A steady state model for predicting performance of small-scale up-draft coal gasifiers

Fuel xxx (2015) xxx–xxx

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A steady state model for predicting performance of small-scale up-draft coal gasifiers Giorgio Cau a, Vittorio Tola a,⇑, Alberto Pettinau b a b

DIMCM, Department of Mechanical, Chemical and Materials Engineering, University of Cagliari, Italy Sotacarbo S.p.A., c/o Grande Miniera di Serbariu, Carbonia, Italy

h i g h l i g h t s  Fixed-bed gasification process is simulated through an Aspen Plus-based model.  Syngas composition and process parameters are assessed in different conditions.  An Alaskan lignite has been considered as reference fuel for the reported simulations.  Model results are compared with experimental results from a pilot-scale unit.  The comparison shows that the model well represents the experimental results.

a r t i c l e

i n f o

Article history: Received 8 August 2014 Received in revised form 14 March 2015 Accepted 17 March 2015 Available online xxxx Keywords: Fixed-bed gasifier Simulation model Steady-state

a b s t r a c t Small-scale fixed-bed coal and biomass gasifiers represent an attractive option for distributed combined heat and power generation. As known, gasification phenomena are very complex, involving drying, devolatilization, pyrolysis, heterogeneous and homogenous reactions, with a large number of intermediate and final products. Gasification processes are also influenced by reaction kinetics and fluiddynamical effects, such as temperature and concentration gradients. For this reason, simulation models are able to predict gasifiers performance under the assumption of thermodynamic equilibrium only if the gasification process takes place at a known temperature and the reaction time is lower than the reactants residence time. As a consequence, for fixed-bed gasifiers equilibrium models must consider drying and devolatilization taking place at lower temperature in the heat transfer zone, where solid feed is heated by syngas. Therefore, moisture and volatiles are not involved in the gasification reactions since they are released before reaching the reaction zone. Several models based on steady-state and one-dimensional representations have been developed to reproduce gasification processes in fixed-bed reactors. These models have been found adequate to provide information for engineering design and process optimization. In this framework a steady-state simulation model has been developed at the Department of Mechanical Chemical and Materials Engineering (DIMCM) of the University of Cagliari by using the Aspen Plus computer code for predicting performance of small-scale up-draft fixed-bed coal gasifiers. The model can be used to evaluate the mass and energy balance in each zone of the gasifier and the main characteristics of the syngas produced by the gasification process (composition, mass flow, temperature, heating value, etc.). This paper describes the model and presents the main results of a parametric analysis, which shows how the gasification process is influenced by the main operating parameters. Moreover, the results of the model have been compared with the experimental results of an up-draft gasifier fed with an lignite from Alaska. The above-mentioned gasifier is part of a pilot gasification and gas treatment plant built at the Sotacarbo Research Centre in Sardinia, Italy. The comparison shows that the model well represents the performance of the pilot-scale unit. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction

⇑ Corresponding author. E-mail address: [email protected] (V. Tola).

Coal gasification is an increasingly attractive option both for power generation and for the production of hydrogen, methanol, dimethyl-ether and other chemicals and clean fuels. In the future

http://dx.doi.org/10.1016/j.fuel.2015.03.047 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

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coal gasification processes integrated with high efficiency energy conversion systems, such combined cycles based on new generation gas turbine and high temperature fuel cells, will allow to achieve efficiencies as high as 50–55% [1]. Other clean coal technologies (CCT), such as, for example, advanced fluidized-bed combustion (FBC) plants are available today, but coal gasification, especially when integrated with combined cycle power plants (IGCC) and carbon capture and storage (CCS) systems is, for several specific applications, the most attractive one [2]. For technical and economic reasons, IGCC plants are currently used chiefly for largescale power generation (300–600 MW), and already achieve efficiencies as high as 43–46%, with very low pollutant emissions. However, research and development activities are now also focussing on small- and medium-scale applications, based on fixed- and fluidized-bed gasifiers, using air as oxidant instead of pure oxygen [3–5]. Among fixed-bed gasification processes, up-draft gasifiers have the advantages of high reliability, high efficiency, low specific emissions and feedstock flexibility, but they typically produce a syngas with a high tar content [6]. In addition, air-blown gasification is more available and simple than the oxygen-blown one [7], with significant advantages in particular for small-scale applications. The DIMCM and Sotacarbo are engaged in a long time cooperation concerning CCT, with an emphasys on small and medium scale coal gasification systems for combined heat and power (CHP) generation. Within this cooperation, DIMCM has developed a zero-dimensional, steady-state, fixed-bed gasification model (SFBG), implemented by using the Aspen Plus simulation tool, with the aims to investigate the gasification process in different operating conditions and to support the design of experiments in pilot gasification units. In particular, it was applied to design the recent experimental campaigns performed in the Sotacarbo gasification pilot plant. After a description of the model, this paper presents the results of a simulation analysis performed to determine the effects of the main assumptions and process parameters on the gasifier’s performance. Moreover, a comparison between the simulation and some corresponding experimental results obtained in the Sotacarbo pilot unit is reported.

