Pilot-Scale Fluidized-Bed Co-gasification of Palm Kernel Shell with

Article pubs.acs.org/EF

Pilot-Scale Fluidized-Bed Co-gasification of Palm Kernel Shell with Sub-bituminous Coal Carlos F. Valdés, Gloria Marrugo, Farid Chejne,* Jorge I. Montoya, and Carlos A. Gómez Universidad Nacional de Colombia, Facultad de Minas, Escuela de Qumı ́ca y Petróleos, Carrrera 80 No. 65-223, Medellín, Colombia ABSTRACT: Currently, there are many studies that analyze the co-gasification or co-combustion of biomass with coal in fluidized-bed reactors. Mixing these two materials takes advantage of particle segregation as a result of the differences in density, shape, and size; this phenomenon requires due attention because it can lead to reactor operation issues. Some common problems seen with biomass are gap flow, plug flow, and large bubbles that reduce mass and energy transport, thus lowering the quality of the syngas product. In this study, we found that mixing biomass palm kernel shell (PKS) with coal greatly reduces these problems. Furthermore, we propose that this technique can be applied to other biomasses than the biomass used in our study without observing fluid dynamic issues. Continuous feeding co-gasification tests were performed with PKS and sub-bituminous coal type A (sub-bA coal) in a pilot-scale fluidized-bed reactor using air as the gasifying agent.

1. INTRODUCTION To add value to typical waste and simultaneously reduce CO2 emissions, it is desirable to use an existing biomass residue to diminish fossil fuel consumption in industrial processes. On the basis of this premise, it becomes relevant to address energy problems using the gasification process, because its intrinsic feedstock versatility can use biomass from the agro-industry. Gasification in fluidized beds operates between 800 and 900 °C, under a chemically reducing atmosphere. Air, oxygen, water, steam, or a mixture of them is usually used as a gasifying agent. The final gasification product is a synthesis gas, which is essentially composed of CH4, H2, CO2, CO, and CnHm; additionally, other products, such as liquid (tar) and solid residues (char and fly ash), are obtained. Many studies have examined gasification of biomass alone, for example, rice husks,1−4 woody biomass,5−7 and others.8−10 However, the main disadvantage of biomass gasification is that the gas product has a low calorific value, implying a low energy content; it also produces large amounts of tar, which could reduce the efficiency of gasification significantly.11−14 However, it has been found that blending coal with biomass enhances gasification beyond levels that can be achieved by gasifying these feedstock alone. This process, known as cogasification, has been well-studied.11,13−28 Using co-gasification, the overall thermal efficiency, coal conversion, and volatile formation were enhanced. It has been hypothesized that these effects are due to improvements in heat transfer, mass transfer, and reactivity.13,14,17,19,29 However, not all researchers have observed positive cogasification effects.30 These mixtures promote the particle segregation phenomenon as a result of density, shape, and particle size differences. This is an interesting problem for research because these help to understand the correct operation and parameter manipulation during the reactor operation. However, these limitations are inherent to the biomass and can cause problems, such as plug flow, large bubbles, slagging, among others, that reduce the heat- and mass-transfer rates affecting the fluidization and syngas quality.8,31−34 Additionally, frequently reported problems of agglomeration and sintering of © 2015 American Chemical Society

