Clean coal: High efficiency low emissions technology leads the way



 

Coal fired power stations will form a substantial part of the energy portfolio of many countries, SA included, into the future. Coal power is seen as the lowest cost reliable generating source to bring electricity to Africa. The IEA held an expert workshop on sustainable coal use at the Eskom research and innovation centre (ERIC) in April 2017. This article covers the major topics discussed.

Coal is an essential part of the global energy mix, but it needs to meet the three parts of the energy trilemma, namely energy security, environment and climate protection, and economic competitiveness and equity. The aim should be to minimise the emissions of CO2 from coal, not cease to use coal itself, through improvements in efficiency and the introduction of carbon capture usage and storage (CCUS). The future for coal appears to be positive in Asia, while Africa and parts of the Middle East show promise. The expectation is that coal will be used for decades to come in significant quantities with a focus on those non-OECD regions [1].

The drivers for coal remain, as a means to lift populations out of poverty and to ensure robust reliable energy sources, for power, industry and chemicals or future fuels production, which will take forward industrialisation in developing countries. The reality is that developing countries are going to use coal, as their driver is security of supply and economic competitiveness [1]. The development work to be undertaken by IEA to establish advanced systems that will provide a step change improvement to well over 50% cycle efficiency is a very exciting prospect.

 

Fig. 1: Technologies for cleaner coal generation (IEA CCC).

Coal fired power comes with several challenges:

  • Greenhouse gas (GHG) emissions
  • Non-GHG emissions
  • Efficiency

High efficiency low emissions (HELE) technology

Hele technology provides the means of reducing the problems associated with CFGs.

HELE coal-fired power generation mitigates more CO2 emissions than renewables per unit of investment. Given the higher capital costs of renewable technologies and their lower load factors, in most regions, conversion to HELE technologies represents the lowest cost CO2 abatement alternative. Increasing the use of these technologies will meet the dual objectives of providing power and realising environmental and social responsibility [1].

HELE clean coal technologies are a key step towards near zero emissions from coal.

The technology is available now and being deployed commercially in Germany, India, Japan, Korea, USA and most especially in China. The technology can be applied now and can readily link with CCS when required. Major transformational technology development programmes are underway to further address carbon emission limitations of existing systems.

Ongoing improvements to coal fired power plants include:

  • Increased operational flexibility
  • Higher efficiency
  • Lower conventional emissions

HELE options include IGCC which offers potential for high efficiency with very low emissions, fuel cells (FC) that can potentially be applied to towards-zero emission, high-efficiency coal power plants and supercritical CO2 alternatives.

Introduction of high efficiency coal power plant will reduce CO2 emissions intensity. However, while the proportion of coal burnt in high efficiency, low emission plants will rise, this will remain a fraction of the total for the foreseeable future, not least because some coal users in developing countries cannot afford the upgrades necessary. There is a need to get international funding sources focused on supporting HELE prospects, which will not be straightforward.

Financing for HELE CFGs

In spite of the impression that financial institutes are unwilling to finance coal fired power stations, there a still considerable resources available for CFPS financing. There has been a move away from commercial banks and traditional financing sources to development finance institutes (DFIs), and a further move away from western finance sources to Asian. The recent signing of R2-billion loan agreement between Eskom and the China development bank is a sign of this move.

 

Fig. 2: Funding for CFPS and mining projects in 2014.

There are three major sources of debt:

  • Commercial banks: Institutes that operate domestically and internationally. They offer a wide range of financial products from project finance to retail customer services. Commercial banks have mixed policies towards investing in CFPS and coal mining. Even western banks are not openly negative towards coal power if it is commercially viable and passes due diligence and environmental impact assessments. Among western banks, mining finance has tightened and pledged to divest.
  • Export credit agencies: These have national strategic goals to encourage exports and trade links with other countries.
  • Development finance institutions: Development finance institutions (DFIs) are specialised development banks set up to support private sector development in developing countries. They are usually majority-owned by national governments and source their capital from national or international development funds or benefit from government guarantees. Multilateral development banks/development funding institutes are private sector arms of international financial institutions (IFIs) that have been established by more than one country. The main multilateral DFIs include the African development bank.