2. Simulation of fixed-bed gasification processes In general, a gasification process includes a wide series of chemical and thermochemical reactions, occurring in different operating conditions depending on the geometry and the fluid dynamic of the reactor. The most significant reactions are summarized in Table 1 [8]. As well known, autothermal gasification is generally carried out by injecting substoichiometric air or oxygen (just to promote the combustion of a portion of fuel to provide heat for the endothermic reactions) and eventually steam. In general, an increasing of air (or oxygen) injection promotes fuel combustion, thus involving an increasing temperature of the process, higher CO2 concentrations and lower CO concentrations in raw syngas. On the other hand, an increasing in steam injection involves a temperature reduction and promotes shift conversion equation, with a subsequent increasing in H2 and CO2 concentrations in raw syngas despite a reduction of CO content. Thermodynamic equilibrium models are barely able to accurately predict performance and syngas composition in fixed-bed gasification processes. The temperature inside the gasifier is not constant and the overall process is very complex, involving drying, devolatilization, pyrolysis, combustion, heterogeneous (solid–gas phase) and homogenous (gas phase) reactions, with a large number

Table 1 Gasification main reactions. Reaction

Reaction name

C + O2 = CO2 2C + O2 = 2CO

Carbon combustion Carbon partial combustion Boudouard reaction Steam gasification CO-shift conversion Steam reforming Metanation Sulphur combustion Hydrogen sulphide formation Carbonyl sulphide formation Carbonyl sulphide hydrolysis NO2 formation Ammonia formation

C + CO2 = 2CO C + H2O = CO + H2 CO + H2O = H2 + CO2 CO + 3H2 = CH4 + H2O CO2 + 4H2 = CH4 + 2H2O S + O2 = SO2 SO2 + 3H2 = H2S + 2H2O CO + S = COS COS + H2O = H2S + CO2 N2 + 2O2 = 2NO2 N2 + 3H2 = 2NH3

Heat of reaction (kJ/mol) 393 221 +173 +131 412 206 165 297 207 +63 34 +66 46

of intermediate and final products. Besides, gasification processes are also influenced by reaction kinetics and fluid-dynamic effects such as temperature and concentration gradients, which depend on reactor characteristics and gasification technology [9, 10]. For this reason, equilibrium models are able to predict more accurately the performance of entrained-flow and fluidised-bed gasifiers, since temperature is almost uniform throughout the reactor [11– 14]. On the contrary, when applied to fixed- or moving-bed gasifiers, equilibrium models lead to large uncertainties in the reaction temperature, even though reactants residence time inside the gasifier is higher than reaction time, insofar as coal is heated by syngas, through countercurrent heat transfer [9,10,15–17]. Therefore, in fixed- and moving-bed gasifiers, coal drying, devolatilization and pyrolysis processes take place in the heat transfer zone at a lower temperature than in the gasification zone; consequently, moisture and volatiles are not generally involved in the gasification reactions since they are released before reaching the reaction zone. Since the 70s of the last century, several models have been developed for studying fixed-bed gasifiers. First models were zero-dimensional [18–21] or one-dimensional [9,13,16,22,23], considering steady-state [18,20,24,25] or transient [23,26,27] operation. Some of these models are listed as homogenous [9,13,19,21,28], some as heterogenous [10,16,22,23], which implies separate solid and gas temperature [29]. A detailed and complete description of these models is summarized by Hobbs et al. [29]. During the 90s, more complex fixed-bed gasifier models were developed. Starting from MBED-1 model by Hobbs et al. [16], Radulovic et al. [30,31] developed the FBED-1 model including an advanced devolatilization sub-model. This model was later modified and improved by Monazam and Shadle [32]. More recent models were developed by Morea-Taha [33], Brundu et al. [34,35], Grana et al. [36], De Souza-Santos [37] and Kulkarni and Ganguli [38]. In recent years more complex CFD (computational fluid dynamics) models were also developed for evaluating fixed-bed gasifier performance, for example, by Murgia et al. [39], Rogel and Aguillon [40] and Yang et al. [41].

3. The steady-state fixed-bed gasification (SFBG) model As mentioned, a computer simulation model for predicting performance of fixed-bed coal gasification processes has been developed over the past few years at the DIMCM [42,43] and implemented through the Aspen PlusÒcommercial software package. The model provides an accurate evaluation of chemical and physical properties of both coal and syngas evolving into the

