Natural gas equipment

8/20/2019 Natural gas equipment

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CHAPTER 7 GAS UTILIZATION EQUIPMENT

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EQUIPMENT FOR POWER GENERATION

Power plants use several methods in converting natural gas to electricity. One process is

to burn up the gas in a boiler to produce steam, which is then used by a steam turbine to

generate electricity. A more common approach is to burn the gas in a combustionturbine to generate electricity.

GAS TURBINE

A gas turbine consists of three segments – (i) compressor (ii) combustor and (iii) turbine.

Ambient air is compressed to 11-30 bar pressure and as a consequence its temperature

rises. Most of this warm air is used in the combustor to burn fuel (natural gas or a liquid

fuel). The resulting hot gas expands through the turbine, doing work and exists at early

atmospheric pressure but a temperature of up to 500-640oC. Work extracted during the

expansion is used to turn the turbine which drives the generator that produces electricity.

Gas Turbine Cycle – Brayton cycle

In a simple gas turbine cycle, low pressure air is drawn into a compressor (state 1)

where it is compressed to a higher pressure (state 2). Fuel is added to the compressed

air and the mixture is burnt in a combustion chamber. The resulting hot products enter

the turbine (state 3) and expand to state 4. Most of the work produced in the turbine is

used to run the compressor and the rest is used to run auxiliary equipment and produce

power.


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The hot gas (~ 500oC) exit from the turbine still has significant amounts of energy which

is used to raise steam to drive a steam-turbine and another generator. This combination

of gas and steam cycle gives rise to the term of ‘combined cycle gas turbine’ or simply

known as CCGT plant.

Power output produced is from 1 to 200 MW with overall efficiencies of 25-30%. It is

suggested that the higher the temperature, the higher the efficiency of the turbine;

however this is limited by the gas turbine (blade) material.

Economics of gas turbines in process applications is depended on the effectiveness

usage of the exhaust energy (recovered energy) which is 60-70% of the inlet fuel

energy. Increase in overall effectiveness of energy use in a gas turbine heat recovery

system is a function of amount of energy transferred from the turbine exhaust gas. Most

common application of the gas turbine exhaust is for the generation of steam in a heat

recovery steam generator.

Advantages

• Minimum cost investment when energy or peak power demand exists

• Low maintenance cost

• Low starting torque, allowing a fast start up

• Fast rate of adjustment to load change/no cooling water required

• On site installation

• Installation time less than for other thermal plants

• Multi-fuel capability

• High power to weight ratio


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Disadvantages

• High initial capital cost (for prime mover)

• Low overall efficiency

• High frequency noise generated

• High gas pressure required

STEAM TURBINE

A steam turbine based power plant consists of raising high pressure steam in a boiler

from the thermal energy and expanding the steam in a turbine to generate shaft power

which in turn is converted into electricity in the generator. Boiler/steam turbine systems

operating through a Rankine cycle in larger, electric utility power plants.

Figure 7-3: Rankine cycle

1-2: reversible adiabatic pumping process in the pump

2-3: constant-pressure heat transfer of heat in the boiler

3-4: reversible adiabatic expansion in the turbine

4-1: constant-pressure transfer of heat in the condenser


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How is electricity generated?

Natural gas and air are burned in a boiler to produce steam. Steam produced in a boiler

is expanded through the power turbine producing mechanical power. However unlike the

gas turbine, in a Rankine cycle, the steam is exhausted from the turbine is usually

condensed and recycled through the feed-water pump to a boiler.

Steam turbine is economically attractive for heat/power ratios of between 0.012-0.8

kg/MJ. Typical power out put is 15 kW to 500 MW with overall efficiencies between 25-

45%.

Advantages

• Very liable prime mover

• Low maintenance cost

Disadvantages

• Require start up system

• Long start up

• Loss of circulation causes serious boiler damage

Types of steam turbine

a) Condensing turbine

Steam at boiler pressure and temperature is expanded from turbine, exhausting to sub-

atmospheric pressure in the condenser

Maximum flexibility for meeting varying process steam and electrical load

Less efficient


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b) Back pressure turbine

Steam is only partially expanded and exhausted from the turbine at pressures above

atmospheric, finally exhausting the whole of the steam flow to the heating process

Maximum economy and simplest installation –most efficient when the power demand

corresponds to the output obtainable from the available steam flow

c) Pass-out condensing turbine

Practical when the process steam is small compared to the power demand or if it varieswidely during the day –back pressure and condensing sections-regulate according to

load and process steam demands


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COMBINED CYCLE POWER PLANT (CCPP)

CCPP describe the combination of gas turbine generator (s) (Bryton cycle) with turbine

exhaust waste heat boiler (s) and steam turbine generator (s) (Rankine cycle) for the

production of electric power.

