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Production of a fuel gas by fluidised bed coal gasification compatible with CO2 capture
Fuel 259 (2020) 116242
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Full Length Article
Production of a fuel gas by fluidised bed coal gasification compatible with CO2 capture
T
Nicolas Spiegl, Cesar Berrueco, Xiangyi Long, Nigel Paterson, Marcos Millan
⁎
Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK
ARTICLE INFO
ABSTRACT
Keywords: Coal gasification Fluidised bed reactor Spouted bed Oxy-fuel Steam/CO2 gasification
A continuously fed, laboratory scale spouted bed gasifier has been used to study oxy-fuel gasification of German lignite. In this paper, the influence of different gasification agents and bed temperature on the process performance, during tests at atmospheric and elevated pressure are studied. Two gasification agents have been used, CO2 (with different CO2/C ratios) and mixtures of CO2/steam. The results show that despite the relatively slow CO2-char reaction, good gasification performance could be achieved with German lignite by adjusting the operating conditions at atmospheric pressure: complete carbon conversion, high energy conversion and a medium heating value fuel gas (8–10 MJ m−3). The CO2/C ratio was found to have a large effect on the gasification performance. Increasing the ratio increased the carbon conversion, but the CO2 conversion decreased. At 950 °C, maximum carbon conversion was already achieved with pure CO2, therefore using steam at this temperature could not increase the conversion, but did increase the H2/CO ratio in the fuel gas. At 850 °C, replacing 25% of CO2 with steam increased the carbon conversion to the level achieved at 950 °C without steam. Replacing more than 25% of CO2 with steam increased the H2/CO ratio further. Therefore, with the addition of steam, the operating temperature could be reduced from 950 °C to 850 °C while maintaining the gasification performance. The changes of gasification performance with steam addition at pressures up to 10 bara followed the same trends achieved at atmospheric pressure.
1. Introduction The higher efficiency, lower emission of greenhouse gases and market flexibility mean that gasification technology is a promising option to improve the efficiency of power generation compared to conventional coal fired pulverised fuel power plants [1]. This can involve Integrated Gasification Combined Cycle (IGCC) technology at large scales or Integrated Gasification Fuel Cell concept at smaller scales. A list of the principal reactions that may occur during gasification is shown in Table 1. Oxygen blown gasification is considered the most promising route for reducing carbon emissions, as it can be combined with carbon capture and storage (CCS), avoiding energy and cost intensive processing for downstream CO2-N2 separation [2,3]. Fluidised bed gasification has potential for this technology option, due to fuel versatility, including the possibility to use low-grade fuels. To combine the advantages of gasification, fluidised bed operation and oxygen blown processes, a novel process configuration proposed in a previous article [4] involves operation of a fluidised bed gasifier with O2 and recycled CO2. In combination with an IGCC power plant, this process concept
⁎
has the potential to produce electricity with a higher efficiency than a combustion based process and to use both fossil and biomass based fuels. It would produce a stream of CO2 ready for sequestration. This process option can be regarded as a further development of the concept of oxy-fuel combustion, where O2 can be supplied together with CO2 to the gasifier or to a separate combustion bed in dual fluidised bed systems. The actual use of recycled CO2 in large scale gasification processes is not very common. It is occasionally used to replace nitrogen for pneumatic fuel transportation - especially when it is desirable to avoid nitrogen contamination of the syngas [5]. More frequently, recycled CO2 is used in syngas production by reforming, to adjust the CO/H2 ratio [6]. The concept of operating a fluidised bed gasifier with pure O2 and recycled CO2 is a novel process configuration. Nevertheless, using recycled CO2 for different purposes was considered by a number of authors proposing new process options, as discussed below. Romano et al. [7,8] used thermodynamic equilibrium calculations to assess the performance of three novel plant configurations for nearzero emissions power generation from coal. One of the plants consisted of an entrained flow gasifier operating at 70 bar. The gasifier was O2/
Corresponding author. E-mail address: [email protected] (M. Millan).
https://doi.org/10.1016/j.fuel.2019.116242 Received 15 July 2019; Received in revised form 17 September 2019; Accepted 18 September 2019 Available online 27 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Normann et al. [14] proposed an oxy-fuel process where fuel would be burned in a slightly sub stoichiometric atmosphere. Producing heat and additional flue gas for subsequent chemical synthesis (dimethyl ether production) would reduce the production cost. The process included co-processing of up to 20% biomass. The study was based on an Aspen Plus simulation. The key issue identified was the oxygen lean combustion or gasification and the related issues in coal conversion and high temperature corrosion. From these references, two driving forces behind the development of gasification processes with CO2 recycle can be identified, production of pure CO and an increase in plant efficiency by using the CO2 available from the CCS integration. The second reason is certainly the dominating one. It also seems that the majority of the studies are theoretical and do not address issues arising from the high partial pressure of CO2 in the gasifier. The reactive nature of CO2 in gasification reactions compared to the use of an inert gas, such as nitrogen, will cause a change in the chemistry of the system [15]. CO2 gasification is inhibited by its product CO and requires temperatures higher than 700 °C for the reaction rate to be significant [16,17]. This paper studies the influence of different gasification agents and bed temperature on the process performance in atmospheres that do not contain N2 and are rich in CO2. The first objective is to obtain data about the overall process performance and to judge the potential of a process based on CO2 gasification. The second objective is to gain a better understanding of the mechanism involved in the fuel conversion when using CO2 and mixtures of steam/CO2 as gasification agents and to determine how this can be used to develop strategies to optimise the process performance.
