Modeling the Combustion of Coal in a 300MW Circulating Fluidized Bed Boiler with Aspen Plus

Zhihui DONG Changqing DONG* Junjiao ZHANG Yongping YANG National Engineering Laboratory for Biomass Power Generation Equipment

North China Electric Power University Beijing, China102206

*Corresponding author:cqdong1@163.com

AbstractA process model was proposed to predict the combustion of coal in a 300MW CFB boiler. The effects of coal feed flow rate, the temperature of preheated air and air flow rate on the exhaust gas temperature were analyzed. The result showed that the exhaust gas temperature and boiler efficiency was consistent with data from literature.

Keywords: CFB; model;exhaust gas temperature;Aspen Plus

I. INTRODUCTION CFBC technologies have been paid more attention in the

past decades due to its advantages, such as a wide variety of solid fuel, high combustion efficiency, low pollutant emissions, smaller combustor cross section, fewer feed points, good turndown and load capability [1]. In order to settle the problem brought from energy utilization and environmental pollution, both capacity and numbers of CFB boilers in use are increasing [2].

The combustion of coal and/or biomass (sludge, wood waste, RDF, etc.) in a circulating fluidized bed has been a commercial topper for over 20 years[3], and references to principles and applications are numerous and widespread.

Afsin Gungor and Nurdil Eskin developed a dynamic 2D model for a CFB combustor to validate against the data from a pilot-scale 50 kW CFB combustor and an industrial-scale 160 MW CFB combustor which used different types of coal. The influence of different operational conditions such as excess air, bed operational velocity and particle diameter on bed temperature and the overall CO, NOx and SO2 emissions from the combustor were investigated with the model [4, 5].Hong Chen, Shuwen Chang studied combustion tests of 300MW CFB boiler with firing Xiao Long tan lignite, providing reasonable optimal operation method:oxygen content at economizer outlet 2%, primary air flow rate 280 kNm3/hratio of inner and outer secondary air 80: 20 and bed temperature 850 [6].

The Aspen Plus process simulator has been used by different investigators to simulate coal conversion, examples included indirect coal liquefaction process, integrated coal gasification combined cycle power plants, atmospheric fluidized bed combustor process, compartmented fluidized

bed coal gasifiers and coal gasification simulation [7]. R.Soutdeh-Gharebaagh proposed a model integrating hydrodynamic parameters, reaction model and kinetic subroutines necessarily to simulate coal combustion in a CFBC.Only considering the important steps of coal combustion, the reaction model was simulated using four Aspen Plus reactor models and several subroutines.The resulting model was applied to predict the performance of The CANMET CFBC pilot plant in terms of combustion efficiency, emission levels of CO, SO2 , NO, O2 and CO concentration profiles. The validity of the model was demonstrated using 14 different sets of operating conditions for the CANMET 0.8 MWth CFBC pilot Plant [8, 9].

But most of research was about coal combustion in the boiler furnace, the work of simulating coal-fired CFB boiler system is still seldom, especially including boiler heating surfaces, cyclones and external heat exchangers [10].

In this work, the performance of 300MW CFB boiler system was simulated by Aspen Plus software, and Xiao Long tan lignite with the property of high volatile and moisture content, low ash component, medium sulfur content, lower ignition point and easier burnout was used. Although few data concerning the operation of large scale CFB-units and ripe operation experience about 300MW CFB burning lignite coal(Vdaf >37%) is presented, the main goal of the modeling of CFB boiler is to constitute a system that predicts boiler efficiency and exhaust gas temperature[11,12]. Some guidance to the 300MW CFB operation will be given.

II. MODEL DESCRIPTION

A. Simulation Diagram In order to show the overall operation of CFB process, the

different stages considered in Aspen Plus simulation, were decomposition of the feed, coal decomposition, coal combustion, gas-solid separation, the heated surfaces at backpass and external heat exchangers. The simulation diagram was given in Fig.1

One DECOM(RYield) block to simulate the decomposition of the coal obtained constituting components including carbon, hydrogen, oxygen, nitrogen, sulfur, ash and moisture in the form of simple substances, by specifying the yield distribution

978-1-4244-4813-5/10/$25.00 2010 IEEE

according to the coal ultimate analysis, proximate analysis and the heat of coal combustion on a dry basis. Coal combustion

used BURN (RGibbs) reactor in conformity with the

Figure1. Simulation diagram based Aspen Plus

assumption that components reactions followed the Gibbs equilibrium. The amount of volatile materials can be specified from the coal approximate analysis, also considering the assumption that char contained only carbon and ash.

Endothermic process of heating surfaces inside boiler chamber was modeled with the module vapor (Heater), by which thermal and phase conditions of outlet stream, and water in the heating surfaces boiled by heat stream from BURN reactor were determined. SEPARATE (Ssplit) block was adopted to provide bottom ash manually [13].