the ashes of the process, caused when the bed temperature exceeds a critical value, the high mineral content of Fe, Na, and K in the carbonaceous material, and the low melting points of these minerals affect the fluidization.12,28,34−36 In this sense, the palm kernel shell (PKS) becomes an alternative that provides a solution to these problems. This is because many factors can affect the performance of co-gasification, including the type of reactor, the operating temperature, the heating rate, the type of biomass and coal, and the way the two components are mixed. For example, they can be pressed into pellets or simply stirred together before feeding.11,12 In short, it is necessary to optimize many parameters to apply this technology in the real world. However, to date, co-gasification is the most promising with the knowledge that we have. In Colombia, the most abundant biomass is PKS and the most abundant coal is sub-bituminous coal type A (sub-bA coal). Furthermore, gasification is an excellent technology for applying to combustion systems that require low ash and tar. On the basis of a thorough literature review, there are no studies on co-gasification with a mixture of PKS and sub-bA coal. The objective of this work is to study co-gasification of PKS and sub-bA coal. To do this, we analyzed two mixtures with percentages of 6 and 10% (w/w) PKS in a pilot-scale fluidizedbed gasifier. This study uses a pilot-scale system. This allows for collection of data to scale up this process for a fluidized bed at atmospheric pressure with a capacity of 700 kg/h power of coal using air as the gasifying agent. The results will be the basis for the development of the industrial prototype for the Colombian ́ Colombia). company Ladrillera San Cristóbal S.A. (Medellin, The same fluid dynamics and thermal conditions will be designed into the industrial prototype with the aim to obtain synthesis gas of suitable quality for the brick-baking process. Received: June 16, 2015 Revised: August 5, 2015 Published: August 25, 2015 5894

DOI: 10.1021/acs.energyfuels.5b01342 Energy Fuels 2015, 29, 5894−5901

Article

Energy & Fuels Table 1. Summary of Physicochemical Properties from Biomass Analyzeda properties

SB

RP

SW

RH

PKS

W

sub-bA coal

rule

high heating value (MJ/kg) C (%, w/w) H (%, w/w) N (%, w/w) S (%, w/w) O (%, w/w) moisture (%, w/w) ash (%, w/w) volatile (%, w/w) fixed carbon (%, w/w) density (kg/m3) Geldart classification29 form

16.90 46.60 5.92 0.14 0.09 43.35 5.25 3.90 82.55 8.30 150 C Fb and C

18.40 48.80 4.78 0.75 0.06 38.22 4.28 7.39 78.39 9.94 176 C Fb and C

18.50 45.29 5.73 0.40 0.06 48.52 6.50 2.81 88.36 2.24 210 C Fi and C

14.60 33.80 4.90 1.40 0.01 40.20 8.41 20.02 58.30 13.27 184 D G

18.90 47.72 5.50 0.96 0.15 45.67 6.88 6.84 67.7 18.58 699 D Gr

5.40 44.44 5.37 0.73 0.07 43.19 7.94 58.71 32.83 0.43 225 C and D G, Fi, and C

22.30 57.78 4.23 1.10 0.46 22.12 10.39 14.31 37.41 37.89 766 B Gr

ASTM D5865 ASTM D5373 ASTM D5373 ASTM D5373 ASTM D4239 by difference ASTM D3173 ASTM D3174 ISO 562 ASTM D3172 own method

SB, sugarcane bagasse; RP, rachis palm; SW, sawdust wood; RH, rice husk; PKS, palm kernel shell; W, agro-industrial waste; Fb, fibrilar; C, cohesive; G, great; Fi, fine; and Gr, granular.

a

The respective ASTM methods used for these measurements can be seen in Table 1. 2.2. Apparatus. The process of co-gasifying blends sub-bA coal− PKS was performed in a pilot-plant system in the Energy Sciences Lab of the Faculty of Mining at the National University of Colombia ́ The reactor (see Figure 1) is a bubbling fluidized (Campus Medellin).

2. EXPERIMENTAL SECTION 2.1. Fuels. The material used low-rank Colombian sub-bA coal extracted from the region of Amagá located within the basin of the Sinifana, Department of Antioquia (see column 8 in Table 1). This was co-gasified with 6 and 10% (w/w) PKS, a residual biomass from the extraction process of palm oil in Colombia, which is a market in permanent growth driven by the rising demand for biodiesel (see column 6 in Table 1). Biomass selection is performed using several criteria; the most practical is to limit this choice to its abundance and availability for the system in use. In Colombia, where many feedstocks are available,37 we can further narrow down the options based on operational performance.38 Performance can be predicted by the physicochemical characterization; Table 1 displays the properties of the most abundant feedstocks in Colombia. On the basis of this knowledge, the PKS biomass as more potential to be used in the process of co-gasification was selected for its high heating value. Because of feedstock availability, storage costs, and transportation costs, it was determined that the feed ratio of PKS to coal will be used. The particle sizes of both the PKS and coal were reduced in size in a hammer mill with an electric motor of 5 kW and 3000 rpm; the crushed material was classified with a homogeneous particle size distribution between bulkhead mesh 8 and 12 (ASTM E11-87). As a result, the average particle diameter was 2 mm. The moisture content of raw materials, classified and subsequently subjected to a drying with direct exposure to a solar radiation, is about 6.88% (w/w) for the PKS and 10.39% (w/w) for sub-bA coal. Once the raw material was prepared, the next characterization was performed. Proximate analysis, physical properties, and heating value measurements (Table 2) were performed on two mixtures, which will be subsequently used in the gasification experiments: 6 and 10% PKS.