Table 1 Principal gasification reactions. Reaction type
Reaction
ΔH, MJ kmol−1
Combustion reactions
C + 0.5 O2 → CO CO + 0.5 O2 → CO2 H2 + 0.5 O2 → H2O C + CO2 ↔ 2 CO C + H2O ↔ CO + H2 C + 2 H2 ↔ CH4 CO + H2O ↔ CO2 + H2 CH4 + H2O ↔ CO + 3 H2
−111 −283 −242 +172 +131 −75 −41 +206
Boudouard reaction Water gas reaction Methanation reaction Water gas shift reaction Methane steam reforming reaction
steam blown and recycled CO2 was used to replace N2 for dry fuel feeding. The fuel gas stream was cooled to 850 °C in a syngas cooler, cleaned via hot gas particle cleaning technology and combusted in a gas turbine using pure O2 and recycled CO2. The technology required for the hot fuel gas cleaning process and the development of a gas turbine operating with CO2 based stream was seen as the main obstacle for the development of such a process. The overall electrical efficiency of the plant was calculated to be to be around 45% (based on the lower heating value, LHV), including the impact of carbon capture and storage. A different proposal from Jillon et al. [9] presented an IGCC plant with two CO2 recycle streams. In the model, the first stream was used to control the fuel gas composition in a Conoco/Phillips type of gasifier by injecting recycled CO2 together with the oxygen into the gasifier. The second CO2 stream was used to replace N2 as temperature moderator in the gas turbine. Only a simplified model of the gasification process was used to simulate the effect of operating conditions on gasification performance. The CFD model showed two effects of the recycled CO2 on the gasification performance: a change in temperature profile as the rate of the endothermic CO2 gasification of coal increased by increasing CO2 recycling rate and the possibility of controlling the syngas composition with the CO2 recycling rate. No information was given whether a real gas turbine was used as a basis for the model. The emphasis of the work was on process control and therefore no further information about the feasibility of the project or possible technical and financial challenges were given. Andries et al. [10] reported plans to modify a pressurised fluidised bed reactor with 1.6 MW thermal capacity for oxy-fuel gasification and combustion with flue gas recycling. The fluidised bed was connected to a combustor, where fuel gas could be combusted with additional oxygen. The objective of the project was to study the effect of operational parameters on fuel gas composition, conversion, in-situ sulphur removal and formation of nitrogen compounds. A second paper published by the same group [11] reported primary results of oxy-fuel fluidised bed combustion at 850 °C with 24% and 27% O2. However, no results were published on the oxy-fuel gasification concept. A new process for the production of CO was proposed by Lath et al. [12]. The key component would be a slagging fixed bed gasifier, which would be oxygen blown and CO2 used as a temperature moderator. The proposed process would operate under slightly elevated pressure and would produce a flue gas high in CO (90–92%). Furthermore, it was claimed that running a gasifier under such conditions would be economic. In another theoretical study, Shao et al. [13] described a power plant concept based on a combined cycle powered either by natural gas or fuel gas from a gasifier. Pure oxygen and steam were used as gasification agent, whereas the gas turbine was operated with pure O2 and recycled CO2 to improve the integration with CCS. The main focus of the paper was the possible integration of the air separation unit with the CO2 capture unit. The problem to operate a gas turbine with a CO2 based stream was not addressed in this report. The performance of the gasifier was calculated based on thermodynamic equilibrium calculations.