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gasifier and of the interactions between coal and syngas flowing countercurrent. It determines raw syngas composition as well as gasifier mass and energy balances, imposing main feed flows and operating parameters. The SFBG model calculates the mean temperature in the gasification and combustion zone as a function of both steam/coal and oxidant/coal ratios, coal composition and temperatures of main inflows (coal, steam and oxidant). The model also provides an accurate evaluation of the thermodynamic characteristics of syngas and ash exiting the gasifier. In particular syngas exit temperature is calculated considering the countercurrent heat transfer processes between syngas and coal inside the different gasifier sections. A suitable minimum temperature difference between gas and solid phases is imposed in the different sections. The model requires to accurately characterize the coal volatile matter since as much as 30–50% of coal mass can be lost by devolatilization. In fact, in fixed-bed gasifiers devolatilization products exit from the top of the reactor, being carried away by the hot syngas flowing countercurrent, as they are released from coal without reacting in the gasification and combustion zone. As a consequence, syngas outlet composition strictly depends on the volatile matter composition. In the developed model, devolatilization process is assumed to occur instantaneously and devolatilized gas composition is taken equal to volatile matter composition. For fixed-bed gasifiers this assumption is justified by the large coal residence time compared to minor time required for devolatilization. The model calculates the composition of volatile gases, in terms of CO, CO2, H2 and CH4, through a mass balance, imposing suitable yields of tar, water and volatile gases in the volatile matter (VM) composition. Fixed carbon (FC), which represents the organic residual that remains after subtracting moisture, volatile matter and ash, determined by proximate analysis, may contain, in addition to carbon, small amounts (parts per thousand) of hydrogen, oxygen and nitrogen, and about a half of the total sulphur contained in coal [44]. Conventionally, the model assumes that fixed carbon does not contain oxygen and hydrogen (assumed to be fully contained in the volatile matter) while nitrogen and sulphur are completely included in the fixed carbon. Starting from these assumptions and imposing a suitable CO/CO2 mass ratio in the VM gases (here assumed equal to 2), the mass balance allows to define a reliable VM gases composition. As for tar, a portion of it leaves the reactor together with produced syngas, whereas the remaining takes part in the combustion and gasification process. To simulate this phenomenon, the model considers the recirculation of a portion of produced tar to the gasification zone. The SFGB model first considers coal as a non conventional solid (NCPSD, non conventional with particle size distribution) as it is composed of a number of chemical elements mixed in a solid which is not present in the Aspen Plus software database. Coal needs to be characterized by both ultimate and proximate analyses and particles size distribution. Enthalpy and density calculation methods must be also specified so as to determine the lower heating value (LHV), energy of formation and heat capacity. For these reasons the compositions of volatile matter (water, carbon dioxide, hydrogen, carbon monoxide, methane, tar, etc.), char (carbon, sulphur, nitrogen, etc.) and ash (SiO2, Fe2O3, SO3, Al2O3, CaO, MgO, TiO2, K2O, Na2O, P2O5, etc.) also need to be known. In Fig. 1 the fixed bed gasification process is schematised into several different blocks: coal drying, coal devolatilization, char gasification and combustion, steam and oxidant preheating [13]. All the blocks operate at the same pressure, thus neglecting pressure drops. A simplified scheme of the SFBG simulation model for fixed-bed gasifiers is given in Fig. 2, showing main process modules (real or fictitious) and flows. The actual Aspen Plus model scheme is far more complex than that shown in Fig. 2. The gasifier

coal

syngas

Coal prehating and drying

Coal devolatilization

Char gasification and combustion

Steam and oxidant preheating

ash

gasification agents

Fig. 1. Simplified scheme of the fixed bed gasification process.

is fed by coal, steam and air and it produces syngas and a mixture of ash and unreacted coal. In the fuel preparation zone (not shown in the simplified schemes of Figs. 1 and 2) coal is milled, if required, and introduced into the gasifier. Coal enters the gasifier at the top of the reactor and is preheated by the exiting syngas losing its moisture. In the SFBG thermal energy required for coal drying is furnished through a fictitious countercurrent direct heat exchanger (DRYING) by the hot syngas leaving the gasification section. Steam from the drying section is mixed with the hot syngas that consequently shows a high moisture content, typical of fixed-bed gasifiers. The simulation of pyrolysis and gasification processes with the Aspen Plus software requires a reactor (DEC-COAL) where dried coal is decomposed in the three remaining components of the proximate analysis: fixed carbon, volatile matter and ash. Coal releases the volatile matter to form another non-conventional solid just composed of fixed carbon and ash. Volatile matter, which is then heated by the hot syngas leaving the bottom of the gasifier, is further decomposed into VM gases, water (vapour) and tar (this process is simulated through the DEC-VOL module). Water vapour and VM gases released from the volatile matter, and also the moisture from coal drying, are mixed with syngas which leaves the gasification zone. Tar and fixed carbon, decomposed into their elementary components (carbon, hydrogen, oxygen, nitrogen and sulphur), are heated and mixed to form the so-called char, that feeds the gasification section. Fixed carbon and tar are heated in series by hot syngas and the heating process is simulated through fictitious heat exchangers imposing a suitable minimum temperature difference. The simulation of gasification and combustion processes with the SFBG model requires the reactions to take place in a reactor (REACTOR), fed by the gasifying agents (water vapour and air)

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Raw Syngas Coal

DRYING

Coal Moisture

Syngas

DEC-COAL

DEC-VOL

Volatile Matter

Gases

Fixed Carbon

Water

TAR

Char

Ash

Hot Syngas Steam

REACTOR

Ash

Air

ASH-COOL Fig. 2. Simplified scheme of the Aspen Plus-based gasification model.

preheated by ash cooling (ASH-COOL). Into this reactor, the combustion reactions provide the heat required by the endothermic gasification reactions, thus producing a syngas with a sufficiently high temperature to sustain the devolatilization and pyrolysis processes. In the REACTOR block, the gasification mass and energy balances are solved considering thermodynamic equilibrium and minimizing Gibbs free energy considering the following 16 chemical species: CO, CO2, H2O, H2, O2, N2, CH4, SO, SO2, H2S, COS, HCN, NH3, NO, NO2, and Ar. Species are selected taking into account the products of the main gasification reactions reported in Table 1. If necessary the Aspen Plus library allows to add other species. 4. Main model assumptions The influence of the main process parameters (gasification temperature and pressure, steam/coal and air/coal ratios, thermal energy losses, carbon conversion rate, etc.) on gasifier performance has been investigated. All the results here reported have been obtained considering a lignite from Alaska (Usibelli coal mine) as feedstock; as a matter of facts, it is currently the best performing fuel among those tested in the Sotacarbo unit [45], mainly due to ist high volatile content (after devolatilization, the porous structure of the fuel particle promotes the gasification reactions) and the relative low ash percentage. As mentioned, the gasifier operates at a pressure slightly higher than atmospheric one (0.14 MPa). As stated, the SFBG simulation model requires the following input data: (i) proximate (fixed carbon, volatile matter, moisture and ash) and ultimate (carbon, hydrogen, sulphur, nitrogen, oxygen, moisture and ash) coal analyses; (ii) coal’s lower heating value; (iii) volatile matter (water, tar and volatile gases), volatile gases (carbon monoxide, carbon dioxide, hydrogen and methane) and tar composition; (iv) oxidant composition, pressure, temperature and oxidant/coal mass ratio; (v) steam pressure, temperature and steam/coal mass ratio; (vi) thermal energy losses; (vii) carbon