Figure 1. Experimental equipment general scheme. bed at atmospheric pressure made of stainless steel, with an internal diameter of 10 cm and a height of 50 cm in the bed (reaction zone); it has an internal diameter of 14 cm and a height of 100 cm in the freeboard (free zone). The power is distributed by three electric heaters, 1.50 kW each, controlled by a proportional−integral− derivative (PID) configuration using K-type thermocouples, which are in direct contact with the particles in the reactor, in both the bed and the freeboard. The reactor is insulated by ceramic material. Its design allows for great versatility in handling various gasifying agents (air, steam, carbon dioxide, oxygen, and nitrogen), and fuels (coal and biomass). It can precisely monitor and record the important physical and chemical parameters (temperatures, flow rates, pressure drop, and gas product composition). The reactor system consists of six modules, some of which are outlined in Figure 1: (1) gasifying agent inlet, (2) screw feeder system with variable feed heights, (3) gasification reactor with electric resistors, (4) gas-cleaning system, (5) monitoring and control system, and (6) peripheral auxiliary systems (compressor, preheaters, boilers, flow meters, etc.).39

Table 2. Chemical Characterization of Mixtures % PKS−% sub-bA coal

6−94

10−90

high heating value (MJ/kg) C (%, w/w) H (%, w/w) N (%, w/w) S (%, w/w) O (%, w/w) moisture (%, w/w) ash (%, w/w) volatile (%, w/w) fixed carbon (%, w/w) density (kg/m3)

22.11 57.18 4.31 1.09 0.44 23.12 10.18 13.86 39.23 36.73 762

21.98 56.77 4.36 1.09 0.43 23.79 10.04 13.56 40.44 35.96 759 5895


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Energy & Fuels Table 3. Summary of Experimental Conditions number of experimental tests parameter

R1

R2

% PKS−% sub-bA coal process temperature (°C) carrier gas (Nm3/h) ER relationship steam/mix

800 2.60 0.30 0.12

850 2.60 0.30 0.12

R3

R4

R5

R6

800 3.40 0.40 0.14

850 3.40 0.40 0.14

800 2.60 0.30 0.12

850 2.60 0.30 0.12

6−94

R7

R8

800 3.40 0.40 0.14

850 3.40 0.40 0.14

10−90

Figure 2. Syngas composition profile in tests. It is known that geometric parameters effect distribution of process products. However, for tests conducted during the experimental stage, bed height, freeboard, and transversal diameter were the same for all trials. With regard to bed material, this was made up of ashes from the process, formed char, and new material that entered the reactor. To keep the height of the material constant in the bed, an equal quantity of ash being generated was regularly removed through the bottom. Therefore, effects of reactor geometry upon product distribution are practically equal for all tests, and they are not a part of the developed parametric study. 2.3. Experimental Procedure. The fluidization gas (air + steam) was preheated to 500 °C with flow rates and quantities shown in Table 3. Once the empty reactor reached the desired experimental temperature (see Table 3), The sub-bA coal−PKS mixture was fed at a rate of 3.25 kg/h. Equipment is previously heated using electric resistances until reaching the operating temperature. Upon reaching the desired temperature, continuous feeding of the combustible mix is started; it causes a temperature reduction. However, a period of 30 min is expected because feeding starts to allow the reactor to increase the temperature to again up to the set value for each test. After 30 min, the reactor was considered to be at steady state, and the measurements reported in this study represent the steady state, which is also verified with syngas composition during the tests, as shown in Figure 2, which corresponds to measurements following the 30 min waiting time to reach the stationary state. It is important to make clear that the stable-state scheme during thermochemistry transformation is not only correlated to the equipment thermal profile, because parallel reactions take place as a result of coexistence of several processes (drying, pyrolysis, combustion, gasification, cracking, among others). These determine

