Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296 283

INVESTIGATION OF PRE-DRYING LIGNITE IN AN

EXISTING GREEK POWER PLANT

by

Michalis AGRANIOTIS a*

, Sotirios KARELLAS b,

Ioannis VIOLIDAKIS a,b

, Aggelos DOUKELIS b,

Panagiotis GRAMMELIS a, and Emmanuel KAKARAS

a,b

a Centre for Research & Technology Hellas, Institute for Solid Fuels Technology and Applications, Ptolemais, Greece

b Laboratory of Steam Boilers and Thermal Plants, National Technical University of Athens, Athens, Greece

Original scientific paper DOI: 102298/TSCI110509120A

The application of lignite pre-drying technologies in next generation of lignite power plants by utilizing low pressure steam as a drying medium instead of hot recirculated flue gas – combined with thermal utilization of the vaporized coal moisture – is expected to bring efficiency increase of 2-4 percentage points in fu-ture lignite power plants compared with today’s state-of-the-art. The pre-drying concept is of particular importance in Greek boilers firing lignite with a high water and ash content. The combustion of Greek pre-dried lignite has been investigated experimentally and via numerical simulations in our previous research. This study focuses on the potential integration of a lignite pre-drying system in an existing Greek power plant with dry lignite co-firing thermal share of up to 30%. The radiative and convective heat fluxes to the boiler and the over-all boiler heat balance is calculated for reference and dry lignite co-firing condi-tions by an in-house calculation code. The overall plant’s thermal cycle is then simulated using commercial thermal cycle calculation software. The net plant ef-ficiency is in this way determined for reference and dry coal co-firing conditions. According to the simulation results the integration of a pre-drying system and the implementation of dry lignite co-firing may bring an efficiency increase of about 1.5 percentage points in existing Greek boilers. It is therefore considered as an important measure towards improving plant efficiency and reducing specific CO2 emissions in existing plants.

Key words: pre-drying, dry lignite co-firing, thermal cycle calculation

Introduction

The development of future lignite-fired power plant technology will be driven by

some of the main goals and challenges regarding climate protection, energy savings, and

resource conservation as well as competitiveness. Therefore, one of the major tasks in power

plant engineering is efficiency enhancement.

*nCorresponding author; e-mail: agraniotis@certh.gr

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant 284 THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296

An essential parameter for increasing efficiency is the drying process. The lignite

drying method applied has critical influence on the efficiency of the power plant, as lignite

drying is an energy-intensive process due to the high moisture content and energy can,

therefore, be saved by optimizing the process. The optimization of the process in future brown

coal plants is calculated to lead to an efficiency increase of 2 to 4 percentage points [1, 2].

The pre-drying method in particular, is predicted to play an important role for all

future types of lignite-based power plants discussed today. Among these are the steam power

plant with optimized efficiency, as well as the IGCC with and without carbon capture, and the

oxy-fuel plant. Lignite pre-drying is based on the idea of utilizing low temperature heat for

drying purposes and recovering the waste heat from the drying process [3].

The fluidized bed drying concept with waste heat utilization (WTA-drying) is one of

the most promising brown coal pre-drying technologies, currently under development. WTA

pre-drying is a technology that can be integrated with all the aforementioned future routes of

lignite based electricity generation – whether to increase efficiency or to effectively capture CO2

or in gasification routes or coal-to-liquid processes, where pre-drying is a vital element, also [4].

The moisture content of raw lignite is high, exceeding 50%. Thus, in conventional

lignite-fired power plants a portion of the fuel’s heat is consumed in the boiler during

combustion and milling/drying to evaporate coal-inherent water in a very high temperature

level, leading in high exergy losses. Additionally, this heat cannot be used in the plant process

and is lost since this coal-inherent water leaves the power plant as steam contained in the flue

gas fig. 1(a) [3].

With the WTA technology, drying is made exergetically more efficient by

performing it at a low temperature level in a separate operating unit upstream of the steam

generator and utilizing the energy of evaporated coal-inherent water in the power plant

process, either in the feed water preheaters or in the WTA dryer for evaporation fig. 1(b) [3].

The “WTA fine grain” drying concept has been successfully demonstrated in a pilot

plant in Frechen, Germany and a prototype fluidised bed dryer has been constructed and

integrated to the steam cycle of the 1000 MWe brown coal power plant in Niederaussem,

Germany.

Figure 1. (a) Conventional drying system and (b) fluidized bed drying concept

Some of the previous pre-drying investigations performed so far at the Laboratory of

Steam Boilers and Thermal Plants in the National Technical University of Athens

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296 285

(LSBTP/NTUA) involve the comparison of different pre-drying technologies based on

thermodynamic calculations in which the WTA-concept has been proven as one of the most

promising concepts, drying tests of Greek lignite in a lab scale fluidised bed dryer in which

the equilibrium curves have been determined, and combustion tests with Greek dry lignite in a

semi-industrial scale facility in which, among others, temperature and emission measure-

ments, fly ash sampling and investigation of slagging and fouling tendency have been carried

out. Recent work includes the examination of possible oxy-fuel configurations for future

Greek power plants burning Greek dried lignite in terms of boiler and steam cycle optimization

and the possibilities of upgrade and economic impact of pre-drying on a state-of-the-art Greek

lignite power plant through the integration of lignite pre-drying technology [5-11].