conversion rate; (viii) temperature of the ash exiting the gasifier; (ix) rate of tar and ash in raw syngas; (x) rate of air not reacted during gasification and combustion process. Table 2 reports the main properties of the Usibelli coal (as received). In particular, proximate, ultimate and thermal analyses have been determined in the Sotacarbo laboratories according to the international standards. With respect to other coals of the same rank, the Alaskan coal presents a higher volatiles content and a lower heating value. Regarding the composition of the volatile matter and volatile gases, as no specific data were available for the Usibelli coals, the composition shown in Table 3 was assumed. Data reported in the literature for similar low-rank coals show that rates of tar and moisture around 15% and 20% respectively could be suitably assumed [21]. Tar was assumed to be composed of 90 wt% by carbon and of 10 wt% by hydrogen. VM gases composition was calculated in order to satisfy the material balances for the VM gases. 5. Results and discussion A sensitivity analysis was carried out in order to evaluate gasifier performance as a function of the main process parameters. In particular, the analysis is based on the following given parameters: (i) air/coal mass ratio a; (ii) steam/coal mass ratio l; (iii) thermal energy losses; (iv) carbon conversion rate; (v) volatile matter composition (water, tar and volatile matter gases). Sensitivity analysis was performed by varying these parameters independently each other and considering, as reference data, the values reported in Tables 3 and 4. Air and steam are fed to the gasifier at a temperature of 120 °C and at a pressure of 0.14 MPa. The reference air/coal a and steam/coal l mass ratios are taken equal to 2.0 (corresponding to an oxygen/coal mass ratio of 0.46) and 0.4, respectively. In addition, a carbon conversion rate of 95% (on the basis of the experimental experience with lignites) and thermal energy losses equal to 5% were considered. Ash is completely removed from the bottom of gasifier and tar from volatiles entirely react into the gasification zone. Fig. 3 shows the mean temperature in the gasification and combustion section, the temperature of syngas exiting the gasifier, the syngas LHV and the gasifier cold gas efficiency as a function of the air/coal mass ratio a. As expected, an increase of a leads to a higher gasification temperature, due to the higher oxygen availability for combustion reaction. In the range 1.0–1.5, oxygen availability is not enough to assure a temperature higher than 700 °C in the gasification and combustion zone. The temperature increases remarkably for a higher than 1.5, since combustion reactions prevail against gasification ones. Temperature higher than 1600 °C can

Table 2 Coal properties. Standard

Value

Proximate analysis (% by weight) Fixed carbon Volatile matter Ash Moisture

ASTM ASTM ASTM ASTM

5142-04 5142-04 5142-04 5142-04

31.33 41.00 10.02 17.64

Ultimate analysis (% by weight) Total carbon Hydrogen Nitrogen Sulphur Oxygen Ash Moisture

ASTM D 5373-02 ASTM D 5373-02 ASTM D 5373-02 ASTM D 4293-05 By difference ASTM D 5142-04 ASTM D 5142-04

48.56 5.96 0.50 0.18 17.14 10.02 17.64

Thermal analysis LHV (MJ/kg)

Calculated

19.95

D D D D

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5

Tar (% by weight) C H

17.67 5.62 65.21 11.50 0.90 0.10

be reached for an air/coal mass ratio of 3.0. A similar trend can be found for syngas temperature at the exit of the gasifier: an increasing of a from 1.0 to 1.5 involves an increasing of syngas temperature from about 60 to 110 °C. For a higher than 1.5, syngas temperature increases remarkably up to 983 °C (corresponding to a = 3.0). As expected, syngas LHV decreases with the air/coal mass ratio, especially if combustion reactions prevail against gasification ones (a higher than 1.5). For an air/coal mass ratio lower than 1.5, a LHV slight reduction is offset by the mass flow increase leading to a maximum value (about 0.91) of cold gas efficiency. For an air/coal mass ratio higher than 1.5, despite the increasing of syngas mass flow, cold gas efficiency is reduced, due to the strong reduction of LHV. Fig. 4 shows the dry syngas composition (in terms of N2, H2, CO, CO2 and CH4 molar fractions) as a function of air/coal mass ratio a. Dry syngas is mainly composed by nitrogen, with a concentration in the range 30–57% by volume; hydrogen and CO stay in the ranges 15–36% and 13–21%, respectively; CO2 concentration varies between 8% and 13% and CH4 is about 2–5%. An increase of air/coal mass ratio obviously leads to a higher nitrogen molar fraction and also to a lower hydrogen molar fraction due to combustion reactions. CO and CO2 reach the maximum and minimum concentration respectively for an air/coal mass ratio of about 1.8. Fig. 5 shows the same parameters reported in Fig. 3 (mean temperature in the gasification and combustion section, the temperature of syngas exiting the gasifier, the syngas LHV and the gasifier cold gas efficiency) as a function of steam/coal mass ratio l. An increasing of steam/coal mass ratio slightly reduces gasification temperature (about 70 °C, from 1115 °C to 1045 °C). On the contrary, outlet syngas temperature is slightly higher (from about 445 °C to 455 °C), despite a lower gasification temperature, due to the higher heat capacity of steam. An increase of the steam/coal mass ratio leads to a lower LHV for dilution and despite a higher syngas mass flow also to a slightly lower cold gas efficiency. An increase of l promotes shift conversion reaction leading to lower CO and higher H2 and CO2 concentrations (Fig. 6). Besides water–