How they work

Combined cycle power plants have two cycles for generating electricity, steam cycle and

the gas cycle. In the steam cycle, fuel is burned to boil water and create steam which

turns a steam turbine driving a generator to create electricity. In the gas cycle, gas is

burned in a gas turbine which directly turns a generator to create electricity. Combined

cycle power plants operate by combining the gas cycle and the steam cycle for higher

efficiency.


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The hot exhaust gases exiting the gas turbine are routed to the steam cycle and are

used to heat or boil water. These exhaust gases typically carry away up to 70% of the

energy in the fuel before it was burned, so capturing what otherwise would be wasted

can double overall efficiency from 30% for a gas cycle plant to 60% using the newest

combined cycle technology.

Advantages

• Flexibility in unit size

• Short construction period

• More efficient than conventional gas fired generator

• Significantly cleaner burning than coal/oil generating plant

Disadvantages

• Produces air emission of NOx and CO2

• Requires water to cool the plant-water pollution

•

Natural gas and light distillate fuels required for low maintenance operation ofgas turbine are expensive

• Heavier distillates and residual oils are also expensive as compared to coal.

COGENERATION – COMBINED HEAT AND POWER (CHP)

Definition: a sequential production of thermal energy and electric energy from a single

energy source. Also known as combine heat and power (CHP)

Generally consist of a thermal energy generation device (e.g. boiler), a prime mover

(e.g. steam turbine), a generator and a heat exchanger.

Improve efficiency of power generation from 30-35% to 65-80%


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Major users of cogeneration include;

• Industrial sectors (e.g. chemical plants, food processing, textiles etc)

• Residential/commercial/institutional sectors (e.g. hospital, office building,

school, colleges, university etc)

• Electricity utility sectors

Benefits

• Economic performance- high efficiency by using the same fuel to provide

electricity & heat

• Reduced fuel consumption

• Reduced fuel cost

• Reduced electric utility bills

• Reduced production or operating cost

• An economic competitive advantage through a maximised return on

investment capital

• Most efficient use of capital investment

• Security of power supply-guarantees continuity and quality of power and

thus high power supply reliability

• Energy independence-production of process steam for internal use &

selling to neighbouring user

• Operating flexibility-electrical and heat load can be managed

• Respecting the environment-royal road to promote an environment

protection policy, reduced air and thermal pollution, no additional

combustion devices needed


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Fuel utilization effectiveness

How cogeneration works

1. fuel burned in a boiler produces steam

2. to drive steam

3. that is linked to an electric generator

4. the generator produces electricity

5. that is either sold to utility

6. used in-house or both

7. the steam expelled from the turbine is used for heating or manufacturing

8. as the steam cools either during use or in a condenser, it becomes water and is piped

back into the boiler


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Two major technologies for co-generating electrical or thermal energies as follow;


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Table 7-5: Comparison of commercially available cogeneration systems


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TRIGENERATION

Trigeneration is the concept of deriving three different forms of energy from the primary

energy source, namely, heating, cooling and power generation. Also referred to as

CHCP (combined heating, cooling and power generation). This option allows having

greater operational flexibility at sites with demand for energy in the form of heating as

well as cooling. A typical trigeneration facility consists of a cogeneration plant, and a

vapour absorption chiller which produces cooling by making use of some of the heat

recovered from the cogeneration system.


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EQUIPMENT FOR SPACE COOLING

STEAM ABSORPTION CHILLER

Steam absorption chillers use thermal input to produce cooling for space conditioning or

for refrigeration. Thermal input (heat source) can be from direct source (usually natural

gas burner) and indirect fired source (low pressure steam, hot water or waste process

heat).

Most common working fluids (refrigerant/absorbent)

• Water/lithium bromide (H2O/LiBr)

Operate under vacuum (10-800 mbar a) – all lithium bromide chillers operate in a

vacuum with the low side pressure equal to the vapour pressure of water vapour

at the evaporator temperature.

Large commercial chillers (nominal capacities ranging from 50 to 2000

refrigeration tons or 175 to 7000 kW) e.g. KLIA

• Ammonia/water (NH3 /H2O)

Operate above atmosphere

Achieve lower cooling temperature (since refrigerant is ammonia not water)

Small refrigerator and residential unitary systems

Basic working principles

• Major component for an absorption chiller includes an absorber, generator,

solution pump, condenser, evaporator and a solution heat exchanger.

• The cycle starts in the generator, where heat from a gas burner, heats a high

concentration ammonia mixture (strong solution) and produces ammonia vapour.


