Advanced Fossil Power Generation

Colin Snape

Energy Technologies Building, Jubilee campusFaculty of Engineering, University of Nottingham,

Nottingham NG7 2RD, UKcolin.snape@nottingham.ac.uk

IEA GHG Summer School, Nottingham, 2013

ICCS&T

NOTTINGHAM

2007

Scope – the established technologies and how we’ve got

here

Pulverised fuel combustion for coal and substitution of coal with biomass. oxyfuel briefly covered since it is not yet commercial.

Fluidised bed combustion, commercial but may not feature strongly with CCS.

Combined cycle gas turbines (CCGTs). Natural gas, methane.

Hydrogen-rich gas, coal gasification and steam-methane reforming.

The history –the first power stations.

The drivers Improved efficiency, reduced CO2 emissions.

Control of SOx, NOx, Hg and particulates.

UK and the Bigger Picture

• Plentiful natural gas drove CCGT deployment.

• Less than half CO2

emissions from NG CCGTs compared to PF combustion.

Combustion Efficiency

• Typical excess air to achieve highest efficiency for different fuels are

- 5 - 10% for natural gas

- 5 - 20% for fuel oil

- 15 - 30% for coal (PF)

Large amounts of excess air with grate combustion of coal lumps giving low temperatures.

Powdered or pulverised fuel gives better mixing and lower levels of excess air.

Fossil and nuclear power stations convert thermal energy into work

• Power stations can be considered as HEAT ENGINES

• A heat engine typically uses energy in the form of heat to do work and then exhausts the heat which cannot be used to do work.

Cold Reservoir

(low T sink)

All real

heat

engines

lose some

heat to the

environme

nt

Q2

Q1

W

Hot Reservoir

(high T source)

Rankine Cycle: Sub Critical

Entropy

Efficiency = 1 –T1/T2 for Rankine cycle.

This tells us that efficiency depends only upon the temperature at which heat is supplied and rejected.

Greater efficiency can be achieved by:

Raising T2 (temp supplied)

Lowering T1 (temp rejected)

Typically only 37% (0.37) for conventional power stations with steam cycles are achieved mainly because heat is rejected above 100 C and is supplied at ca. 500 C.

Rankine Efficiency – why is conventional power / steam

generation limited?

Coal-fired Power Plant: History

• End of the 19th century, it was by no means unusual for individual companies or individuals to install their own supply, driven by steam.

• First coal power stations - stream engines, district supplies.

• Extremely low efficiencies, <10%.

• PF combustion, introduced 1920s.

• UK, national grid since 1938.

• Improvements with increasing steam temperatures and pressures.

Coal PF Combustion

Maximise efficiency.

Fuel and operational flexibility.

Particulate, SOx and NOx control.

UK, construction wave early 70s through to Drax in mid-80s.

Hence, most plants are now over 40 years old.

Burner Types

(Plan view)

Ash Handling

Fly ash – 80 to 95% of coal mineral matter, very fine, removed in ESP, saleable, different grades, needs low UBC, used in cement and concretes.

Furnace bottom ash – 5 to 20% of coal mineral matter, dense dark, coarse, glassy material. Used in block making.

Stack dust <1% of mineral matter. Very finely divided material not collected by ESP. Can contain toxic elements, ‘fume’.

ESP plan view during

construction

ESP in PF power plant

Longannet Power plant

Coal PF combustion

Over 600 super-critical coal-fired units (SC) have been under commercial operation worldwide, of which over 60 units are ultra-supercritical units (USC).

Subcritical plants (180 bar, 570ºC)

efficiency 36% (traditional)

Supercritical plants (240 bar)

efficiency 44% (now)

Ultra-supercritical (>275 bar,

>590ºC) efficiency >47% (future)

For every 1% gain in efficiency

there is a 2-3% reduction in CO2

emissions.