Table 4 Main operating process parameters. Operating conditions Steam/coal mass ratio Steam conditions (°C/MPa) Air/coal mass ratio Air conditions (°C/MPa) Gasifier pressure Gasifier energy losses (%) Carbon conversion rate (%) Temperature of exiting ash (°C) Rate of tar in raw syngas (%) Fraction of fly ash (%) Rate of air not reacted (%) a

Function of gasifier temperature.

0.4 120/0.14 2 120/0.14 Atmospheric 5.0 95.0 496a 0.0 0.0 10.0

1800

0.9

1600

0.85

1400

0.8

1200

0.75

1000

0.7

800

0.65

600

Syngas exit temperature Gasification temperature Syngas Lower Heating Value Cold Gas Efficiency

0.6 1000 900 800 700 600 500 400 300 200 100 0 1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

400 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2

2.8

Gasification temperature (°C)

Volatile gases (% by volume) CO CO2 H2 CH4

15.00 20.00 65.00

0.95

Syngas LHV (MJ/kg)

Water TAR Volatile gases

Syngas exiting temperature (°C)

Volatile matter (% by weight)

3

Air/coal mass ratio α Fig. 3. Mean temperature in the gasification and combustion section, temperature of syngas exiting the gasifier, syngas LHV and gasifier cold gas efficiency as a function of air/coal mass ratio a.

gas shift conversion reaction increases number of moles (anything but H2O), thus reducing nitrogen molar fraction in the dry syngas. Fig. 7 shows the same parameters of Figs. 3 and 5 as a function of thermal energy losses. In the analysis thermal energy losses were considered in the range 0% (adiabatic gasifier) and 10% (referred to the chemical energy of feeding coal). An increase of thermal energy losses leads to a remarkable reduction of mean temperature in the gasification and combustion zone and, consequently, of syngas temperature. The gasification temperature drops

0.6

0.55

0.5

0.45

Syngas molar fractions, dry basis

Table 3 Volatile matter, volatile gases and tar composition.

Cold gas efficiency

G. Cau et al. / Fuel xxx (2015) xxx–xxx

CO CO2

0.4

H2 CH4

0.35

N2 0.3

0.25

0.2

0.15

0.1

0.05

0 1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

Air/coal mass ratio α Fig. 4. Syngas composition as a function of air/coal mass ratio a.

Please cite this article in press as: Cau G et al. A steady state model for predicting performance of small-scale up-draft coal gasifiers. Fuel (2015), http:// dx.doi.org/10.1016/j.fuel.2015.03.047

1600

0.9

1400

0.85

1200

0.8

1000 800

0.75

Syngas exit temperature Gasification temperature Syngas Lower Heating Value Cold Gas Efficiency

0.7 600

600 6

550

5.5

500

5

450

4.5

400

Syngas LHV (MJ/kg)

Syngas exiting temperature (°C)

0.95

Gasification temperature (°C)

G. Cau et al. / Fuel xxx (2015) xxx–xxx

Cold gas efficiency

6

4 0.3

0.32 0.34 0.36 0.38

0.4

0.42 0.44 0.46 0.48

0.5

Steam/coal mass ratio μ Fig. 5. Mean temperature in the gasification and combustion section, temperature of syngas exiting the gasifier, syngas LHV and gasifier cold gas efficiency as a function of steam/coal mass ratio l.

from about 1200 °C (adiabatic gasifier) to 950 °C (10% of thermal losses), whereas corresponding syngas outlet temperatures are about 635 °C and 260 °C respectively. An increase of thermal energy losses leads to a slightly lower LHV, and a subsequent reduction of cold gas efficiency. No substantial variations in syngas composition can be noticed by increasing thermal energy losses (Fig. 8): due to lower gasification temperature, water–gas shift reaction is promoted, just leading to a slight increase of H2 and CO2 and a reduction of CO molar fractions. The gasification performance parameters are also reported in Fig. 9 as a function of carbon conversion rate. An increasing of this parameter leads to a lower gasification temperature. In fact, due to 0.6

0.55

limited oxygen availability inside the gasifier, endothermic gasification processes lead to a temperature reduction. In particular an increase of carbon conversion rate from 80% to 100% reduces gasification temperature from 1262 °C to 1017 °C. Also the syngas temperature is reduced from about 570 to 410 °C. An increasing of carbon conversion rate, which promotes gasification reactions against combustion ones, allows to reach a higher LHV and consequently a higher cold gas efficiency. An increasing in the carbon conversion rate also reduces nitrogen molar fraction for dilution (Fig. 10). The increasing of hydrogen and CO molar fractions causes the raise of LHV, as reported on Fig. 9. Gasifier performance, as well as syngas outlet composition, are strongly dependent on the volatile matter composition. The latter, as previously specified, is not known for the reference Usibelli lignite and it has been assumed taking into account literature data for tar and water molar fractions in similar fuels [21]. For this reason, a sensitivity analysis was performed for the volatile matter composition. In order to meet mass balance, tar molar fraction varies in the range 0.10–0.25 and H2O molar fraction in the range 0.05–0.20. Table 5 shows the volatile gases composition as a function of volatile matter composition, whereas Table 6 reports gasifier performance for the same values of volatile matter composition. As results from Table 5, volatile gases composition is strongly influenced by the volatile matter composition, especially considering H2 and CH4 molar fractions. Table 6 shows that syngas composition is largely influenced by the VM composition, as appears also analyzing the H2/CO molar fraction, that varies in the range 1.02– 1.46. Also gasification temperature and syngas outlet temperature are strongly variable with tar and H2O molar fractions. In particular, gasification temperature increases reducing tar and water content in the VM. Lower variations result for LHV and cold gas efficiency. 6. The Sotacarbo gasification pilot plant As mentioned above, one of the aims of the SFBG model is to support the design of the gasification experimental campaigns carried out in the Sotacarbo pilot plant, tested for more than 2000 h since 2008. The plant, located in the Sotacarbo Research Centre in Carbonia town (South-West Sardinia, Italy), is based on an up-