The present work is involved with the numerical investigation of the potential

integration of the WTA pre-drying concept in an existing Greek brown coal power plant and

the impact of its implementation on the boiler performance and the overall plant efficiency. In

this case, design data of the 339 MWe Agios Dimitrios V unit in Kozani are used for modeling

the project. Thermal performance and behavior of the plant for raw and dry lignite co-firing

cases is evaluated and predicted through an in-house built code in the LSBTP/NTUA. The

raw lignite firing case is used as a reference case for confirmation and comparison and

thereafter, some cases of thermal share substitution of the boiler input through co-firing dry

lignite are modeled. The cases of thermal share substitution examined are those of 10, 20, and

30% of the initial input. The steam extracted from lignite pre-drying in each case is used as a

heating medium in the power plant’s feed water pre-heaters. The overall plant’s thermal cycle

is afterwards simulated by using commercial thermal cycle calculation software.

Methodology

Description of the 339 MWe Agios Dimitrios unit V facility

Agios Dimitrios V in Kozani is a unit with a boiler consisting of the furnace where

the pulverized fuel is burnt and the circulated water is evaporated, and of six more gas and

water/steam heat exchangers, forming altogether the economizer (ECO), superheater (SH),

and reheater (RH) surfaces. The superheater is constituted of three parts, while the reheater of

two and the economizer of one. The set-up of the facility and the order of the heat exchanger

surfaces after the furnace, according to the gas flow, fig. 2, is: 1. SH2,

2. SH3-RH2,

3. SH1,

4. RH1, and

5. ECO.

The last parts of the superheater and

reheater (SH3 and RH2) come along as a pair.

There is also a hot gas recirculation for drying

the raw lignite.

The unit also includes 3 steam turbines

(high and intermediate pressure-1 HP/2 IP)

and 7 water preheaters. Two streams of water

downstream of the preheaters are used as

Figure 2. Layout of the furnace and the heat exchanger surfaces of Agios Dimitrios V unit

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant 286 THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296

cooling spray water between the different parts of the superheater and another one downstream of the pump for the reheater. The full water/steam cycle with all the auxiliary units is shown in fig. 3.

In-house built code

The code developed in the LSBTP/NTUA for the calculation of the boiler’s thermal cha-racteristics uses the theory according to the German FDBR regulation [12].

Furnace calculations

For the furnace calculations, the combus-tion chamber is separated into three sections, according to fig. 4. Section 1 includes the bottom of the furnace (region below the flame), section 2 is the flame region and sec-tion 3 includes the upper part of the furnace and the combustion chamber “neck”.

The total heat flux radiated from the flame and the hot gas to the sections 1, 2, and 3 of the furnace, is:

tot 1 2 3 3GQ Q Q Q Qtot 1 2 3 3Gtot 1 2 3 3Gtot 1 2 3 3Q Q Q Q Qtot 1 2 3 3Q Q Q Q Qtot 1 2 3 3tot 1 2 3 3Gtot 1 2 3 3Gtot 1 2 3 3Q Q Q Q Qtot 1 2 3 3Q Q Q Q Qtot 1 2 3 3GQ Q Q Q QGtot 1 2 3 3Gtot 1 2 3 3Q Q Q Q Qtot 1 2 3 3Gtot 1 2 3 3

where 1Q1Q1Q1 , 2Q2Q , 3:Q3:Q3Q3 is the radiated heat flux from the flame towards sections 1, 2, and 3, respectively, and 3GQfrom the flame towards sections 1, 2, and

3GQGQG – the radiated heat flux from the gas that occupies the space above the flame section (section 3).

This total heat flux is equal to the thermal power offered by the gas in the furnace:

F F,otot max( )G G G GQ Q m h hF F,o

( )F F,o

( )F F,oF F,oG G G GF F,o

( )G G G G( )F F,o

( )F F,oG G G GF F,o

( )F F,o

( )Q Q m h h( )( )G G G G( )Q Q m h h( )G G G G( )F F,o

( )F F,oG G G GF F,o

( )F F,o

Q Q m h hF F,o

( )F F,oG G G GF F,o

( )F F,oF F,otot maxG G G GF F,oG G G GF F,otot maxG G G Gtot maxF F,otot maxF F,oG G G GF F,otot maxF F,o

Q Q m h htot maxQ Q m h htot maxtot maxtot maxG G G Gtot maxF F,otot maxF F,oG G G GF F,otot maxF F,oQ Q m h htot maxQ Q m h htot maxG G G GQ Q m h hG G G Gtot maxG G G Gtot maxQ Q m h htot maxG G G Gtot maxF F,otot maxF F,oG G G GF F,otot maxF F,oQ Q m h h

F F,otot maxF F,oG G G GF F,otot maxF F,oF F,oF F,oG G G GF F,oF F,otot maxF F,oG G G GF F,otot maxF F,oF F,oG G G GF F,oF F,otot maxF F,oG G G GF F,otot maxF F,oF F,o( )

F F,oG G G GF F,o( )

F F,oF F,otot maxF F,o( )

F F,otot maxF F,oG G G GF F,otot maxF F,o( )

F F,otot maxF F,o( )Q Q m h h( )( )G G G G( )Q Q m h h( )G G G G( )

F F,o( )