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• This causes ammonia, to separate and rise up to leave the generator, while the

lower concentrated mixture (weak solution) falls downwards to the bottom of the

generator

• This ammonia moves into a condenser coil & cooling water condenses it to a

liquid refrigerant

• The refrigerant then moves into the evaporator where it cools a circulating water

system (chilled water)

• The chilled water is pump into the building for air conditioning

• The refrigerant, now an ammonia vapour again, moves into the absorber section

where it meets the weak solution and they are re-joined together again as strong

solution.

Chiller types

• Absorption chillers are commonly classified by both their type of thermal input

and by the internal heat utilization capability


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• Thermal input can be provided by steam, hot water, natural gas or oil – other hot

liquid or vapour heat sources can be used in custom industrial heat recovery

applications

• The natural gas or oil-fired units are commonly known as direct-fired chillers,

while the steam or hot water driven units are referred to simply as indirect-fired

chillers

PRESENT COMMERCIAL CHILLERS

Single effect chiller – as previously discussed in the basic working principle of absorption

chiller


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• A concentrated solution of lithium-bromide is sprayed into the absorber (2)

that is connected to the evaporator

• The lithium-bromide has a strong attraction for the (water) vapour that is

produced in the evaporator

• The lithium-bromide solution ‘pulls’ refrigerant (water) vapour that is produced

in the evaporator

• This creates a vacuum that in turn causes the refrigerant (water) vapour to

evaporate at a low temperature

• This low temperature is transmitted to the chilled water that returns to the

building

• As the concentrated solution of lithium bromide absorbs more refrigerant

vapour, it dilutes and loses its affinity for the refrigerant vapour- the diluted

solution, containing lithium bromide and refrigerant vapour, is pumped (3)

from the absorber through heat exchanger (5A & 5B) to a high temperature

generator (4A) to boil off the refrigerant vapour and increases system

efficiency

• The refrigerant vapour is cooled in the condenser (6) and then passed

through an expansion valve (7) to the evaporator/absorber vessels where the

cycle continues

COMMENT ON NATURAL GAS COOLING

Although natural gas absorption cooling technologies have progressed in recent years,

they still have a few critical downfalls that often make electric cooling more attractive.

Natural gas absorption units can only obtained a Coefficient of Performance (COP) of

1.1 whereby electrical centrifugal units can obtain COPs up to 6.0.

Gas absorption chillers generally cost 1.5 to 2.5 times that of electric chillers and often

require larger cooling towers and pump modifications. With high initial cost and

comparably low system efficiency, natural gas absorption chillers are only economically

feasible in areas with high electrical demand rates, low natural gas summer rates and

rebate and incentive plans offered by local gas utility companies.


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VAPOUR COMPRESSION CYCLE (VCC)

VCC is able to transfer heat from low T system to high T system. Refrigerant used such

as R11, R122, CFC, R22

Non flammable, non toxic but … ozone depletion

New type is being introduced e.g. R134a

- Prime mover : electric motor (conventional)

engine (gas, petrol or diesel)

- Compressor : reciprocating (small size system), centrifugal

a) refrigerant vapour is compressed causing its T and P to rise

b) the hot vapour passes through a condensing HEX and turn liquid at high

P. Heat is rejected to a water cooling circuit or air-cooled radiators

c) the liquid is expanded through a throttling valve and cools

d) the cool refrigerant passes through an evaporator where it absorbs heat

from the air conditioning chilled water or ambient air and vapourize at low

pressure.

e) The vapour returns to compressor to begin a new refrigeration cycle


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GAS UTILIZATION EQUIPMENT – FURNACE

A type of heat exchanger where the fuel is burned in an enclosure to heat a process

material contained in tubular coils, tubes or in an enclosure itself. Other common names

are direct-fired heater, heaters, fired heaters and tube-still heaters. Mostly used in high

temperature industries such as ceramic, ferrous or non-ferrous, glass and refining

industries.