the product distribution and, specially, if parallel catalyst reactions take place, which are relevant when carbonaceous materials are processed, because they determine the interaction between formed char and ashes. Multiple studies are conclusive on catalytic parallel reactions arising during gasification,13,14,17,28,40−42 promoted by mineral composition of the materials.42,43 In fact, the occurrence of catalytic effects and their influence on reaction and conversion rates are another factor indicating that co-gasification has been very attractive,13,14,17,28,40−42 because from those effects, it is possible to increase the process conversion at a given temperature or keep high reaction rates at lower temperatures.26,42,44,45 The increase in production of a gaseous species in particular is also possible, such as the case of H2, which is favored by the presence of K and Ca oxides but in the absence of silica sand or silica oxides,42 because the later inhibits gasification and catalytic activity.42,46 Additionally, tar formation may be reduced,47 which is interesting for the particular case of biomass gasification, because the high content of volatile material allows for production of many liquids during cogasification that, dependent upon the equipment thermal profile, may be problematic during operation. Then, catalytic reactions taking place may help in removal of process inefficiencies and make the process viable by increasing the output of the produced gas. The experimental set consisted of 8 tests [runs (R)] performed in duplicate for a total of 16 runs, lasting 2 h each. Gas sampling was taken every 10 min, for a total of 12 chromatographic analyses per run. The results were considered satisfactory when the error between the measured parameters between each test and its respective repetition does not exceed 5%. Therefore, the results reported are average values of the characteristic syngas compositions for each assay conducted in accordance with the procedure detailed in Table 3. 5896


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Energy & Fuels Table 4. Summary of Experimental Results from Gasification Test Mixturesa mixture test CO H2 CH4 CO2 N2 high heating value (kJ/Nm3) kg of syngas/kg of blend thermal efficiency (%) carbon conversion (%) mixture test CO H2 CH4 CO2 N2 high heating value (kJ/Nm3) kg of syngas/kg of blend thermal efficiency (%) carbon conversion (%) a

6% (w/w) PKS−94% (w/w) sub-bA coal R1 21.26 20.64 0.80 0.73 56.47 4225.36 1.74 49.14 51.70

± ± ± ± ±

R2

± ± ± ± ±

R4

0.20 1.71 0.21 0.20 2.00

22.06 ± 1.52 19.40 ± 0.53 18.16 ± 2.78 16.91 ± 2.03 1.04 ± 0.20 1.04 ± 0.26 0.55 ± 0.14 0.87 ± 0.09 58.01 ± 2.00 61.79 ± 1.81 4191.33 3811.99 1.83 2.69 50.08 75.55 54.40 79.90 10% (w/w) PKS−90% (w/w) sub-bA coal

0.86 1.37 0.16 0.10 1.71

16.47 14.53 1.05 1.04 66.90 3305.47 1.55 37.48 36.20

R5 17.54 15.19 1.12 1.06 65.09 3491.25 1.54 37.72 42.30

R3

R6 ± ± ± ± ±

R7 1.05 2.21 0.18 0.07 2.30

17.90 15.79 0.89 1.05 64.36 3516.30 2.29 55.23 61.00

± ± ± ± ±

18.45 17.90 0.77 0.16 62.72 3737.40 2.49 52.98 60.60

± ± ± ± ±

0.70 0.66 0.05 0.02 1.00

R8 0.47 2.23 0.19 0.05 2.36

21.02 12.90 0.59 0.19 65.30 3444.54 2.50 53.41 60.90

± ± ± ± ±

0.56 1.56 0.11 0.03 1.21

Gas yields are presented in volume percent concentration.