F F,oG G G GF F,o( )

F F,oQ Q m h h

F F,o( )

F F,oG G G GF F,o( )

F F,otot max( )tot maxG G G Gtot max( )tot maxQ Q m h htot max( )tot maxG G G Gtot max( )tot maxF F,otot maxF F,o( )

F F,otot maxF F,oG G G GF F,otot maxF F,o( )

F F,otot maxF F,oQ Q m h h

F F,otot maxF F,o( )

F F,otot maxF F,oG G G GF F,otot maxF F,o( )

F F,otot maxF F,oF F,oF F,oG G G GF F,oF F,otot maxF F,oG G G GF F,otot maxF F,oF F,otot maxF F,oG G G GF F,otot maxF F,oF F,otot maxF F,o( )

F F,otot maxF F,oG G G GF F,otot maxF F,o( )

F F,otot maxF F,oQ Q m h h( )Q Q m h h( )G G G GQ Q m h hG G G Gtot maxG G G Gtot maxQ Q m h htot maxG G G Gtot maxF F,otot maxF F,oG G G GF F,otot maxF F,oQ Q m h h

F F,otot maxF F,oG G G GF F,otot maxF F,o( )G G G G( )Q Q m h h( )G G G G( )tot max( )tot maxG G G Gtot max( )tot maxQ Q m h htot max( )tot maxG G G Gtot max( )tot maxF F,otot maxF F,o( )

F F,otot maxF F,oG G G GF F,otot maxF F,o( )

F F,otot maxF F,oQ Q m h h

F F,otot maxF F,o( )

F F,otot maxF F,oG G G GF F,otot maxF F,o( )

F F,otot maxF F,o

where GmGm is the gas mass flow, maxGh – the enthalpy in adiabatic flame temperature, and

F,oGh – the enthalpy corresponding to the combustion chamber outlet temperature.

The radiated heat flux from the flame towards sections 1, 2, and 3 are calculated as follows:

4 41 s 1 o1 f W( ) ( )Q C A T T1 s 1 o1 f WQ C A T T1 s 1 o1 f WQ C A T T1 s 1 o1 f W

4 44 4( ) ( )4 4( ) ( )4 4( ) ( )Q C A T T( ) ( )4 4( ) ( )4 4Q C A T T4 4( ) ( )4 4( ) ( )Q C A T T( ) ( )Q C A T T( ) ( )1 s 1 o1 f W1 s 1 o1 f W( ) ( )1 s 1 o1 f W( ) ( )1 s 1 o1 f W1 s 1 o1 f W( ) ( )1 s 1 o1 f WQ C A T T1 s 1 o1 f W( ) ( )1 s 1 o1 f W1 s 1 o1 f W1 s 1 o1 f W( ) ( )1 s 1 o1 f W1 s 1 o1 f WQ C A T T1 s 1 o1 f W1 s 1 o1 f W( ) ( )1 s 1 o1 f WQ C A T T1 s 1 o1 f W( ) ( )1 s 1 o1 f W4 4( ) ( )4 4( ) ( )4 4

1 s 1 o1 f W( ) ( )1 s 1 o1 f W( ) ( )Q C A T T( ) ( )4 4( ) ( )4 4Q C A T T4 4( ) ( )4 41 s 1 o1 f W( ) ( )1 s 1 o1 f WQ C A T T1 s 1 o1 f W( ) ( )1 s 1 o1 f WQ C A T T1 s 1 o1 f WQ C A T T1 s 1 o1 f W( ) ( )Q C A T T( ) ( )1 s 1 o1 f W( ) ( )1 s 1 o1 f WQ C A T T1 s 1 o1 f W( ) ( )1 s 1 o1 f W1 s 1 o1 f WQ C A T T1 s 1 o1 f WQ C A T T1 s 1 o1 f W1 s 1 o1 f WQ C A T T1 s 1 o1 f WQ C A T T1 s 1 o1 f W1 s 1 o1 f WQ C A T T1 s 1 o1 f WQ C A T T1 s 1 o1 f W

Figure 3. Water-steam cycle

Figure 4. Combustion chamber sections andmain dimensions

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296 287

4 42 s 2 o2 f W( ) ( )Q C A T T2 s 2 o2 f WQ C A T T2 s 2 o2 f WQ C A T T2 s 2 o2 f W

4 44 4( ) ( )4 4( ) ( )4 4( ) ( )Q C A T T( ) ( )4 4( ) ( )4 4Q C A T T4 4( ) ( )4 4( ) ( )Q C A T T( ) ( )Q C A T T( ) ( )2 s 2 o2 f W2 s 2 o2 f W( ) ( )2 s 2 o2 f W( ) ( )2 s 2 o2 f W2 s 2 o2 f W( ) ( )2 s 2 o2 f WQ C A T T2 s 2 o2 f W( ) ( )2 s 2 o2 f W2 s 2 o2 f W2 s 2 o2 f W( ) ( )2 s 2 o2 f W2 s 2 o2 f WQ C A T T2 s 2 o2 f W2 s 2 o2 f W( ) ( )2 s 2 o2 f WQ C A T T2 s 2 o2 f W( ) ( )2 s 2 o2 f W4 4( ) ( )4 4( ) ( )4 4

