Energy Technology Expert

Is Advanced Clean Coal Technology the Answer to our Global Power Problem?

Remaining Life of Fossil Fuels (oil, natural gas, coal)

Recent events have thrust lately renewed interest in “advanced clean coal” technologies to provide additional power generation capacity in view of dwindling and expensive oil supplies (remaining life 39 years), natural gas (61 years). World wide coal reserves are expected to last over 231 years (remaining life = reserves / extraction rate).

However, due to concerns arising from pollution (emission of sulfur as SO2, toxic ash and heavy metals) and climate change (emission of CO2 greenhouse gases), the utilization of coal for power generation has spurred researches leading to the development and commercialization of so called “advanced clean coal” technologies.

The reader is encourage to read on and share his views, comments and suggestions on how best we could have a healthy balance between utilizing coal as a bridge fuel when oil, natural gas and geothermal resources are exhausted.

The bridge fuel will buy the world economy some time as the world shifts gradually from a carbon economy (fossil fuel based) to hydrogen economy (hydrogen from water dissociated using renewable off-peak energy or electricity) and when this is not yet enough for the highly populated earth, to ultimately the nuclear breeder economy (nuclear technology that creates more fuel than what is utilized in the nuclear power plant).

ADVANCED COAL-BURNING POWER PLANT TECHNOLOGY

Traditional pulverized coal-fired power plant suffers from two primary drawbacks:

• overall thermal efficiency limited (38 – 47%)
• major source of pollution (SO2, NOx, ash, metals)
• major source of greenhouse gas (CO2)

There are strategies to reduce levels of pollution immediately in traditional plants. However, little can be done to raise its efficiency, being limited by thermodynamic constraints.

Efficiency of 49-50% feasible within 20 years for advanced clean coal technologies. Two advanced clean coal technologies are discussed below:

A) Fluidized Bed Combustion

Layer of sand, finely ground coal or any fine solid material is placed in a container and high pressure is blown though it from below.

Small particles become entrained in the air and form a floating or fluidized bed of solid particles that behaves like a fluid that constantly move and collide with one another

Bed contains only 5% coal and the balance are inert materials like ash or sand. The low temperature of bed (950 C) significantly lowers NOx formation.

Limestone (CaO) may be added to the bed to capture sulfur and form gypsum, thus reducing SO2 emissions:

SO2 + ½ O2 + CaO –> CaSO4 + 6733 BTU/lb S

Boiler pipes immersed in the bed captures the heat given off and raises thermal efficiency.

Fluidized Bed Combustion Efficiency

A bubbling bed can achieve 70-90% sulfur removal.

While a circulating fluidized bed (CFB) can achieve higher removal of 90-95% with C/S mole ratio of 2.0-2.5

The thermal efficiency similar to traditional pulverized coal plant is low (47%).

With pressurization, capturing the vented exhaust gases thru a gas turbine will raise efficiency to 50%. Usual capacity of 200 MW though larger 350 MW units have been developed.

Applicability of Fluidized Bed / Circulating Fluidized Bed

Fuel preparation - fluidized bed accepts crushed solids less than 6.4 mm (between stoker firing and pulverized firing), thus avoiding costly pulverizing system.

Lower temperature – needs less refractory, cheaper unit.

Reduced emissions – with lower temperature, cheap limestone or dolomite can be used as a sorbent to remove SO2 without the need for sulfur removal equipment like FGD; air-staging and post-combustion techniques even lower NOx emissions.

Fuel flexibility – variety of fuels from very low-BTU coal cleaning tailings, municipal solid wastes, biomass, high-BTU solid fuels like coal, and fouling and slagging fuels may be burned efficiently with little difficulty.

B) Integrated Gasification Combined Cycle (IGCC)

IGCC is an advanced coal burning plant based on the gasification of coal, an old technology used to produce town gas until natural gas came.

Modern gasifiers convert coal into a mixture of hydrogen [H2] and carbon monoxide [CO], both of which are combustible

C + O2 –> CO2 (complete combustion)
C + ½ O2 –> CO (incomplete combustion)
CO + H20 –> CO2 + H2 (water-gas shift)

Gasification takes place by heating the coal with a mixture of steam [H2O] and oxygen [O2] or air [21% O2, 79% N2]. This can be carried out in a fixed bed, fluidized bed or an entrained flow gasifier.

