the year 2020 are shown in Fig. 1. It can be seen that the costs and availability of wind energy depend greatly on wind farm siting criteria. Figure 1 Costs of electricity generation by wind energy in the EU The quantities of electricity available from wind energy in the EU, shown in Figure 1, are substantial proportions of the total demand. The electricity demand in the EU in 2020 is predicted to be 3030 TWh/y. Wind is an intermittent energy source. Typical electricity grids can accommodate small amounts of wind energy without significant effects but as the proportion of energy supplied by wind increases, the overall system effects increase. To cope with times when wind energy availability is low, extra fossil fuel fired back-up generating capacity has to be installed and also the fossil fuel fired plants on the grid have to spend more time operating as peaking plants, at lower efficiencies. When the potential amount of wind energy exceeds the electricity demand, some of the potential wind generation has to be curtailed. These system effects add to the overall cost. This is not taken into account in Figure 1 but is included in the calculation of greenhouse gas abatement costs, described below. Costs of greenhouse gas abatement The net costs of avoiding CO2 emissions were calculated by comparing total generation system costs with and without wind and emissions from the power sector with and without wind. Wind energy is used to meet new generation requirements (net growth plus retirements), as the overall costs of such plants are usually higher than the marginal operating costs of existing plants. This means that wind energy tends to displace new fossil fuel-fired generating capacity. The displaced 0 2 4 6 8 10 12 0 200 400 600 80