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the chemical engineer|issue 853|july 2012

OLYMPIC TECHNOLOGY | MINING | SENSORS & INSTRUMENTATION| REGIONAL FOCUS

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july 2012 www.tcetoday.com 23

CAREERS tceOPINION

John Griffiths looks

at the engineeringchallenges of the UKgovernments draftenergy policy

The UK government published its draft

energy policy1 in May confirming

that nuclear and renewable power

will form the basis of its future carbon-free

electricity supply.

In the shorter term for transition, and in

the longer term for security of supply, natural

gas will be the preferred alternative energy

source.

An emissions performance standard (EPS)

will be established, restricting CO2

emissions

to 450 g/kWh. So-called grandfathering (a

legal practice which allows old regulationsto apply to some existing situations) will

guarantee continued existence at this level

until 2045. The limit permits the use of natural

gas fired turbines but prohibits unabated coal

burning.

Both nuclear power and basic natural

gas combined cycle technologies are well

understood and dealt with in the draft bill.

Intermittent renewable power will continue

to have access to the electrical power supply

grid and a capacity market is to be established

to determine the total amount of capacity

needed to ensure security of supply. This will

be contracted through a competitive central

auction. The legal framework for the capacity

market will be put in place as soon as possible

and the first capacity auction could, if needed,

be run by the system operator National Grid

as early as 2014 with new capacity in place by

2015 if necessary.

On the surface this is a logical strategy but

problems arise when we look at the tactics

available to achieve the governments desired

energy mix and 80% reduction in emissions.

It is generally understood that the onlyoption which could be used to fill the gaps

in a renewable power supply (backup) is

gas turbines fed with natural gas. But this is

fraught with major commercial difficulties.

a question of backupTaking wind power as an example of a leading

intermittent renewables power source,

there is no general agreement on how much

backup will be needed. For security of supply,

it would be a majority of the current wind

power. If this backup were open cycle natural

gas fed gas turbines then these would have tobe on standby all the time a highly expensive

option.

And the gas turbines would negate the

reductions in CO2

emissions achieved by

installing the wind turbines in the first place.

The challenge of providing flexible backup

for renewables was discussed by the Energy

Research Partnership (ERP), co-chaired by

David MacKay, chief scientific advisor to

the UKs Department of Energy and Climate

Change, in its report Delivering flexibility

options for the energy system: priorities for

innovation.

The report says that conceptually, storing

electrical energy at times of over-supply for

later use is an ideal solution for a system with

more electrified energy demand and variable

renewable generation.

batteries and PSH not

enoughERP and The Economistin its latest Technology

Quarterlyreport both examined the stored

energy options for wind. Both came to the

conclusion that conventional backups of

batteries and pumped storage hydro (PSH)

would be insufficient to provide the cover

needed for the proposed extent of new wind

generation.

PSH accounts for more than 99% of bulk

electricity storage capacity worldwide,

amounting to an installed capacity of

127,000 MW, according to the US Electric

Power Research Institute (EPRI).

The UK has two major PSH stations atFfestiniog and Dinorwig in the relatively

mountainous region of North Wales and two

larger installations in Scotland.

However, the two Welsh stations can only

generate a combined 635 MW, the Scottish

stations 700 MW, for around 45 hours. With

renewables planned to rise to some 30 GW of

capacity by 2020, this constitutes only 2% of

the flexible supply needed to balance wind

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24 www.tcetoday.com july 2012

tce OPINION

fluctuations, and these stations are already

committed to the day-to-day management of

matching electricity demand to supply.

PSH is expensive to build and is dependent

on suitable topography to fit the two

required reservoirs and connect them with

a reasonably short pipe. Several countries,especially in Europe, have already exhausted

their capacity for environmentally acceptable

PSH systems. But to function as a reasonable

backup to an intermittent power source,

many more would be required.

could ARES do the job?Daily, weekly, and seasonal fluctuations

in demand are well understood by grid

controllers and anticipated in good time

using existing energy sources modulated

in accordance with a large library of past

experience and records.But as the proportion of renewable

power generation increases in response to

government and EU strategies, it will become

more difficult to obviate sudden shortages of

power supply or the wasteful production of

surplus power for which there is no market.

The search is on for an alternative solution

to PSH and batteries. The best prospect

reported byThe Economistwas advanced

rail energy storage (ARES), based in Santa

Monica, California. Its system uses modified

railway cars on a specially-built track. Off-

peak electricity is used to pull the cars tothe top of a hill. When energy is needed, the

cars are released, and as they run back down

the track their motion drives a generator.

Like PSH, the ARES system requires specific

topography. ARES delivers more power for

the same height differential. Its also more

efficient, with a round-trip efficiency the

ratio of energy out to energy in of more

than 85%, compared with 7075% for PSH.