The tests R1−R4 were performed for a mixture of solid fuels containing 6% (w/w) PKS, and tests R5 and R6 were performed for a content of 10% (w/w) of the same biomass (see third row of Table 3). For each fuel mixture, two temperatures (800 and 850 °C) and two fluidization carrier gas flows (2.60 and 3.40 Nm3/h) were performed. The content of water entered the process through the moisture in the fuel as well as steam fed with the air. The effect of the carrier gas has been evaluated using the parameter commonly called the equivalence ratio (ER), which indicates the stoichiometric mass fraction of oxygen to solid fuel fed. The carrier gas flow rates assured fluidization in the steel reactor using conditions that were previously found experimentally in the team at room temperature (cold) in an acrylic reactor of the same dimensions. These results found that the minimum fluidization velocity was 0.50 and 0.35 m/s for mixtures of 6 and 10% (w/w) PKS, respectively. Because of the small size of the reactor, extra heat needs to be provided to the system to match conditions that can be achieved in a commercial size reactor as a result of inefficiencies associated with the insulation. If all of the heat necessary to reach isothermal conditions was provided by air, the fluidization would not be stable in the bubbling region. To compensate for this common scale-up issue, electrical resistance heaters were provided along the external walls of the reactor inside the insulation. 2.4. Product Characterization. The syngas sampling procedure was carried out using a cleaning system consisting of a particulate filter, a system for bubbling in isopropanol cooled to capture tar, and an impinger with silica gel to remove moisture from the prior sample for its chromatographic gas analysis. The analyzes are performed on a micro gas chromatograph (GC) Agilent model 3000 with a thermal conductivity detector (TCD), which has a molecular sieve 5A column of 10 m × 0.32 mm with gas from an argon 5.0 carrier and column PLOT U of 8 m × 0.32 mm with helium 5.00 carrier gas. The system will be configured to an appropriate analytical method to quantify concentrations of H2, O2, N2, CH4, CO, and CO2 (v/v). The fly ash was captured in the cyclone, and the heavy ash was collected on the fluidizing medium inside the reactor. To these solids and elemental analysis was performed CHN using the ASTM D5373 standard, with elemental analysis equipment EXETER 490 CE, and the

results are used to estimate the conversion reached. The gas yield products are estimated from elemental nitrogen balance. In each test, equipment is heated at the set temperature pursuant to the experimental plan using outer electric resistances; tar generation is insignificant up to the point that it was not possible to retrieve condensate material during any test because it was not visible.

3. RESULTS AND DISCUSSION It was previously determined that no steam needed to be added to this system as a result of the native content of water in the biomass−coal and in the air;48 in other studies, steam is commonly added to gasification systems to offset the amount of oxygen in the system. The amounts of water involved, in both the air and the PKS−sub-bA coal mixture were sufficient to maintain a steam/carbon ratio of 0.12 and 0.14 for the experimental tests, as reported in Table 3. Table 4 shows the average results of eight experiments, from which the effects of parameters on the gas quality were studied with the intention to produce a sufficient quality gas with minimal thermal requirements for the process at the industrial level. According to the results reported in Table 4, several important conclusions were made. One experimental condition achieved a very high thermal efficiency (ETh) of 76% (calculated as indicated by eq 1), but the rest of the experiments were below 55. The ETh difference between R1 and R4 (ETh = 55%) arises from a higher production of syngas per kilogram of combustible (Yg; eq 2), as a result of a higher conversion of carbon in syngas containing more H2 and CH4. With a low air/carbon ratio (ER), the combustion was not able to achieve enough combustion to maintain the heat and more electrical energy was required; as a result, ETh values were low for these trials. Similarly, operating with high equivalent ratios (ERs) provided high carbon conversion efficiency (ECC; eq 3) around 80%; lower ERs gave lower conversions (around 55%). This shows that the largest air share provides oxygen directed toward the combustion process. 5897


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Figure 3. Variation of the calorific value of syngas variable ER.