The material flow from SEPARATE block entered into CYCLONE (Cyclone) block to represent gas-solid separation at the riser outlet of furnace by setting the diamter and numbers of CYCLONE to reach proportion of gas-solid separation.CYCLONE separated an inlet gas stream containing solids into a solid stream and a gas stream carrying the residual solids. The flue gas stream heated water in the heated surfaces at backpass simulated by a series of module MHEATX, which ensured an overall energy balance but did not account for the exchanger geometry.

The solid stream from CYCLONE was divided into three flows by CYCL-SEP (Fsplit) block as a solid drain valve. CYCLE-1 stream directly entered into BURN reactor, and another two streams were going to external heat exchangers to release heat to steam in order to make fuel and desulfurizer circulating and burning several times.External heat exchangers included low temperature superheater, middle temperature superheaters and high temperature reheater [14].

B. Model Assumptions The following assumptions were considered in the

simulation:

Process was steady state and isothermal Coal decomposition took place instantaneously and

products mainly consisted of H2, N2, O2, H2O, S, C and ASH

Char only contained carbon and ash

Carbon completely reacted C. Initial Parameters

To validate the CFB model, two kinds of coal were selected as the model analysis data from the literature, as listed in TABLE. The input variables of the process at rated load were summarized in TABLE.

TABLE I. INITIAL PARAMETERS AT RATED LOAD

Fuel Mcoal(t/h) MCaO(t/h) Mair(t/h) Ca/S

DESIGNED 226.5 14.87 1142.0 2.0

CHECKED 236.7 17.16 1095.5 2.0 Notes: Mcoal was coal feed flow rate; MCaO was mass flow of CaO in limestone;

Mair was theoretic air flow rate; Ca/S was calcium to sulphur mole ratio.

III. RESULTS AND DISCUSSION

A. Model Results Two kinds of lignite were applied to calculate boiler

efficiency and exhaust gas temperature with comparisons and errors listed in TABLE. The result from the model had shown good agreement with literature data. Discrepancies were less than 9% owing to the fact that the algorithm of this model was based on the equilibrium calculations. Complex combustion in the furnace was simplified, all gaseous products were assumed to behave ideally and all condensed products were treated as pure phases except ash. Heat transfer process of all heating surfaces differed from actual behavior.

B. Model Analysis The designed coal was selected as model analysis data

presented at rated load of boiler. Exhaust gas temperature versus air preheater inlet temperature, combustion air flow rate and coal feed flow rate were studied in this paper.

As can be seen in Fig.2, exhaust gas temperature increased linearly with air preheater inlet temperature.When air preheater inlet temperature changed from 20 to 80, the exhaust gas temperature varied from 78 to 130. The reason of exhaust gas temperature increasing with air preheater inlet temperature was that great temperature

difference between gas and steam improved much exchange heat.

TABLE II. PROXIMATE AND ULTIMATE ANALYSIS OF FUEL

Fuel Moisture

% as received

Proximate analysis % as dry basis Ultimate analysis % as dry basis

Volatile Fixed carbon Ash C H O N S

DESIGNED 34.7 43.46 39.01 17.53 56.23 2.86 19.28 1.57 2.54

CHECKED 36.12 40.88 37.13 21.99 51.89 3.98 18.5 0.81 2.83

TABLE III. COMPARISION BETWEEN LITERATURE DATA AND MODEL CALCULATION

Comparison

Combustion conditions

Exhaust gas temperature () Error (%)

Boiler efficiency (%) Error (%)

lit cal lit cal

DESIGNED 135 123 8.89 93.4 89.4 4.3

CHECKED 133 130 2.25 94.22 89.05 5.5 The boiler efficiency was defined as follows:

( ) ( )' 'gr gr gr zr zr zrcoal

D I I D I IQ M

+

=

Symbol Meaning expression boiler efficiency, % Q low heat value as received, kJ/kg

Mcoal coal feed flow rate, kg/h

Dgz flow rate of superheated steam, kg/h

Dzr flow rate of reheated steam, kg/h

Igr enthalpy of superheated steam outlet, kJ/kg

Igr enthalpy of feed water, kJ/kg

Izr enthalpy of reheated steam outlet, kJ/kg

Izr enthalpy of reheated steam inlet, kJ/kg

20 30 40 50 60 70 80

80

90

100

110

120

130

Designed value Checked value

Exha

ust g

as te

mp.

()

Air preheater inlet temp. ( )

Figure2. Effect of air preheater inlet temperature to exhaust gas temperature

Considering two kinds of coal with the same air preheater inlet temperature of 75, the checked fuel containing higher moisture and ash, lower value heat resulted in increasing flue flow rate and exhaust gas temperature.

The effect of air flow rate to the exhaust gas temperature was showed in Fig.3. From the figure it can be observed air

100 102 104 106 108 110 112 114 1160

20

40

60

80

100

120

140

160

180

200

Exha

ust g

as te

mp.