ESP FGDCO2

captureESP FGD

CO2

capture

Post-

combustion

Increasing cycle temperature and improving efficiency – materials

challenge

Flue gas desulphurisation: Wet, Semi-Dry & Dry Processes

Process Contact SO2 removal

Example

Wet Liquor/slurry in absorber tower

90 – 99%

Limestone gypsum

Semi-dry Solid & water injected into gas path

50 – 99%

Spray dry, CFB

Dry Solid injected into gas path Up to 70%

Sodium bicarbonate

Flue Gas Desulphurisation -FGD

• Absorption is used for flue gas desulphurisation (FGD) with limestone scrubbers being the most widely used technology:

Limestone-Gypsum (LG) Process

• One mole of CaCO3 in the limestone slurry adsorbs two moles of SO2 in a contacting (spray tower) i.e.

CaCO3 + 2SO2 + H2O -> Ca2+ + 2HSO3- + CO2

limestone calcium ions hydrogen sulphite ion

• More limestone is added to the effluent to generate calcium sulphite:

CaCO3 + 2HSO3- + Ca2+ -> 2CaSO3 + CO2 + H2O

FGD continued

• Calcium sulphite can be oxidised further to calcium sulphate (gypsum):

CaSO3 + O2 + 2H2O CaSO4.2H2O

• Gypsum product used for plasterboard manufacture.

• Since the bisulphite precipitates, this reaction must be avoided in the scrubber.

Advantages

• Up to 98% removal.

• Removes all HCl from flue gas.

NOx Control

In-furnace – controlled air introduction.

Reburn Effective (up to 50% reduction) – but seldom room to install on

existing furnaces.

Application in a few, niche sites.

Selective Non-Catalytic Reduction - SNCR.

Cheap to install – but difficult to control.

Limited effectiveness (20-40% Reduction)

Application in a few, niche sites.

Selective Catalytic Reduction - SCR

No UK installations – but imminent.

Large installed base in Germany, Japan, US

High Effectiveness - >90%.

Post Combustion NOx Control:Selcective Catalytic reduction (SCR)

• Ammonia injection into flue gas.

• Reacts with NOx to give nitrogen and water.

SCR module during

construction

Tightening conventional emissions regulations for coal-fired power

generation

Continuing pressure to reduce NOx, SO2 and particulates emissions, for example EU large Combustion Plant Directive (LCPD) limits

SO2: 200mg/m3 for new plant

400mg/m3 for existing plant Jan 2008800mg/m3 for restricted life plants

NOx: 500mg/m3 from Jan 2008

200mg/m3 from Jan 2016

600mg/m3 from Jan 2008 for restricted life plants

Dust: 50mg/m3, 25mg/m3 if fitted with FGD

This has resulted in a number of power stations closing in the UK

New coal plant – CO2 Emission Performance Standards.

Biomass firing

UK legislation (ROCs) previously encouraged co-firing at relatively low levels but now incentives are for dedicated firing.

Biomass has lower CV than coal and generally lower bulk density.

More difficult to grind than coal.

Different mineralogy.

Energy density can be increased by Torrefaction –mild thermal treatment to improve CV for transportation.

Key issue is to guarantee supply – N. America is the focus for the UK generators.

Biomass handling and quality

Biomass degrades quite quickly on storage. Tilbury to closes following fire in 600 tonne feed hoppers. Slagging and fouling due to more alkali/alkaline metals in ash.

15 weeks in storage

Fungal growth but also structural damage and stickiness

Impact of Biomass on Emissions:

• SOx:

– Low content in biomass (<0.5%) leads to reductions overall

• NOx: (more complex)

– Low content in fuels (<0.5%)

– Can enhance the performance of low NOx burners (more fuel rich flame)

– Co-firing for reburn is also a benefit.

Oxyfuel Technology

Air firing typically 15%v/v dry basis. Oxyfuel firing typically >75%v/v dry

basis.

Pulverised fuel combustion produces a flue gas CO2 concentration…

OxyCoal 2 – Demonstration of an Oxyfuel Combustion System

Start-Up / Light-Up

Air Heavy Fuel Oil Firing

Air Coal Firing

Transition

OxyCoal Firing

Shutdown

Heavy Fuel Oil

- 3000 litres

Kellingley Coal

- up to 50 tonnes

Liquid Oxygen

- up to 100 tonnes

Full-scale testing of the oxyfuel combustion process on Doosan Babcock’s 90MWt Clean Combustion Test Facility (CCTF)

OxyCoalTM Burner Testing

Air Firing Oxyfuel Firing

Flame structure and shape were found to be similar for Air and Oxyfuel firing operation but need oxygen levels of 30%.