Trials with higher syngas yield correspond to those with a higher ER, although higher conversion. However, these conditions also produce a syngas with a lower calorific value as a result of the fact that combustion enhanced the production of CO2; furthermore, air contains nitrogen, which dilutes the product gas E Th =

Yg =

CnHm +

ECC =

(1)

⎛ m⎞ CnHm + nH 2O → nCO + ⎜n + ⎟H 2 ⎝ 2⎠

(2)

CnHm + nCO2 → 2nCO +

⎛ 12xCO + 44xCO2 + 16xCH4 ⎞ ⎟ Fg ⎜ PM g ⎝ ⎠ Ff wC(1 − wm − wash)

(5)

When having proper thermal conditions (temperature exceeding 750 °C50,51), reformation reactions take place with water vapor and dry steam with CO2 (eqs 6 and 7), even when only quantities of these gases are available in the reactor in drying, pyrolysis, and devolatilization processes, which implies that CO and H2 increase.

Fg Ff

(4)

C + nO2 → (2 − 2n)CO + (2n − 1)CO2

Fg HHVg Ff LHVf (1 − wm)

n m O2 → nCO + H 2 2 2

⎛m⎞ ⎜ ⎟H ⎝2⎠ 2

(6)

(7)

Another reaction taking place that restricts the quantity of CO2 produced by partial CO oxidation (eq 8) is the Boudouard heterogeneous reaction (eq 9), which indicates that, despite being too slow, there is too much evidence that it is favored under high heating rate and energy-transfer conditions, such as those present in fluidized-bed reactors from temperatures exceeding 700 °C.52−54

(3)

where Fg, HHVg, and PMg are flow, gross calorific value, and mole weight of the syngas, respectively, Ff and LHVf are flow and lower heating value of the combustible, respectively, xCO, xCO2, and xCH4 correspond to molar fractions of CO, CO2, and CH4 of the syngas, respectively, and wC, wm, and wash are the mass fractions of the carbon, volatile material, and ashes of the combustible, respectively. In all tests, the concentrations of CO exceed 16% (v/v) and the concentrations of H2 exceed 12%. In this study, the hydrogen content is lower than CO because no water was added to the system other than that which was native to the coal−biomass and air in the feed. Interestingly, when the biomass ratio increased (thus increasing the feed of moisture), the H2/CO ratio decreased. This is contrary to typical observations that feeding more water enhances H2 production as a result of the shift reaction. It is possible that excess hydrogen produced by the biomass went on to react with coal to enhance the tar production, which was unfortunately not measured.13,49 It may be difficult to identify one controlling phenomenon to decrease hydrogen production because of the complexity of this system. High ER values involve a higher oxygen content available for oxidization reactions of volatile compounds and light gases, which are the pyrolysis output (eq 4) and formed char (eq 5, which leads to having high CO concentrations during the transition stage and the stable state. Then, oxidation reactions control the CO/CO2 ratio and determine the thermochemistry process stability, being mainly CO when n = 0.50, which is a typical gasification condition (partial oxidation).

CO +

1 O2 → CO2 2

C + CO2 → 2CO

(8) (9)

Admission of a small fraction into the process (in our case, only the combustible moisture), at bed temperatures exceeding 750 °C, promotes other reactions, such as gasification (eq 10) and methanation (eq 11) heterogeneous reactions as well as shift homogeneous reactions (eq 12) and reformed methane−vapor (eq 13), with which syngas composition stability of the produce gas is reached. C + H 2O → CO + H 2

(10)

C + 2H 2 → CH4

(11)

CO + H 2O → CO2 + H 2

(12)

CH4 + H 2O → CO + 3H 2

(13)

3.1. Effect of Variable ER. The effect of ER on the quality of the synthesis gas product can be seen in the resulting calorific values (see Figure 3). The results show that the calorific values decrease with increasing air fed (i.e., increase in ER); this is consistent with results reported in other studies.15,55 5898


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Figure 4. Variation of syngas composition with variable ER for mixing with 6% (w/w) PKS.

Figure 5. Variation of the composition and syngas HHV (%) of PKS in the mixture.