Funding or financial guarantees to coal and power companies totalled $152-billion in 2014, of which $70-billion was allocated to power projects and approximately $82-billion to mining and infrastructure [5].

MDBs currently make a small financial contribution to CFPS financing, but there is a shift towards development funding for CFPS, as many of the proposed new plants are in developing countries. It is foreseen that many of the CFPS projects in Africa, SE Asia and India, for which a growth in demand of 1200 Mtce is foreseen, will be funded by DFIs.

Western commercial banks are unlikely to fund coal mining projects but may fund CFPS.

Demand for CFPS funding may slow but remains robust. Since 2010, 526 GW of new coal fired plant have been installed worldwide. Plant with a combined total of 280 GW is under construction in 37 countries, all committed and funded through to 2022. A number of countries, with a cumulative capacity of 660 GW, are in the planning stage. In Africa 30 GW is planned across 18 African economies.

Western banks have retreated from all infrastructure funding since 2000, and the gap has been filled by Asian banks, especially in the CFPS sector. China and japan head overseas investment in HELE plant, so Asian finance supports construction of efficient plant. There are many new coal plants in the planning stages which will need funding.

Asian banks are some of the largest in the world. Fig. 3 shows the funding ability of several Asian banks compared to western banks. In addition to this list are the credit and development agencies of Japan which have funded numerous CFPS or have pledged to fund new plants.

 

Fig. 3: Funding ability of Asian banks (IEC CCC).

Funding affects the technology choice. Supercritical and ultra-supercritical are the technology of choice when it comes to funding from Asian sources. More funding from Japan and Korea could lead to a rise in the fleet efficiency of developing countries.

Reducing non-GHG emissions

Non-GHG emissions (SO2, mercury, NOx and particulates) have a direct effect of air quality and its impact on human health and therefore are of prime importance. The reduction of non-GHG emissions has a measurable impact on air quality. Much of the present effort is focussed on SO2 reduction systems which can be retrofitted to existing plant.

Flue gas desulphurisation (FGD) systems

These comprise:

  • Wet scrubbers: FGD systems are the second largest users of water, accounting for 10 to 15% of plant’s evaporative water losses, 40–70% of total site water use with dry cooling or sea water cooling systems. Treating FGD waste water for reuse is expensive. Cooling flue gas before entering wet scrubber lowers evaporative water losses by approximately 40 to 50% recover moisture in flue gas.
  • Semi-dry scrubbers: Spray dry scrubbers and circulating dry scrubbers that consume about 60% less water than wet scrubbers, and produce no waste water are commercially available. They have a lower SO2 removal efficiency and are used with low to medium sulfur coals.
  • Dry FGD systems use a sorbent injection processes and consume no water or minimal amount if sorbent needs hydrating or flue gas is humidified to improve SO2 removal efficiency. The process removes <80% SO2 and is used to capture SO3 and mercury.

Multipollutant control systems

Emission control systems on conventional coal-fired power plants typically employ technologies designed to remove one specific pollutant. These are then combined, in series, to remove several pollutants in order to meet the emission regulations. Multi-pollutant systems remove two or more of the pollutants (SO2, NOx, mercury and particulate matter) in a single reactor or a single system designed for the purpose.

Multi-pollutant control systems are more cost effective and have lower water consumption than separate systems. An example is the ReACT (activated coke) system which uses 1% of water required by wet FGD scrubbers, removes >99% SO2 and SO3, between 20 and 80% NOx, >90% mercury, and approximately 50% particulates (Fig. 4).

 

Fig. 4: React multi pollutant removal system (IEA CCC).

Upgrading the South African coal fleet to reduce CO2 emissions

Power generation is heavily reliant on coal, with the South African coal fleet accounting for approximately 2% of the global coal-fired capacity, and for approximately 2% of global carbon dioxide emissions from coal through the production of electricity. The majority of the units in the SA coal fleet are of subcritical design, with supercritical units coming into service. The profile of the South African coal fleet is shown in Fig. 5, from which it is clear that the majority of the CFGs are older than 25 years.