2 s 2 o2 f W( ) ( )2 s 2 o2 f W( ) ( )Q C A T T( ) ( )4 4( ) ( )4 4Q C A T T4 4( ) ( )4 42 s 2 o2 f W( ) ( )2 s 2 o2 f WQ C A T T2 s 2 o2 f W( ) ( )2 s 2 o2 f WQ C A T T2 s 2 o2 f WQ C A T T2 s 2 o2 f W( ) ( )Q C A T T( ) ( )2 s 2 o2 f W( ) ( )2 s 2 o2 f WQ C A T T2 s 2 o2 f W( ) ( )2 s 2 o2 f W2 s 2 o2 f WQ C A T T2 s 2 o2 f WQ C A T T2 s 2 o2 f W2 s 2 o2 f WQ C A T T2 s 2 o2 f WQ C A T T2 s 2 o2 f W2 s 2 o2 f WQ C A T T2 s 2 o2 f WQ C A T T2 s 2 o2 f W

4 43 s 3 o3 f W( ) ( )Q C A T T3 s 3 o3 f WQ C A T T3 s 3 o3 f WQ C A T T3 s 3 o3 f W

4 44 4( ) ( )4 4( ) ( )4 4( ) ( )Q C A T T( ) ( )4 4( ) ( )4 4Q C A T T4 4( ) ( )4 4( ) ( )Q C A T T( ) ( )Q C A T T( ) ( )3 s 3 o3 f W3 s 3 o3 f W( ) ( )3 s 3 o3 f W( ) ( )3 s 3 o3 f W3 s 3 o3 f W( ) ( )3 s 3 o3 f WQ C A T T3 s 3 o3 f W( ) ( )3 s 3 o3 f W3 s 3 o3 f W3 s 3 o3 f W( ) ( )3 s 3 o3 f W3 s 3 o3 f WQ C A T T3 s 3 o3 f W3 s 3 o3 f W( ) ( )3 s 3 o3 f WQ C A T T3 s 3 o3 f W( ) ( )3 s 3 o3 f W4 4( ) ( )4 4( ) ( )4 4

3 s 3 o3 f W( ) ( )3 s 3 o3 f W( ) ( )Q C A T T( ) ( )4 4( ) ( )4 4Q C A T T4 4( ) ( )4 43 s 3 o3 f W( ) ( )3 s 3 o3 f WQ C A T T3 s 3 o3 f W( ) ( )3 s 3 o3 f WQ C A T T3 s 3 o3 f WQ C A T T3 s 3 o3 f W( ) ( )Q C A T T( ) ( )3 s 3 o3 f W( ) ( )3 s 3 o3 f WQ C A T T3 s 3 o3 f W( ) ( )3 s 3 o3 f W3 s 3 o3 f WQ C A T T3 s 3 o3 f WQ C A T T3 s 3 o3 f W3 s 3 o3 f WQ C A T T3 s 3 o3 f WQ C A T T3 s 3 o3 f W3 s 3 o3 f WQ C A T T3 s 3 o3 f WQ C A T T3 s 3 o3 f W

where Cs is the Stefan-Boltzman constant (= 5.6697 10–8 Wm2/K), fT – the average flame temperature, wT – the average wall temperature, ε1, ε2, ε3 are the emissivity for sections 1, 2, and 3, respectively, Ao1, Ao2, Ao3 the – equivalent radiated surface of sections 1, 2, and 3, respectively, while the radiated heat flux from the gas that occupies section 3 is:

4 43 s 3 3 f 3 f W( ) ( )GQ C v A T T3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f W

4 44 4( ) ( )4 4( ) ( )4 4( ) ( )Q C v A T T( ) ( )4 4( ) ( )4 4Q C v A T T4 4( ) ( )4 4( ) ( )Q C v A T T( ) ( )Q C v A T T( ) ( )3 s 3 3 f 3 f W3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f W3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f W3 s 3 3 f 3 f W3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f W3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f W3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f W4 4( ) ( )4 4( ) ( )4 4

3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f W( ) ( )Q C v A T T( ) ( )4 4( ) ( )4 4Q C v A T T4 4( ) ( )4 43 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f W( ) ( )Q C v A T T( ) ( )3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f W( ) ( )3 s 3 3 f 3 f W3 s 3 3 f 3 f WGQ C v A T T3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f WGQ C v A T TG3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f W3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f WQ C v A T T3 s 3 3 f 3 f W

where fA l bA l bA l b is the area of the flame as seen in fig. 5, and ν3 – a coefficient depending on the average gas temperature of section 3.

The total radiated heat flux, the sum of the aforementioned heat fluxes, is only partly absorbed by the evaporating water, as there is heat loss in the combustion chamber. The absorbed part of the heat flux is:

W tot wQ Q nW tot wQ Q nW tot wQ Q nW tot wW tot wQ Q nW tot wQ Q nW tot w

where w tot a tot/n Q Q Qw tot a totn Q Q Qw tot a totn Q Q Qw tot a totw tot a totw tot a tot/n Q Q Qw tot a totn Q Q Qw tot a tot/n Q Q Q/w tot a tot/w tot a totn Q Q Qw tot a tot/w tot a tot is the relative thermal coefficient that indicates the heat loss to the environment and aQ

w tot a totw tot a totw tot a totn Q Q Qw tot a tot

aQ – the heat loss of the combustion chamber.The absorbed heat flux is also calculated as the heat flux received from the water in

the furnace during vaporizing:

SD D D,F W( )Q m h hSD D D,F WSD D D,F WS

( )S

( )SD D D,F W( )D D D,F WSD D D,F WS

( )SD D D,F WS

( )Q m h h( )D D D,F W( )D D D,F WQ m h hD D D,F W( )D D D,F WD D D,F WQ m h hD D D,F WQ m h hD D D,F WD D D,F WQ m h hD D D,F WQ m h hD D D,F WD D D,F WD D D,F WD D D,F W( )D D D,F W( )Q m h h( )D D D,F W( )D D D,F WQ m h hD D D,F W( )D D D,F WD D D,F WD D D,F WD D D,F W( )D D D,F WQ m h hD D D,F WQ m h hD D D,F W( )Q m h h( )D D D,F W( )D D D,F WQ m h hD D D,F W( )D D D,F W

where DmDm is the steam production mass flow, D,Fh – the furnace outlet steam enthalpy, and SWh – the furnace inlet water enthalpy.

Heat exchanger calculations

The total amount of thermal power transferred at each heat exchanger is the amount of heat power received from the water/steam which is equal to the amount of heat power removed from the gas passing through the heat exchanger:

i iexch,i D gQ Q Qi iexch,i D gi iexch,i D gi i

Q Q Qexch,i D gQ Q Qexch,i D gi iexch,i D gi iexch,i D gi iQ Q Qexch,i D gQ Q Qexch,i D gi iexch,i D gi iQ Q Q

i iexch,i D gi i

where i

out inDD D D( )Q m h hout in( )out in( )out in

D D( )D D( )Q m h h( )out in( )out inQ m h hout in( )out inD D( )D DQ m h hD D( )D Di

Q m h hi

Q m h hiDQ m h hDQ m h hout in( )Q m h h( )out in( )out inQ m h hout in( )out in

D D( )D DQ m h hD D( )D DQ m h hDQ m h hD( )Q m h h( ) is the heat flux received from the water/steam at each heat exchanger i,

iin out

g g g g( )Q m h houtg g g g( )out( )outg g g g( )g g g g

( )D D( )D DD D( )D DQ m h hD D( )D D( )( )Q m h h( )D D( )D DQ m h hD D( )D D( )Q m h hDQ m h hD( )Q m h h( )Q m h hQ m h hg g g gQ m h hg g g gig g g giQ m h h

ig g g gi

( )( )Q m h h( )D D( )D DQ m h hD D( )D DQ m h hQ m h hg g g gQ m h hg g g g

( ) is theg g g g

( )inin

( )in

( )out

g g g g( )out( )outg g g g( )g g g g( )Q m h h( )in( )inQ m h hin( )ing g g g( )g g g gQ m h hg g g g( )g g g g

( )D D( )D DD D( )D DQ m h hD D( )D DD D( )D DQ m h hD D( )D D

g g g g

( )D D( )D D( )Q m h h( )D D( )D DQ m h hD D( )D D( )( )Q m h h( )D D( )D DQ m h hD D( )D D

g g g gg g g g( )g g g gQ m h hQ m h hg g g gQ m h hg g g g( )Q m h h( )( )Q m h h( )g g g g( )g g g gQ m h hg g g g( )g g g g – the heat flux removed from the gas at each heat exchanger i, exch. mQ KA

g g g gg g g g

exch. mQ KAexch. mQ KAexch. m

g g g g

exch. mig g g gig g g gi

Q m h hg g g gig g g giQ m h hg g g gQ m h hg g g gig g g giQ m h h

ig g g gig g g gg g g gQ m h hg g g gg g g gg g g gg g g g( )g g g gg g g gQ m h hg g g gg g g g( )g g g gQ m h hg g g g( )g g g g

exch. mQ KAexch. mQ KAexch. m

g g g gg g g gg g g gg g g g

exch. mQ KAexch. mQ KAexch. m – the total amount of thermal power exchanged, A – the total heat exchanger surface, ž θm – the average logarithmic temperature, K = fKth – the (corrected) total heat transfer coefficient of the heat exchanger, Kth – the (theoretical) total heat transfer coefficient of the heat exchanger, and f – the heat exchanger pollution factor.

Τhe average logarithmic temperature is expressed by the inlet and outlet tempera-tures of gas and water/steam through the following equation:

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant 288 THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296

Fig

ure

5.

Flo

wsh

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f A

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s D

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ref

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ce c

ase

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296 289

in in out outg D g D

in ing D

out outg D

m in out out ing D g D

in outg Dout ing D

,for co-current flowln

,for counter-current flowln

,for co-current flowin ing D

out outg D

,for counter-current flowin outg Dout ing D

in in out outin in out outg D g D ,for co-current flowin in out outg D g Dg D g Dg D g Dg D g Dg D g D

in ing Dlnln out out

g Dm in out out in

g D g D ,for counter-current flowin out out ing D g Din out out inin out out inin out out inin out out ing D g Din out out inin out out ing D g Dg D g Dg D g Dg D g D

in outg Dlnlnln out ing D

while the theoretical total heat transfer coefficient is expressed through:

tho o o

g i i w

11 1ln

2

K d d da d d ag i i wa d d ag i i wa d d ag i i w

1 1d d d1 1d d d1 1o o o1 1o o o1 1o o o1 1o o o1 1o o olno o olno o oo o o1 1o o olno o o1 1o o o1 1d d d1 1o o o1 1o o od d do o o1 1o o o1 1ln1 1d d d1 1ln1 1o o o1 1o o olno o o1 1o o od d do o o1 1o o olno o o1 1o o o

a d d a2a d d a2ln

a d d a

where ag = g,conv + s is the heat transfer coefficient by convection and radiation of gas,aw – the heat transfer coefficient of water/steam, λ – the thermal conductivity of the material of pipes, and do and di – the external and internal diameters of the pipeline, respectively.