With increasing ER, the yield of H2 decreases, as shown in Figure 4. For higher ER, partial combustion reactions prevail that produce CO and CO2, instead of gasification reactions to generate H2. This is consistent with observations reported in other studies.51,55 These researchers, however, observed a lower CO2 yield than us because they had a higher water content in their feed. The absence of a steam feeding draft into the process does not allow for the shift reaction (eq 12), producing meaningful effects on syngas composition. 3.2. Effect of the PKS Content in the Fuel. The effect of the PKS biomass content in the fuel on the quality of the synthesis gas was also evaluated. We found that increasing the

amount of biomass for a given temperature and ER reduces the quality of produced synthesis gas. It is difficult to identify one explanation for this observation because many competitive phenomena occur simultaneously (see Figure 5). For an ER = 0.30, the amount of H2 and CO decreases with increasing the content of PKS in the co-gasification mixture, as seen in panels a and c of Figure 5; the HHV also decreases as seen in Figure 5e. These results are consistent with reports in the literature.12,15,51 However, other researchers have also reported contrary results, where HHV and H2 increase with more biomass fed.12,14 This may be due to several possible causes. First, with greater biomass quantity, there are more 5899


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reported in the literature, to show that a rise in the temperature promotes the H2 formation through the cracking of tar generated during devolatilization.51,55,58 Trials with ER = 0.40 favored CO instead of H2, which is likely due to the enhancement of partial combustion that favors this instead of H2. Similar results were observed for tests with 6% (w/w) PKS, which implies that the tar residence time for devolatilization is too short for cracking reactions. This is the likely explanation because our reactor has biomass fed close to the top, near the freeboard, and pyrolysis tar products can more easily leave the reactor before passing through the whole heated region (see Figure 1). This system feeds near the top to match the industrial-scale reactor with which these results will be compared in future work.

heavy volatile compounds comprising tar. The path of the tar depends upon the residence time because it may or may not be cracked to lighter compounds, such as CO and H2, produced; therefore, increasing the height of the reactor is required to provide enough reaction time to promote the cracking reactions of tar and hydrocarbons.51,56 Our reactor did not vary the height of the reactor. A second possible hypothesis is related to the fact that the lower biomass content has a smaller total moisture (a gasifying agent); this reduces the effectiveness of the gasification reactions and, as a result, lowers the contribution of potential biomass reactivity toward carbon reactivity.41 Another possible explanation is the presence of inhibitory and catalytic effects caused by the mineral content of the combustible as reported by other papers.46,57,42 On the other hand, when ER = 0.40, the hydrogen content is slightly lower at 850 °C than at 800 °C (see panels b and d of Figure 5), which is contrary to other reports, where H2 formation is favored with increasing temperature.12,13,15,51,55 In this case, an increase occurs in the CO content with increasing the percentage of PKS in the fuel, which causes the syngas HHV to increase (see Figure 5f). This may be explained as a result of the increased presence of volatile material and reaching tar to be cracked in the reactor with an increased CO content in the syngas.58 3.3. Temperature Effect. The temperature reaction is one of the most important variables that affects the coal and biomass gasification, because the gasification reactions are endothermic and, therefore, energy availability in the reaction zone determines the occurrence degree of each one of them. In Figure 6, the process performance on the synthesis gas occurs, which takes place at 800−850 °C, with 6 and 10% PKS in the fuel and ER of 0.30 and 0.40. However, results with 10% (w/w) PKS in the fuel and ER = 0.30 are consistent with those

4. CONCLUSION It can be concluded that the PKS−sub-bA coal blend cogasification with air provides the best synthesis gas quality close to HHV of 4 MJ/Nm3 for ER = 0.30 and 3.50 MJ/Nm3 for ER = 0.40 with a 6% blend of biomass. The presence of biomass with coal appears to create a synergism. For example, the biomass enhances CO and H2 production, likely as a result of volatiles released by the biomass that can react with the coal. Additionally, the use of PKS in cogasificaction processes on fluidized beds does not show fluid dynamic issues, such as other biomasses.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +5744255333. Fax: +574234102. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the following financial institutions from the project with Contract 38411 0527 of 2013: Colciencias, InterAmerican Development Bank (IDB), National University of ́ Universidad Pontificia BolivariColombia (Campus Medellin), ana, and Ladrillera San Cristóbal S.A.

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REFERENCES

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Figure 6. Effect of the temperature on the co-gasification gas for mixtures with 6 and 10% (w/w) PKS. 5900


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