H2 CH4

0.35

N2 0.3

0.25

0.2

0.15

0.1

0.05

0

0.95

1300

0.9

1200

0.85

1100

0.8

1000

0.75

900

Syngas exit temperature Gasification temperature Syngas Lower Heating Value Cold Gas Efficiency

0.7 700

800 6

600

5.6

500

5.2

400

4.8

300

4.4

Syngas LHV (MJ/kg)

Cold gas efficiency

CO CO2

0.4

Syngas exiting temperature (°C)

Syngas molar fractions, dry basis

0.45

Gasification temperature (°C)

0.5

4

200

0

1

2

3

4

5

6

7

8

9

10

Thermal energy losses (% LHV) 0.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

0.46

0.48

Steam/coal mass ratio μ Fig. 6. Syngas composition as a function of steam/coal mass ratio l.

0.5

Fig. 7. Mean temperature in the gasification and combustion section, temperature of syngas exiting the gasifier, syngas LHV and gasifier cold gas efficiency as a function of thermal energy losses.

Please cite this article in press as: Cau G et al. A steady state model for predicting performance of small-scale up-draft coal gasifiers. Fuel (2015), http:// dx.doi.org/10.1016/j.fuel.2015.03.047

7

G. Cau et al. / Fuel xxx (2015) xxx–xxx 0.6

0.55

0.5

Syngas molar fractions, dry basis

0.45

CO CO2

0.4

H2 CH4

0.35

N2 0.3

0.25

0.2

0.15

0.1

0.05

0 0

1

2

3

4

5

6

7

8

9

10

Thermal energy losses (% LHV)

metallic grate, which supports the fuel bed and allows ash discharge. Air and steam flow to the top of the gasifier, thus cooling the hot ash and passing through the different zones of the gasifier, in which combustion, gasification, pyrolysis, devolatilization and drying processes take place [16,46]. This is revealed by the temperature profile, determined by 45 K-type thermocouples distributed in a metallic probe located near the reactor’s vertical axis, in the wall and in the grate,. All the process takes place at a pressure slightly higher than the atmospheric (around 0.14 MPa). Raw syngas composition is measured by three different systems: an oxygen probe, a micro gas chromatograph and a real-time gas analyser. The oxygen probe provides a continuous analysis of O2 concentration, with the double role of performance indicator and safety control (in order to avoid explosive atmosphere into the syngas treatment lines) [47]. The micro gas chromatograph (Agilent 3000) allows to evaluate, every three minutes, the concentrations of the main syngas compounds: H2, CO, CO2, N2, O2, CH4, H2S, COS, C2H6 and C3H8. The real-time gas analyser provides syngas composition by using four independent modules: a URAS 26 infra-red module is used to analyse CO, CO2 and CH4; a thermal conductivity-based CALDOS25 module measures H2 concentration; a paramagnetic Magnos 206 module indicates O2 concentration; an ultra violet Limas module is used to evaluate H2S content [47]. Due to its specific operating conditions, the fixed-bed gasification process is particularly suitable for high reactive fuels, such as biomass (mainly wood chips), lignites and sub-bituminous coals; bituminous coals and anthracites, typically characterized by a low reactivity, are not suitable for atmospheric fixed-bed gasification.

Fig. 8. Syngas composition as a function of thermal energy losses.

7. Comparison with the experimental results draft and air-blown gasifier, which technology has been derived by the so-called Wellman-Galusha, specifically scaled down for pilotscale experimental tests. The reactor, which wall is covered by refractory material, is characterized by an inner diameter of 300 mm and by an overall height of 2000 mm. Solid fuel is manually charged into a hopper and then periodically introduced through the top of the reactor in order to maintain constant the height of fuel bed (typically 1000–1200 mm). On the other hand, gasification agents (air and steam, both typically preheated at 250 °C) are injected in the cone located below the

In order to preliminarily verify the reliability of the SFBG model, it was tested with the same operating parameters (steam/coal and air/coal mass ratios, steam and air input temperatures and pressures) measured during a specific experimental campaign carried out in the previously described Sotacarbo pilot gasifier Fig. 11. In 0.6

0.55

1400

0.8

1300

0.75

1200 1100

0.7

Syngas exit temperature Gasification temperature Syngas Lower Heating Value Cold Gas Efficiency

0.65 600

1000 6

550

5.5

500

5

450

4.5

400

4 80

82

84

86

88

90

92

94

96

98

100

0.45

Syngas molar fractions, dry basis

0.85

Gasification temperature (°C)

1500

Syngas LHV (MJ/kg)

Syngas exiting temperature (°C)

Cold gas efficiency

0.5 0.9

0.4

CO CO2

0.35

H2 CH4 N2

0.3

0.25

0.2

0.15

0.1

0.05

0

Carbon conversion rate (%) 80

Fig. 9. Mean temperature in the gasification and combustion section, temperature of syngas exiting the gasifier, syngas LHV and gasifier cold gas efficiency as a function of carbon conversion rate.