Case study

The fuel used as a reference case is raw lignite with 54% moisture. A certain amount of the fuel is dried to a point of 12% moisture and part of the thermal input is substituted by dry lignite. Apart from the raw-lignite-firing case, which is considered as a reference case,during the co-firing simulation, three cases of thermal substitution are considered:– 10% substitution of the boiler's initial thermal input, – 20% substitution of the boiler's initial thermal input, and– 30% substitution of the boiler's initial thermal input.

In each case, the mixture of raw and dry lignite is considered as an equivalent fuel with a composition as shown in tab. 1.

Table 1. Fuel composition

Fuel ultimate analysis

Referencefuel

Pre-dried lignite,12% moisture

Thermal share of substitution (% thermal)10% 20% 30%

C 18.00 34.43 18.75 19.60 20.58H 1.45 2.77 1.51 1.58 1.66N 0.50 0.96 0.52 0.54 0.57O 8.50 16.26 8.85 9.26 9.72S 0.44 0.84 0.46 0.48 0.50

Ash 14.60 27.93 15.21 15.90 16.69Water 54.00 12.00 52.08 49.90 47.41CO2 2.51 4.80 2.61 2.73 2.87

Hu [MJkg–1] 5418.00 12593.609 5746.07 6118.05 6543.38

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant 290 THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296

Fig

ure

6.

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s D

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30

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her

ma

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itu

tion

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296 291

Some of the various other modeling parameters are the fuel, air, gas, and working medium (water/steam) conditions (tab. 2). The code calculates the furnace first and then the heat exchangers in the direction of water/steam flow, setting initial values for gas and water/steam temperatures. The procedure is then repeated until the values of the temperatures and heat fluxes converge. A parametric investigation was first carried out, so that the results of the code at the reference case can accurately match the data from the technical report of the facility. In the co-firing cases, cooling water mass flow was adjusted to keep the temperature of the steam below a threshold, which is 540

°C for both the last part of the superheater (SH3) and the reheater (RH2)

and 520 °C for the intermediate part of the superheater (SH2). Mass flow, temperature, and pressure of the feed water entering the economizer were also held constant in each case, as well as the temperature and pressure of the steam entering the reheater (tab. 2).

Table 2.

Thermal share of substitution (% thermal) 0%

(Reference) 10% 20% 30%

Fuel

conditions

Fuel mass flow [kgs–1] 167.5 157.94 148.33 138.69

Fuel mass flow [tnh–1] 603 569 534 499

Air

conditions

Ambient temperature Ta [°C] 20

Temperature of preheated air [°C] 268.2

Oxygen mass content in air [kgkg–1] 0.2321

Humidity [kgkg–1 dry air] 0.01

Air ratio 1.2

Air mass flow [kgs–1] 448.89 440.03 431.17 422.31

Gas

conditions

Total mass flow [kgs–1] 643.39 623.52 603.65 583.79

Main mass flow to the heat exchangers [kgs–1] 590.56 572.320 554.083 535.85

Mass flow to the gas recirculation ducts [kgs–1] 52.83 51.201 49.570 47.938

Water/

Steam

conditions

Steam boiler (economizer) inlet mass flow [kgs–1] 249.87

Steam boiler (economizer) inlet temperature [°C] 236.3

Steam boiler inlet pressure [bar] 233.3

Reheater inlet temperature [°C] 287.7

Reheater inlet pressure [bar] 34.2

The ΗP and IP turbine steam extraction mass flows were considered constant in each case for the extractions 4-7 (fig. 3), but a greater amount of steam is extracted from the steam extraction 3 for lignite drying, while the extraction 1 is blocked and isolated in the 10% substitution case, and both the extractions 1 and 2 likewise in the other cases (20% and 30%) as the preheaters which used those extractions are now heated from the dried lignite vapour.

The overall net efficiency of the plant is then calculated manually using the known

mass flow and enthalpy of steam through the steam turbines, the steam extraction mass flows

and the efficiency of the generator [13].

Commercial code (IPSE PRO) model and assumptions

During the commercial software IPSE PRO modelling, the same co-firing cases as before are examined and the fuel composition is the same as mentioned in tab. 1. In this case

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant 292 THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296

though, the full water/steam cycle is modelled, including every part of the facility (steam turbines, pre-heaters and extractions, condenser etc.).

Based on the reference case given plant data, the heat exchanger transfer coefficients are calculated. The particular coefficients are taken as constant in the dry coal co-firing cases

Ιn this model,feed water mass flow entering the steam boiler (economizer inlet) is also considered constant while cooling spray water mass flow is adjusted according to the needs in order for the temperature of the superehaters and reheater to be kept below the same threshold in each case (540

oC) and subsequently feed water through the water pump is also

adjusted and it is not held. The steam boiler inlet water temperature is also different in each case and it is determined by the previous upstream preheaters.