With a mainly subcritical coal fleet there is a significant potential for carbon dioxide emissions reduction through a HELE upgrade pathway:

  • Base case: Existing coal fleet with additional USC to meet demand (if required)
  • Fifty year retirement scenario: Review in 2020, 2030 and 2040. Retire capacity over 50 years old and replace with USC
  • Twenty-five year retirement scenario: Review in 2020, 2030 and 2040. Retire capacity over 25 years old and replace with USC in 2020, AUSC in 2030 and 2040

The base case level of 402 Mt falls to 356 Mt and then to 328 Mt under the 50-year and 25-year plant retirement scenarios, respectively (reductions of 11 and 18% against a rising demand curve). If the most effective carbon dioxide abatement pathway is followed (25-year plant retirement, AUSC upgrades after 2025, CCS installation) emissions could fall to 49 Mt in 2040.

Improving the flexibility of coal-fired power plant

Coal will remain important for many years, but there is a need to increase plant flexibility, lower emissions, and reduce costs. Many CFPS were designed for base load operation, but are now required to operate in load following mode. Plants are having to adapt to the changing circumstances. An improvement in operational flexibility, and reduction in environmental impact can be achieved by:

  • Combining solar energy with coal-fired power generation
  • Co-firing natural gas in coal-fired power plants

Fig. 5: Profile of the South African CFG fleet (IEA CCC).

Coal solar hybrid

This system uses solar-generated steam, which can be fed into different locations in a thermal power plant’s steam cycle, such as:

  • Boiler feedwater heating
  • Low pressure cold reheat
  • HP steam between the evaporator and superheater
  • Production of intermediate pressure steam, or main steam
  • Hot reheat or main steam circuits

The system is used for coal saving or solar boosting, the only extra cost is that of the solar field:

  • Coal saving replaces part of the coal feed with solar energy, giving
    the same electricity output but with less coal
  • Solar boosting provides additional steam from solar system fed through plant’s steam turbines, generating more power for the same amount
    of coal

Coal/gas co-firing

Many plants use natural gas for start-up and warming operations but amounts are often relatively small. Co-firing with gas provides the following advantages:

  • Adaptation/re-use of existing infrastructure and control systems: some already in place
  • Enhanced fuel flexibility: no longer reliant on single source
  • Improved operational flexibility: faster warm up, ramp up, and low load operation
  • Cost savings: use the cheapest fuel or change fuel ratio
  • Environmental aspects: less coal equates to lower emissions and waste

Retro fitting may involve the following possible technical issues:

  • Different flame characteristics can result in heat transfer imbalances throughout steam generator
  • Poor gas burner placement
  • Inadequate original heat transfer surfaces

Fig. 6: 25 year retirement scenario (IEA CCC).

CCS: Global status and challenges

The concept of carbon capture and storage has been around for many years, as a solution to GHG emissions. More recently evolved into CCUS or carbon dioxide capture usage/storage with the realisation that the carbon is a valuable commodity and could be used as feedstock rather than being sequestrated. Carbon dioxide capture from CFPS has opened new possibilities for industry.

Twenty-one projects are in operation or construction phase (40,3 Mt/y) This includes three coal power plants, six projects in advanced planning stage (includes three coal power), eleven more in earlier stages of planning (includes six coal power). There has been a recent deployment surge, but not much in pipeline. Fig. 7 shows the planned capacity.

CCS activity and incentives grew from 2005 to 2010 with $30-billion state funding committed in the period 2007 to 2014, but only $2,8-billion was spent. The financial crash and “failure” of COP19 heralded CCS decline, but COP21 gave an indication of a turnaround.

Economic barriers to CCS adoption are capital cost and operating costs. CCs has high entry costs and lead times relative to other low-carbon technologies, requires large plant (not modular), storage appraisal, transport and storage infrastructure. There is a high cost of finance for first of a kind projects, largely due to unusual risks. Most demonstrations have relied on state grants or loan guarantees.

CCS imposes a 70% increase in LCOE and needs some means of compensation (pull) and/or penalisation of non-CCS plant (push) examples would be power purchase agreements (feed-in tariffs, tax credits for stored CO2, CO2 market, CO2 emissions tax or a CO2 cap. A CO2 price alone may be insufficient to promote CCS.