82

84

86

88

90

92

94

96

98

100

Carbon conversion rate (%) Fig. 10. Syngas composition as a function of carbon conversion rate.

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8

G. Cau et al. / Fuel xxx (2015) xxx–xxx

Table 5 Volatile gases composition as a function of volatile matter. TAR/volatiles/H2O

25/55/20

Volatile gases (% by volume) CO 0.1652 CO2 0.0526 H2 0.6821 0.1001 CH4

25/60/15

25/65/10

20/65/15

20/70/10

20/75/05

15/70/15

15/75/10

15/80/05

10/75/15

10/80/10

10/85/05

0.1732 0.0551 0.7188 0.0529

0.1795 0.0571 0.7479 0.0155

0.1767 0.0562 0.6521 0.1150

0.1827 0.0581 0.6888 0.0703

0.0975 0.0310 0.8081 0.0634

0.1803 0.0574 0.5827 0.1795

0.1861 0.0592 0.6275 0.1271

0.1908 0.0607 0.6637 0.0849

0.1842 0.0586 0.5104 0.2469

0.1896 0.0603 0.5639 0.1861

0.1940 0.0617 0.6069 0.1373

Table 6 Main performance of gasifier as a function of volatile matter composition. (Reference values in bold). TAR/volatiles/ H2O

25/55/ 20

25/60/ 15

25/65/ 10

Syngas composition (molar fractions, dry basis) CO 0.2065 0.2036 0.2004 CO2 0.0831 0.0856 0.0883 H2 0.2512 0.2709 0.2897 0.0184 0.0107 0.0034 CH4 N2 0.4351 0.4237 0.4127 Ar 0.0052 0.0050 0.0049 H2S 0.0004 0.0004 0.0004 H2O 0.1747 0.1626 0.1505 H2/CO LHV (MJ/kg) Cold gas efficiency Gass. temp. (°C) Syngas temp. (°C)

20/65/15 (reference)

20/70/ 10

20/75/ 05

15/70/ 15

15/75/ 10

15/80/ 05

10/75/ 15

10/80/ 10

10/85/ 05

0.1996 0.0884 0.2451 0.0235 0.4376 0.0052 0.0004 0.1744

0.1971 0.0907 0.2649 0.0157 0.4262 0.0051 0.0004 0.1624

0.1944 0.0931 0.2838 0.0082 0.4152 0.0049 0.0004 0.1505

0.1941 0.0923 0.2183 0.0372 0.4521 0.0054 0.0004 0.1859

0.1923 0.0941 0.2391 0.0287 0.4400 0.0052 0.0004 0.1739

0.1903 0.0960 0.2589 0.0207 0.4285 0.0051 0.0004 0.1620

0.1871 0.0975 0.1906 0.0518 0.4670 0.0055 0.0004 0.1969

0.1858 0.0988 0.2125 0.0426 0.4544 0.0054 0.0004 0.1848

0.1844 0.1002 0.2333 0.0340 0.4423 0.0053 0.0004 0.1730

1.2166 5.0716 0.8313

1.3305 5.1207 0.8393

1.4452 5.1693 0.8473

1.2277 5.0636 0.8307

1.3435 5.1139 0.8388

1.4597 5.1620 0.8468

1.1243 5.0051 0.8219

1.2432 5.0552 0.8301

1.3610 5.1049 0.8382

1.0192 4.9451 0.8128

1.1435 4.9958 0.8211

1.2652 5.0462 0.8294

1040.3 426.4

990.9 391.9

941.5 357.7

1078.3 451.6

1028.7 417.0

979.1 382.6

1166.5 512.1

1116.7 477.1

1066.9 442.2

1255.2 573.1

1205.4 537.9

1155.4 502.7

Fig. 11. Sotacarbo pilot gasifier.

particular, the comparison considers the experimental results obtained by the gasification of the high reactive Usibelli lignite, which properties are summarized in Table 2.

Before each experimental run, fuel samples are crushed and sieved to obtain a particle size between 5 and 15 mm. Each run is characterized by a duration of about 16 h and by four main phases: (i) plant preparation; (ii) start-up; (iii) plant operation; (iv) shut-down. The experimental results considered in this work were obtained in a specific run. In particular, results are measured during the plant operation phase, when the fuel bed level is maintained about constant by introducing fresh fuel into the reactor (every 30 min) and by discharging ash through the grate (every 60–120 min). It involves that the process does not operate in steady-state, due to the complex fluid dynamics and the discontinuous test procedures. As specified below, this discontinuous charging can involve a slight difference between the injected coal and the reacted coal. Moreover, during the operation phase, the maximum gasification temperature has to be maintained at a value which typically, for the considered fuel, ranges between 950 and 1050 °C [45]; temperature control is performed by small variations of air and steam flows. As a consequence, the actual air/coal and steam/coal mass ratios can be slightly different with respect to the considered reference values (2.4 and 0.398, respectively), leading to slight uncertaines in the gasifier’s performance, reported in Table 6. With reference to syngas experimental composition, it can be noticed the relatively high oxygen concentration (1.6% by volume, dry basis); this value is due to the pilot scale of the reactor, in which the internal diameter is just one order of magnitude larger than coal particle size. It frequently involves the formation of hot spots and preferential paths, with a subsequent high unreacted O2 concentration in syngas. The reactor’s geometry also involves a significant difference between the calculated and the experimental values of syngas outlet temperature: the experimental tests have been performed with a very high fuel bed, that involves a low syngas outlet temperature (this is not considered by the model, which is referred to conventional units). No data were available on thermal energy losses in the gasifier, but, in accordance with previous experimental campaigns, they were estimated around 10% of the chemical energy introduced with the coal.