This model also includes lignite drying which is modeled through a heat exchanger. The high temperature stream in the heat exchanger is the steam extraction from the steam turbine, and the low temperature stream is the water content of lignite that is evaporated in the drying process. Lignite is dried through this extra stream of steam extracted from extraction 3 (fig. 3) and extractions 1 and 2 used to heat the first and second preheater are replaced by the evaporated moisture of lignite, as mentioned previously.

The layout of the facility is represented in fig. 5 for the reference case and in fig. 6

for the 30% thermal substitution case.

Figure 7. Q-T diagram comparison of the reference case as calculated and predicted by

two methods

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296 293

Results

The predicted results for the refer-

ence case during calibration are quite

accurate for both methods. In the

following Q-T diagrams (fig. 7), a com-

parison between the technical speci-

fications manual data and the calculated

results is demonstrated.

The heat fluxes on the boiler heat

exchanger surfaces for the reference and

the dry coal co-firing cases are given in

fig. 8. An increase of the total useful heat

and the useful heat produced in the furnace section is predicted for the dry-coal firing cases,

due to the higher adiabatic temperature and the increase of radiative heat flux. The total useful

boiler heat flux increases from 824 to 840 MWt.

The calculated flue gas temperature at the furnace exit and the boiler exit is

presented in fig. 9. The furnace exit temperature increases from 957 °C in the reference case

to 1005 °C in the 30% co-firing case, while the boiler exit temperature decreases from 280 °C

in the reference case to 270 °C in the 30% co-firing case. The elevated flue gas temperatures

lead to an increase of the required cooling water mass flows (fig. 10).

Figure 9. Calculated flue gas temperature at furnace and boiler exit

Figure 10.Calculated mass flows of cooling water at super heater and reheater steam coolers

Taking into account that the total

thermal input remains constant in all

cases considered, the increasing total

useful heat leads to increased boiler

efficiency. The boiler efficiency calcu-

lated by the direct method increases ac-

cordingly 1.5 absolute percentage points

(fig. 11). This improvement is also con-

firmed by the lower flue gas temperature

values at the boiler exit. The Q-T dia-gram (fig. 12) illustrates more vividly

the changes in temperatures and useful heat due to thermal substitution. The calculated plant

efficiency rate also increases 0.8 absolute points from 35.4% to 36.2% (fig. 11).

Figure 8. Prediction of the produced useful heat at each heat exchanger

Figure 11. Calculated boiler efficiency rate and net plant efficiency rate

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant 294 THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296

Figure 12. Q-T diagram for the reference case and the 10% and 30% substitution cases

A detailed analysis of thermal and electric power self-consumption is given in tab. 3.

Finally the calculated specific CO2 emissions decrease from 1272.048 kg/MWe in the

reference case to 1173.326 kg/MWe in the 30 % co-firing case (fig. 13).

Table 3. Thermal and electric power self consumption

Thermal share of substitution

[% thermal] Ref. case 10% 20% 30%

Electricity

Mills [kW] 14.446 13.001 11.557 10.112

Feed water circulation

(pumps) [kW] 11.332 11.444 11.526 11.612

Total (kW) 25.778 24.446 23.082 21.724

Heat Drying steam heat

[kW] 0.000 5.752 11.514 17.267

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296 295

Conclusions

Dry lignite firing will play an important role

in the next generation of lignite power

plants. The pre-drying concept is of

particular importance in Greek boilers firing

lignite with a high water and ash content

[14-17]. In the current work, the integration

of a WTA dryer in an existing Greek power

plant is evaluated through thermodynamic

cycle calculations. The moisture evaporated

from the drying process is used as heating

medium and partially replaces the steam

bleed utilised for the first low pressure water

pre-heaters. The steam cycle of Agios Dimitrios V unit is taken into account for the

examination. According to the calculation results, increasing the share of dry coal in the total

thermal input, an increase in plant efficiency of up to 0.8 absolute percentage points is

predicted, and, therefore, reduced CO2 emissions can be achieved [8, 18-21].

To sum up, the overall evaluation of the investigations performed indicates that the

application of the WTA pre-drying concept is feasible in an existing Greek power plant

without great risks to the operational behaviour of the boiler. The installation of dry lignite

burners in an existing Greek power plant by replacing the start up oil burners with dedicated

dual burners firing oil and dry lignite dust could be a next step towards the application of the

particular technology in Greek power plants [22, 23]. Detailed CFD simulations on the impact

of such a co-firing concept on combustion related aspects of the furnace may be carried out.

These investigations should focus on the evaluation of the ash slagging propensity in the walls

near the dry coal burners and the estimation of the NOx emissions increase tendency due to

the adiabatic temperature increase after the implementation of dry coal co-firing. Moreover,

the design of the new generation of lignite boilers, where firing 100% of pre-dried lignite will

be implemented, is the further development step required for the full exploitation of the pre-

drying technology.