Natural gas Combined Cycle Gas Turbines (NG CCGTs)

Fuel is burned and the resulting energy in the gas turbine turns the generator drive shaft.

Exhaust heat from the gas turbine is sent to a heat recovery steam generator (HRSG)

The steam turbine delivers additional energy to the generator drive shaft.

Roughly the steam turbine cycle produces one third of the power and gas turbine cycle produces two thirds of the power output of the CCCGT.

Overall efficiency: 52-58%.

CCGT Power Plant and gas turbines

GE LM5000 machine - 6.2 m and 12.5 tonnes. It produces maximum shaft power of 55.2 MW (74,000 hp) at 3,600 rpm.

Direct drive configuration where the l.p. turbine drives both the l.p.compressor and the output shaft.

Gas turbines for power generation – early history

• 1791 First patent for a gas turbine (John Barber, United Kingdom)

• 1906 GT by Armengaud Lemale in France (centrifugal compressor, no useful power)

• 1910 First GT featuring intermittent combustion (Holzwarth, 150 kW, constant volume combustion)

• 1939 World’s first gas turbine for power generation (Brown BoveriCompany), Neuchâtel, Switzerland(velox burner, aerodynamics by Stodola) – 4MW.

West Burton 2GW coal and 1.6

GW natural gas power plants

• Fuel efficiency, 55% ca. efficiency

• Low capital costs - two thirds the capital cost of a comparable coal plant.

• Commercial availability and proven

• Abundant methane supply for many nations, but not all –energy security,

• Flexibility with other fuels, notably hydrogen, not exactly 100% proven.

• CCGTs – optimisation is fuel specific.

Combined cycles

T2

A

T1

B

Q2A

Q1A = Q2B

Q1B

WA

WB

A = WA

Q2A

B = WB

Q2B

Overall efficiency of 2 heat engines (A and B) in series:

=WA + WB

Q2A

Where Q2A is the total heat input into the first engine WA and WB being the work obtained.

The overall efficiency for two heat engines: = A + B - AB

10

20

30

40

50

60

70

80

0 10 20 30 40 500

B = 50%

30%

40%

20%

10%

p

erc

en

tag

e

A percentage

Production of hydrogen from natural gas is widespread in the refining and chemical industry.

CO2 made available at moderate concentrations and pressures (partial pressure 5-15 bar).

CO2 is generally vented!! - production of urea is a notable exception.

Natural Gas Reforming: This is how we make hydrogen now

IGCC – Pre-combustion Capture

Coal gasification is a mature and cross-cutting technology - can raise power and provide hydrogen.

Gas at pressure for capture so efficiency loss (ca. 6-7%) and associated cost lower than for other technologies.

Pre-combustion

Shift reactor

CO2 capture

Essentially a hydrogen producer and has advantages over PF combustion if poly-generation is required.

Gasification - Product Flexibility and Power

Gasification Chemistry

Gasifier Types - Moving bed

Entrained Flow Gasifiers

Entrained Flow Gasifiers

Fluidised-bed combustion –fit for purpose for CO2 capture?

Fuel flexibility, circulating fluidised beds, atmospheric pressure, 300 MW scale.

Pressurised fluidised bed combustion, combined cycle, developed in the 80s.

Difficult to match efficiency of ultra-supercritical PF plant.

Coal PF Combustion vs. NG-CCGTs and IGCC

NG CCGTs , lower CO2 emissions and capital costs but need abundant and cheap supply of natural gas – shale gas revolution.

PF combustion has lower capital costs than IGCC. Many more units and USC plant gives higher efficiencies just for power generation.

PF combustion can ramp up and down quicker for meeting peak demand but gas can be stored. Also, coal in the UK is relatively cheap

IGCC has good reliability and hydrogen can be piped much more cheaply than electricity!

Overall efficiency with CO2 capture is similar for all 3 technologies.

Questions?

Colin Snape

Energy Technologies Building, Jubilee campusDept. of Chemical & Environmental Engineering,

Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD

colin.snape@nottingham.ac.uk

Acknowledge Trevor Drage and Chenggong Sun.

ICCS&T

NOTTINGHAM

2007