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G. Cau et al. / Fuel xxx (2015) xxx–xxx Table 7 Gasifier performance and syngas composition. Experimental

Calculated 8% energy losses

Calculated 10% energy losses

Calculated 12% energy losses

Air/coal mass ratio Steam/coal mass ratio

2.4 0.398

2.4 0.398

2.4 0.398

2.4 0.398

Raw syngas composition (% by volume, dry basis) CO CO2 H2 CH4 N2 O2 Ar H2S + COS H2/CO

20.1 6.70 22.5 1.49 47.0 1.60 0.55 0.04 1.12

17.6 8.63 22.5 2.11 47.0 1.63 0.56 0.04 1.28

17.3 8.89 22.7 2.10 46.9 1.63 0.56 0.04 1.31

16.9 9.18 22.9 2.09 46.7 1.62 0.55 0.04 1.37

Raw syngas properties (dry basis) LHV (MJ/kg) Syngas Outlet Temperature (°C)

5.29 79.4

5.17 378.2

5.15 307.1

5.14 235.1

Main gasifier performance Mean gasification temperature (°C) Cold gas efficiency Gasifier yield (Nm3/kg)

992.6 0.844 3.08

994.1 0.834 3.07

948.2 0.833 3.08

902.4 0.832 3.09

Table 7 reports a comparison between experimental and numerical results for three different values of the energy losses (8%, 10% and 12%). Considering that the micro gas chromatograph used for the experimental tests does not provide argon concentration, experimental syngas composition was adjusted in order to consider also this specie, calculated on the basis of the injected air. Overall, the comparison shows that, in the considered operating conditions, the model well represents the experimental behaviour of the pilot-scale gasifier. The calculated syngas composition is similar to the measured one, even if carbon monoxide concentration is underestimated, whereas carbon dioxide and methane concentrations are slightly overestimated. Hydrogen and nitrogen concentrations are nearly the same than experimental values measured by gas chromatograph. As a consequence, the syngas lower heating value calculated by the model (about 5.14– 5.17 MJ/kg) is slightly lower than the experimental one (5.29 MJ/ kg). A lower LHV leads to a lower cold gas efficiency calculated by the model (slightly higher than 0.83, to be compared with the experimental value of 0.84). As previously shown in Fig. 8, LHV and cold gas efficiency are not greatly influenced by thermal energy losses. On the contrary the calculated gasification temperature is largely dependent on this parameter. A temperature reduction from 994.1 °C to 902.4 °C has been calculated increasing thermal energy losses from 8% to 12%, whereas experimental gasification temperature is 992.6 °C. On the other hand, syngas outlet temperature calculated by the SFBG model is significantly higher than the measured one. As a matter of facts, the model underestimates the thermal losses into the upper part of the pilot-scale gasification reactor where the heat exchange between hot syngas and coal takes place. The calculated value is comparable to the typical values of commercial-scale fixed-bed up-draft gasifiers, whereas the Sotacarbo pilot unit operates with a fuel bed level proportionally higher than the conventional reactors. Finally, the SFBG model well estimates the gasifier yield. 8. Conclusions In this paper a steady-state fixed-bed gasification (SFBG) simulation model developed at the DIMCM using the Aspen Plus computer code for predicting performance of small-scale up-draft coal gasifiers is presented. Considering the potential of steady-state models, the developed computational model simulates fixed-bed gasifier performance accurately. The influence of the main process

parameters has been also evaluated. The study shows that gasifier performance is extremely sensitive to air/coal and steam/coal mass ratios and to thermal energy losses and carbon conversion rate. In particular, an increasing of air/coal mass ratio involves a reduction of syngas LHV, mainly due to the dilution with nitrogen, whereas gasification and syngas outlet temperatures decrease. In parallel, cold gas efficiency raises up to 91% (when a = 1.5) and then it decreases. On the other hand, an increasing of steam/coal mass ratio promotes water–gas shift reaction, thus involving an increasing of H2 and CO2 concentrations and a decreasing of CO one in raw syngas. In parallel, cold gas efficiency is characterized by a slight reduction. Carbon conversion rate and energy losses into the gasifier also impact both the gasification performance and syngas composition. Accuracy of performance prediction of the simulation model could be enhanced improving the devolatilization model and optimising adjustable parameters as internal minimum DT between syngas and coal, amount and distribution of thermal energy losses, tar and ash flow rate in the syngas, and carbon conversion rate. Finally, the model well represents the experimental behaviour of the Sotacarbo pilot gasifier, when it is fed with high reactive coals (lignites and sub-bituminous coals). Therefore, it can be usefully used to support the design of the experimental campaigns.

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Please cite this article in press as: Cau G et al. A steady state model for predicting performance of small-scale up-draft coal gasifiers. Fuel (2015), http:// dx.doi.org/10.1016/j.fuel.2015.03.047