Five EU-funded CCS projects failed during 2010 to 2012. The only EU CCS project still active is the 2009 venture of Uniperand Engie which involves a post-combustion capture to be installed on newly built 1 GW Maasvlaktecoal/biomass plant, with storage in depleted gas reservoir.

The project stalled due to low ETS CO2 price but was “reactivated” in 2016. A new financial model seeks income from diverse sources (greenhouses etc.). There are hopes that the project will come online in 2020.

 

Fig. 7: CCS project status (IEA CCC).

Reasons for market failures:

  • Regulatory: CCS needs a framework to transfer liability for stored CO2 to government (in place in US, Canada, Norway, EU)
  • Infrastructure: It is challenging to promote shared transport and storage infrastructure without certainty of emission sources, or reliance on ‘full-chain projects’
  • Multi-sector nature: Full-chain projects need complex partnerships between the oil and gas (O&G) industry and power industry as different expertise is involved.
  • Storage: “Uninsurable” cost risk of leakage. The O&G sector is used to high returns for offshore exploration, not ‘waste disposal’ economics
  • Policy risk

Carbon capture on conventional coal plant is challenging due to:

  • Large flue gas volume
  • Dilute CO2
  • Contaminants (particulates, acid gases)

State-of-the-art systems include:

  • Post-combustion capture: Bespoke amine solutions from Cansolv, Fluor, BASF, KHI, IHI, (has steadily improved but levelling off)
  • Pre-combustion capture: Selexolor Rectisol solvents
  • Oxyfuel: No full-scale demo, but uses largely commercial technologies from Alstom, Linde, Air Liquide etc.
  • Cost reductions will come from build experience by lowering capex

Future possibilities:

  • Sorbents: Potentially lower energy penalty and cost, less toxic
  • Membranes: Modular, no steam use, better for partial capture
  • Advanced solvents: Regenerate at higher pressure (non-aqueous) or with lower grade heat (enzymes).

Next generation carbon capture systems incorporate capture into the power generation cycle. In the Allam cycle supercritical exhaust from oxyfuel combustion of syngas drives a CO2 turbine, with surplus CO2 being captured. The chemical looping combustion system is based on two circulating fluisided bed reactors, uses cheap oxygen carriers
like limestone and requires no gas separation step.

Fig. 8: Next generation CC systems include the Allam cycle and chemical looping combustion (IEA CCC).

The need for CCS remains, especially under ambitious climate targets like 1,5°C. CCS is needed for “deep” CO2 cuts once low-hanging fruit is decarbonised. State investment in CCS has often failed, and is even more challenging during a financial crash. The few successful projects are based on enhanced oil recovery and/or strategic investment in cheap coal.

There is a need for a steady policy and some form of CO2 pricing, but, CCS seems to also need additional incentives: greater state backing of risks; mechanism for appropriate sectors to separately invest in infrastructure with confidence. De-risking will open access to lower cost finance and significantly cut costs. Technological innovation can also cut costs and big gains are possible for transformational plant designs, with less room for improvement in post-combustion capture retrofit.

References

[1]  Dr. A Minchener: “The IEA Clean Coal Centre and a focus on High Efficiency Low Emissions coal utilisation options”, IEA Clean coal workshop, Johannesburg, April 2017.
[2]  P Baruya: “Trends in international lending for coal fired power plants”, IEA Clean coal workshop, Johannesburg, April 2017.
[3]  A Carpenter: “Advances in multi-pollutant control for coal fired power plants”, IEA Clean coal workshop, Johannesburg, April 2017.
[4]  I Barnes: “Upgrading the coal fleet to reduce CO2 emissions, focus on South Africa”, IEA Clean coal workshop, Johannesburg, April 2017.
[5]  Dr. S Mills: “Innovative approaches to improving flexibility and emissions from coal-fired power plants”, IEA Clean coal workshop, Johannesburg, April 2017.
[6]  T Lockwood: “Status and outlook for CCS with coal power plant”, IEA Clean coal workshop, Johannesburg, April 2017.

Send your comments to energize@ee.co.za 

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