Process Refining Furnace – Main Component

Burner

- deliver heat to the furnace

- capable of providing uniform heating environment

- oil, gas or oil/gas burners

Radiant section

- hottest area in a furnace

- heat transfer to process materials radiation primarily from flames and flue gases,

through hot refractory on walls also radiate heat. Secondarily is by convection (5-

10% of total radiant section

Convection section

- typically preheats process materials before they go to the radiant section

- convection heat transfer from flue gases


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Process Refining Furnace

The physical size and heat absorption capabilities of a furnace vary depending on its

design-the capacity of a furnace is expressed as its heat-absorption capability expressed

in million BTU per hour (MMBtu.hr). Furnaces are sometimes grouped by the following

characteristic:

• orientation of the tubes in the radiant section – horizontal or vertical

• types of air supply and flue gases removal methods – induced, forced or natural

draft


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Table 7-7: Modern furnace vs traditional furnace

Traditional furnace Modern furnace

• large furnace chamber for heat

release/transfer

• mainly radiation

• densely refractory lining

• high thermal capacity

• slow heat up, difficult controllability

• stratification

• forced draught firing

• well-stirred furnace interior

• high exit velocity burner

• good temperature uniformity

• jet-driven recirculation

• high convection heat transfer

• lightweight construction

• rapid heat up, good

controllability

• heat recovery


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GAS FIRED FURNACE

Classification

a) Method of firing –

i) directly fired: process materials directly contact with hot heating gases

ii) indirectly fired: process materials contact with radiation heat transferred

from solid partition, e.g. refractory or some other metals, muffle, radiant

tube etc

b) Method of load handling – intermittent (batch) or continuous

GAS FURNACE - REFRACTORY

Function: To isolate or form a heat barrier by means of resisting materials in the form of

lining within structure in such a way that maximum heat amount of initial heat input will

be contained and passed only to workload

The properties required from a refractory material are:

1. As high fusion or melting point as possible (primary requirement)

2. Resistant at working temperature to deformation under load

3. Minimum volume change with temperature

4. Adequate mechanical strength and insensivity to temperature change

5. High resistance to wear

6. Adequate chemical resistance to slag attack

7. Suitable thermal properties

8. Electrical non-conductance, if necessary9. Uniform and constant composition

Refractories are materials that retain their shape and chemical stability for an acceptable

period when subjected to high temperature.


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Table 7-8: Properties of refractory brick

GAS UTILIZATION EQUIPMENT - BOILER

A boiler is a pressure vessel where water is heated to produce hot water or steam. This

hot water or steam is utilized for space heating or commercial and industrial buildings or,

in the case of high pressure steam, for process application.

The boiler derives its heat energy from the burning of the fuel in the furnace of the boiler

– the resulting flue gases transfer their heat energy to the water contained within the

boiler as they circulate through the tubes of the boiler.

CLASSIFICATION

Boilers are classified as

a) steam or hot water

b) fire tube or water tube

c) horizontal flue or vertical flue

d) type of boiler and burner combinations (package and non-package)


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Steam boilers

• low pressure for building space heating

• or high pressure to generate process steam for dry cleaning or food processing

Hot water boilers

• for conventional space heating of commercial and industrial buildings are limited

to a relatively low pressure of 160 psig and water temperature < 121oC

• however for large commercial space heating applications involving long runs of

distribution piping (e.g. at airports, universities and hospitals), operating

temperatures and pressure from 121-221oC and 55-350 psig

Fired Tube boiler

• Hot flue gases generated in the furnace of the boiler, flow through the fire tubes

transferring their heat to the boiler water within the shell surrounds the fire tubes

• The water or steam pressure on the boiler tubes is exerted on the outside which

tends to collapse the tubes

• This requires the tube to be made relatively thick and heavy, placing practical

limits on the size of fire tube boilers

Figure 7-8: Fired Tube boiler


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Water tube boilers

• Consists of one more drums (or headers) with connecting tubes, arranged so that

water is contained in the drums and tubes

• Unlike fire tube boilers, the hot flue gases flow around the outside of the water

tubes

• The pressure on the tubes is from the inside

• This arrangement allows water tube boilers to contain high operating pressures

as compared to fire tube boilers

• Water tube boilers can be built in large sizes to operate at highly operating

pressures, such as large steam generators for power plant operation

Figure 7-9: Water Tube Boiler


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Horizontal flue boiler

• The flue gases are drawn horizontally across the boiler’s heating-surface in a

number of passes

•

In a two pass boiler the flue gases have a relatively short travel time between theburner and the flue outlet of the boiler resulting in a relatively low pressure drop

or a draft loss across the boiler

• However, less heat is transferred and lower efficiency is obtained than for boilers

having three and four passes

• As the number of passes in the boiler is increased, the efficiency of the boiler is

increased

• However, greater burner fan power or draft developed by the boiler stack is

required to overcome the higher pressure drop or draft loss across the boiler dueto the restrictive design of the boiler

Vertical flue boilers

• The flue gases flow vertically up through

the boiler in a single pass arrangement

• Because of this distinct single pass

arrangement, most vertical flue boilers

are fired by an atmospheric burner,unlike horizontal flue boilers which are

usually fired by a fan assist or a forced

draft type of burner.

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