References

[1] Klutz, H-J., Moser, C., Block, D., WTA Fine Grain Drying – Module for Lignite-Fired Power Plants of the Future, Development and Operating Results of the Test Plant, VGB PowerTech, PT 11, Essen, Ger-many, 2006

[2] Agraniotis, M., et al., Experimental Investigation on the Combustion Behaviour of Pre-Dried Greek Lignite, Fuel Processing Technology, 90 (2009), 9, pp. 1071-1079

[3] Schwendig, F., Klutz, H-J., Ewers, J., The Dry Lignite-Fired Power Plant, RWE-VGB, PT 12, Essen, Germany, 2006

[4] ***, RWE Power: WTA TECHNOLOGY-A Modern Process for Treating and Drying Lignite, http://www.rwe.com

[5] Agraniotis, M., Substitution of Coal by Alternative and Supporting Fuels in Pulverised Fuel Boilers Towards Reducing CO2 Emissions, Ph. D. thesis, NTUA, Athens, 2010

[6] Kakaras, E., Ahladas, P., Syrmopoulos, S., Computer Simulation Studies for the Integration of an External Dryer into a Greek Lignite-Fired Power Plant, Fuel, 81 (2002), 5, pp. 583-593

[7] Agraniotis, M., et al., Numerical Investigation on the Combustion Behaviour of Pre-Dried Greek Lig-nite, Fuel, 88 (2009), 12, pp. 2385-2391

Figure 13. Specific CO2 emissions

Agraniotis, M., et al.: Investigation of Pre-Drying Lignite in an Existing Greek Power Plant 296 THERMAL SCIENCE, Year 2012, Vol. 16, No. 1, pp. 283-296

[8] Agraniotis, M., et al., Dry Lignite Co-Firing in a Greek Utility Boiler: Experimental Activities and Numerical Simulations, Energy & Fuels, 24 (2010), 10, pp 5464-5473

[9] Agraniotis, M., et al., Investigation of Lignite Pre-Drying in a Modern Greek Power Plant Towards Zero CO2 Emissions, Proceedings, 24th International Conference on Efficiency, Costs Optimization, Simula-tion, and Environmental Impact of Energy Systems, ECOS 2011, Novi Sad, Serbia, 2011, pp. 619-629

[10] Kakaras, E., et al., Ultra-Supercritical Power Plant Fired with Low Quality Greek Lignite, Proceedings, 20th International Conference on Efficiency, Costs Optimization, Simulation, and Environmental Impact of Energy Systems, ECOS 2007, Padova, Italy, 2007

[11] Kakaras, E., et al., Oxyfuel Boiler Design in a Lignite-Fired Power Plant, Fuel, 86 (2007), 14, pp. 2144-2150

[12] ***, FDBR-HANDBUCH: Warme und Stromung Technik, Vulkan Verlag, Essen, Germany, 1975 [13] Violidakis, I., Development of a Computational Code for the Prediction of Pulverised Fuel Boiler's

Thermal Data-Application in a Greek Steam Boiler for the Case of Raw and Dry Lignite Co-Firing (in Greek), Diploma thesis, National Technological University of Athens, NTUA, Athens, 2008

[14] Maier, J., et al., Investigation of Particle Behaviour of Rawand Pre-Dried Brown Coal and Bituminous Coal in a 20 kW and in a 500 kW Test Facility, Proceedings, 23rd International Conference on Coal Uti-lization & Fuel Systems, Clearwater, Fla., USA, 1998, pp. 9-13

[15] Maier, J., et al., Milling, Drying and Combustion of Low Rank Coal in a 500 kWth Pulverized Coal Test Facility, Proceedings, International Symposium on Clean Coal Technology, Xiamen, China, 1997, pp. 322-328

[16] Maier, J., Hein, K. R. G., Effect of Pre-Drying on P. F. Combustion, Fly Ash and Emission Behavior of Different European Low-Rank Fuels, Tagungsband: 18th Annual International Pittsburgh Coal Confe-rence, Newcastle, N. S. W., Australia, 3-7, 2001

[17] Chatel-Pelage, F., et al., Applications of Oxygen for NOx Control and CO2 Capture in Coal-Fired Plants, Thermal Science, 10 (2006), 3, pp. 119-142

[18] Koukouzas, N., et al., CO2 Capture and Storage in Greece: a Case Study from Komotini NGCC Power Plant, Thermal Science, 10 (2006), 3, pp. 71-80

[19] Ravi Kumar, N., et al., Thermodynamic Analysis of Heat Recovery Steam Generator in Combined Cycle Plant, Thermal Science, 11 (2007), 4, pp. 143-156

[20] Kakaras, E., et al., Fluidized Bed Combustion with the Use of Greek Solid Fuels, Thermal Science, 7 (2003), 2, pp. 33-42

[21] Harkin, T., Hoadley, A., Hooper, B., Process Integration Analysis of a Brown Coal-Fired Power Station with CO2 Capture and Storage and Lignite Drying, Energy Procedia, 1 (2009), 1, pp. 3817-3825

[22] Kaß, H., Tappe, S., Krautz, H. J., The Combustion of Dry Lignite under Oxy-Fuel Process Conditions in a 0.5 MWth Test Plant, Energy Procedia, 1 (2009), 1, pp. 423-430

[23] Bosoaga, A., The Combustion of Pulverised Low Grade Lignite, Fuel, 85 (2006), 10-11, pp. 1591-1598

Paper submitted: September 5, 2011 Paper revised: August 22, 2011 Paper accepted: August 22, 2011