()

Air flowrate (x104kg/h)

Designed value Checked value

Figure3. Effect of air flow rate to exhaust gas temperature

flow rate for combustion ranged from 1000 to 1150 kg/h, while exhaust gas temperature varied from 122 to 132, overall tend of which had a little drop.The reason above was that with air flow rate increasing, the air absorbed much heat of flue gas and overall tend of exhaust gas temperature decreased. Comparing designed coal with checked coal, at the same value of air flow rate, exhaust gas temperature of designed coal was lower because the checked fuel containing higher Moisture and ash, lower value heat resulted in increasing flue flow rate and exhaust gas temperature.

Exhaust gas temperature effected by coal feed rate was presented in Fig.4. When the designed coal feed rate was 226500 kg/h at rated load of boiler, exhaust gas temperature was 123. It can be seen the amount of coal 260000 kg/h was a knee point, While exhaust gas temperature increased to the maximum value 330 .The fact that exhaust gas

temperature decreased with coal feed rate above 260000 kg/h was that coal had burned incompletely with certain amount of air and unreacted fuel absorbed much heat in the furnace, so less heat was released to flue gas at backpass. From the curve it can be observed the checked coal feed rate was 236700 kg/h at rated load of boiler, exhaust gas temperature was 130. Flow rate of checked coal 257500 kg/h was a knee point, while exhaust gas temperature increased to a maximum value 267 and then decreased with coal feed flow rate increasing.

210 220 230 240 250 260 270

0

50

100

150

200

250

300

350

Designed value Checked value

Exha

ust g

as te

mp.

()

Coal feed flowrate (x103kg/h)

Figure4. Effect of coal feed flow rate to exhaust gas temperature

IV. Conclusions A model was developed for the operation process of CFB

using Aspen Plus simulator. To provide such a CFB operation model, Several Aspen Plus unit operation blocks were combined and initial parameters were quoted from literature. The result model was used to predict the performance of two kinds of coal in terms of boiler efficiency and exhaust gas temperature, and the calculating results satisfied well with quoted data. Exhaust gas temperature increased linearly with air preheater inlet temperature, which was predicted by the model ranged from 20 to 80, while Exhaust gas temperature varied between 78 to 130 .When air flow rate for combustion ranged from 1000 to 1140 kg/h, Exhaust gas temperature varied from 122 to 132, overall tend of which had a little drop. With the increase of coal feed rate, Exhaust gas temperature was growing up to a top value, then coal burning incompletely with certain amount of air and unreacted fuel absorbing much heat led to a decrease of exhaust gas temperature. The agreement between the model prediction and experimental data is satisfactory but more experimental data are still required to confirm the proposed CFBC model in order to make it more comprehensive and reliable.

ACKNOWLEDGMENT The authors thank the financial support for this work

provided by National Basic Research Program of China (2009CB219801), National High Technology Research and Development Program (2008AA05Z302), National Natural Science Foundation of China (50976032), Nature Science Foundation of Beijing (3101001, 3083027), the key project of

Ministry of Education of China (108033, 107119), and Doctoral Program of North-China Electric Power University (200822015).

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[4] Afsin Gungor, Two-dimensional coal combustion modeling of CFB,FUEL 87 (2008) 14531468.

[5] Qinhui Wang and Zhongyang Luo, A mathematical model for a circulating fluidized bed (CFB) boiler, Energy 24 (1999) 633653.

[6] Hong Chen and Shuwen Chang, Experimental study of burning brown coal in a 300MW CFB boiler,BOILER MANUFACTURING. 213 (2009)1-4.

[7] Mehrdokht B.Nikoo and Nader Mahinpey,Simulation of biomass gasification in fluidized bed reactor using ASPEN PLUS, BIOMASS AND BIOENERGY 32 (2008)12451254.

[8] R.Sotudeh-Gharebaagh,R.Legros,J.Chaouki and J.Paris, Simulation of circulating fluidized bed reactors using ASPEN PLUS, Fuel Vol. 77, No. 4, pp. 327-337, 1998.

[9] Legros, R., Brereton, C. M. H., Lim, C. J., Li, H., Grace, J. R.and Anthony, E. J. , Combustion characteristics of different fuels in a pilot scale circulating fluidized bed combustor,12th International Conference on FBC,Proceedings, Vo1.2, ed. Lynn N. Rubow. ASME, NewYork, 1993, pp. 66 l-666.

[10] Senior R. C., Circulating fluidised bed fluid and particle mechanics: modelling and experimental studies with application to combustion. Ph.D. dissertation,The University of British Columbia, Vancouver, Canada, 1992.

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[13] Yasemin Bolkan, Franco Berruti.Modeling circulating fluidized bed downers, Powder Technology 132 (2003) 85 100.

[14] Aspen Plus User Guide (Version 10. 2) . Aspen Technology ,Inc.ors

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