A demonstration system is being built in

California, and should become operational

in 2013.But all these concepts use electro-

mechanical technology which requires

recharging and therefore are continuously

available only for a few hours.

swing and stashA chemical engineering solution called

swing and stash (swing to ammonia to

store hydrogen) has recently been proposed.

This is based on the swing technique which

makes two products in parallel from a single

feedstock. The common feed is selectively

swung between the two products to gainthe best return on investment. For example,

[xxxxcompany here xxx] Gelsenkirchen site

in Germany has been successfully swing-

producing hydrogen, ammonia and methanol

from the gasification of refinery residues for

more than 40 years. The proportions of the

products are determined by the market.

This principle is equally applicable if one of

the products is electricity.

Hydrogen is synonymous with energy

if it is to be used to generate electricity. A

gas turbine could be fired with diverted

hydrogen plus nitrogen made available fromthe air separation units associated with the

gasification plant and its flue gas would

contain no carbon.

Hydrogen storage is generally accepted as

non-commercial because of the expense of

storing very large quantities of hydrogen gas.

This requires large underground chambers

which are expensive to excavate.

Swing and stash (see Figure 1) avoids the

need for pressurised hydrogen storage, yet

facilitates the instant availability of hydrogen

as fuel for power production. The process

also avoids the thermally inefficient route

of manufacturing an intermediate product

which is then combusted as required to yield

extra power.

The idea is to use gasification to produce

a continuous fixed quantity of a hydrogen

and nitrogen mixture either for use as a gas

turbine fuel or for the synthesis of a hydrogen-

rich chemical such as ammonia. The ratio of

the by-products of power and chemicals may

be varied within the fixed output of syngas.

A key factor is that the chemical product

should be capable of being stored in a bulk

storage container that could accommodate

large fluctuations in the filling rate, allowing

the liquid level in the storage vessel to rise

and fall against the wind. This is a feasible

proposition for liquid ammonia and,

alternatively, for methanol which would be

Figure 1: Swing and stash

Swing and stash (swing to ammonia to store hydrogen) is based on the

swing technique which makes two products in parallel from a single feedstock.

The common feed is selectively swung

between the two products to gain

the best return on investment.

CoalGasifier

Oxygenfrom ASU

Shiftsystem

PSA 2

PSA 1 CCU

ASU

Chemical

synthesis

Carbon dioxideto treatment

Oxygen togasification

Electricity

Exportchemical

Carbon dioxide

to treatment

Impurehydrogen

Swinghydrogen

Purehydrogen

Nitrogen

As the proportion of

renewable power generation

increases in response

to government and EUstrategies, it will become

more and more difficult to

obviate sudden shortages of

power supply or the wasteful

production of surplus power

for which there is no market.

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july 2012 www.tcetoday.com 25

synthesised from high purity chemical-grade hydrogen plus CO2

produced by the gasification process.

This means an immediate availability of fuel for power

generation by reducing the ammonia production rate. The

variation is kept within limits and at least some power and

ammonia are always simultaneously produced. Exact figures are

dependent on the nature of the ammonia synthesis loop and the

gas turbine characteristics

A worked example was given in a paper at the Eleventh European

Gasification Conference, coupling a 175 t/h gasifier with a 500 MW

combined cycle and a 2,000 t/day ammonia loop. The availableswing is 237 MW and the captured CO

210, 580 t/day (see Figure 2).

This is a capture rate of 91%, reducing CO2

emissions by more than

twice that proposed for gas turbines.

The equipment is proven and available from a number of

internationally reputable vendors.

The backup feedstock is coal, for which the UK has a well

established infrastructure (it still satisfies nearly a third of the

countrys energy needs) and it may be stockpiled on the ground to

provide several months of immediate energy reserve.

Backing up wind with coal in this process would maintain a UK

foothold in a significant raw energy market and would generate

less than half the carbon emissions of a natural gas combined cycle

backup.Exposure to highly-competitive gas markets served by

increasingly fickle and capricious suppliers would be reduced and

process engineers would be encouraged to think out-of-the-box to

devise other process means of achieving a low carbon backup for

intermittent renewables, a subject for which government has not

yet allocated funding.

Do we have the courage and initiative to try new ways to

apply the skills we already have and do we have the lines of

communication and the communication skills to convince those

that make the final decisions? tce

John Griffiths (john@livewest.com), is a senior consultant for

Jacobs Engineering and chairman of IChemEs Gasificationconference series

further reading1. Draft Energy Bill, http://bit.ly/LyEN8z

Figure 2: Swing & stash forelectricity?Basis

F-class gas turbine in combined cycle

2,000 t/d ammonia loop

50,000 t of refrigerated ammonia storage

captured CO2

is compressed to 100 bar(g) for export

minimum gas turbine loading 50%

wind backup averages 33% of swing power available

constant 91% CO2

capture

Performance

Coal consumption: 175 t/h

Wind backup power: 0237 MW

Total export power: 60297 MW

Export ammonia: 5222,000 t/d

Export CO2: 10,580 t/d

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