Integrated Gasification Combined Cycle Efficiency

Partial combustion of coal takes place in the gasifier, releasing considerable amount of heat that is used to generate steam to drive a turbo-generator:

C + ½ O2 –> CO + 4347 BTU/lb C

Gas produced is cleaned and burned in a GT to produce more electricity and the heat from GT exhaust is recovered in a waste heat boiler to raise additional steam for power generation (combined cycle).

IGCC can already achieve 45% efficiency and will reach 50-51%. It can remove 99% of sulfur from coal and reduce NOx emission to below 50 ppm.

Applicability of IGCC

Fuel preparation and flexibility – a new way of utilizing coal, wastes and biomass has been developed – by first gasifying it, then purifying the synthetic gas like natural gas, and using it in a CCGT for the cleanest and most efficient way of generating power

High efficiency – after the coal/water slurry and oxygen have reacted at high temperature and pressure to produce a medium temperature synthetic gas in a gasifier, the gas goes to a heat recovery unit to cool the gas and generate high-pressure steam for power generation.

Reduced emissions – the cooled gas is water-washed for particulate removal, then a COS hydrolysis reactor converts sulfur prior to feeding in a conventional amine sulfur removal system (97% sulfur capture); cleaned gas is reheated and fired in the CCGT.

Environmental Impacts:

Uncontrolled coal combustion is generally a filthy process.

Like oil, the obvious contaminants are SO2, NOx, CO, CO2, unburnt HC and particulates (fly ash)

Typical emissions for CFB are low:

100-200 ppm NOx
< 200 ppm CO
< 20 ppm UHC
Fly ash <44 microns (requires bag filters)

Coal generates more CO2 than natural gas which contains less carbon and more hydrogen, aside from being more efficient.

Fluidized bed combustion results in lower temperature, hence lower NOx emission.

Addition of limestone to capture sulfur before it goes to the stack is a positive benefit that results in 70-90% sulfur removal for bubbling bed and higher 90-95% removal for circulating bed.

IGCC achieves much higher sulfur removal of 99% and attains NOx emissions below 50 ppm compared to traditional coal firing

Because of the higher thermal efficiency of fluidized bed combustion and IGCC, the emission of greenhouse gas CO2 is much lower per unit kWh:

CO2 = (HR kJ/kWh) / (LHV kJ/kg) * (% C/100) * (mw CO2/mw C)
= (3600 / eff.) / (LHV, kJ/kg) * (% C/100) * (mw CO2/mw C)

Risks:

Technology risk (minor)

- Advanced coal-burning technologies like fluidized bed combustion and IGCC have only achieved commercial status only recently; some technology risk

- Atmospheric fluidized bed combustion have been extensively demonstrated in power generation and their reliability is generally proven; technology risk would be minimal

Fuel supply risk (low)

- Fluidized bed combustion and IGCC have added flexibility of being able to burn different coals with ease, including low-BTU wastes and biomass

- Local coal could be cleaned and blended with imported high quality coal to achieve desired performance and costs

Technology risk (moderate)

- Pressurized fluidized bed combustion (PFBC) and IGCC are still in the demonstration and early commercialization stage; long-term reliability has to be established and some component developments still remain; full-scale commercial implementation has to be monitored to see its performance

- The advanced coal-fired systems require a higher level of technological expertise to manufacture and maintain; core components will often have to be imported.

Now, as a concerned reader, do you think that “advanced clean coal” technology is the answer to our global power problem?

Is it the logical choice as “bridge fuel” while the world converts gradually to renewable energy (geothermal, solar, wind, biomass, ocean) or to stored energy fuel (hydrogen from dissociation of water using off-peak energy or electricity)?

Take note that it is possible that fossil oil and natural gas may be exhausted in our lifetime (39 and 61 years based on proven reserves and current extraction rates), hence we need a “clean bridge fuel” to bring us to the future as we develop renewable energy resources.

Ultimately, though, some scientists are of the opinion that there will be a “nuclear renaissance” with the use of uranium based fuels (which is also finite) that the world will have to deal later on using “breeder reactors” to produce more fuel from spent uranium in the form of plutonium.

Please share your thoughts by leaving a comment.

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Marcial T. Ocampo

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