2. Experimental 2.1. Spouted-Bed gasifier A bench-scale fluidised-bed reactor was modified to allow continuous operation under oxy-fuel gasification conditions, i.e. in CO2 rich environments with temperatures up to 1000 °C and pressures up to 2 MPa. O2 was not used in this study, as the main objective was to investigate the influence of CO2 rich gas, as gasification agent, on its performance. The heat for gasification was provided by electrical resistance heating. This reactor has been successfully operated in several configurations [18], and the details of the modifications to the reactor to enable operation under CO2 rich oxy-fuel conditions and the experimental procedure are described in an earlier paper [4]. Briefly, the reactor consists of a 504-mm-long (34 mm internal diameter) Incolloy 800 HT column, which is heated by a high-current, low-voltage alternating current (AC) with two copper electrodes connected to the reactor body. A fluidised sand bed is held inside a quartz liner (28 mm i.d.), with a spout jet inlet at the apex of the inverted cone shaped base. The solid fuel is continuously injected into the bed through the spout jet, which provides effective mixing with the bed material and avoids agglomeration at the base of the gasifier, with feed rates from 0.5 to 6 g min−1. 2.2. Fuel gas analysis Online gas analyzers (Servomex Xentra 4200 for O2 and CO2; ADC analyzer for CO and CH4 and Hitech K1500 for H2) were used to quantify the major components in the fuel gas produced during the experiments. The analyzers were calibrated before each experiment using a certified calibration gas mixture (BOC Ltd.). All gas concentrations are presented on a dry basis. The volumetric flow rate of the fuel gas was measured using a dry gas meter (Model G6, Meters UK Ltd). After measuring the fuel gas flow rate and fuel gas concentrations, the CO2 conversion (conv.CO2, %) was calculated from the inlet and outlet CO2 flowrates by Eq. (1). 2
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conv .CO2 =
CO2 in CO2 out × 100% CO2 in
Table 3 Operating Conditions for Gasification of GL with Different Temperatures, Pressures and Gasification Agents.
(1)
The LHV of the fuel gas (LHVfuel gas, MJ m−3) was calculated from the contribution of each gas by Eq. (2), which involves the heating value of the gas components (LHVCO, LHVH2, LHVCH4, MJ m−3) and their volumetric concentration (CCO, CH2, CCH4, %vol.).
LHVfuel gas = LHVCO × cCO + LHVH2 × c H2 + LHVCH4 × cCH4
(2)
The C conversion (conv.C, %) to gas products excluding tars was calculated from the mol flowrates of C in the coal and inlet and outlet gas by Eq. (3).
conv .C =
C outlet gas C inlet gas × 100% C in the coal
(3)
To gain an insight into the underlying reactions taking place, the hydrogen product distribution (fraction of hydrogen in the fuel gas as H2, H2O and CH4) was estimated. The fraction of H2O was determined by difference, performing a balance between the hydrogen input in the coal and steam and the ouput in H2, CH4 and steam. 2.3. Feedstock German lignite coal (GL) was used for all the experiments in this study (Table 2). The fuel sample was ground using a swinging hammer mill and then sieved to recover fuel in the size range of 200–300 µm. Acid-washed silica sand (particle size of 100–300 µm, VWR) was used as the initial bed material in the FBR. 3. Results and discussion
Table 2 Feedstock Analysis. German lignite 1
Prox. Analysis Moisture % Ash % Volatile Matter % Fixed Carbon %2 Ult. Analysis1 Sulphur % Chlorine % Carbon % Hydrogen %3 Oxygen %2, 3 Nitrogen % 1 2 3
Pressure [bara]
Gas
CO2/C or steam/ CO2 ratio
Fuel Feed Rate [g min−1]
Flow [NL min−1]
750 750 750 850 850 850 950 950 950 750 750 850 850 850 850 950 950 950 850 850 850 850 850 850 850 850 850
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5 5 5 10 10 10
CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/
0.9 1.1 1.3 1.0 1.4 1.8 0.9 1.1 1.4 0 0.3 0 0.3 1.0 2.3 0 0.3 1.0 0 0.25 0.67 0 0.25 0.67 0 0.25 0.67
1.7 1.7 1.8 1.6 1.7 1.0 1.8 2.3 1.7 1.8 1.6 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.0 1.0 1.0 3.9 3.9 3.9 3.9 3.9 3.9
1.8 2.0 2.6 1.8 2.6 2.0 1.8 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.7 2.6 2.6 2.6 2.4 2.4 2.4 9.0 9.0 9.0 9.0 9.0 9.0
CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2
CO2/C ratios for the first set of experiments and several CO2/steam ratios (0, 0.3, 1 and 2.3) for the second set of experiments. Different CO2/C ratios were achieved by altering the amount of CO2 injected into the reactor and keeping the fuel feeding rate constant. This changed the superficial velocity (Us), but maintained the contribution of pyrolysis to the overall performance due to the constant fuel feeding rate. Additional tests were performed to test this assumption and these showed that the performance was not affected by the change in fluidising velocity over the range studied [19]. The different steam/CO2 ratios were obtained by replacing between 20 and 75% of CO2 by steam (molar basis). The total flow rate (Fin) entering the reactor was kept constant: Fin = 2.6 NL min−1 and Us = 0.24–0.27 m s−1 for the second set of experiments. In the third set of experiments the pressures tested were 1, 5 and 10 bara. As gas density increases with pressure, keeping constant coal and gas (steam and CO2) mole flow rates would cause a decrease in the gas volumetric flow rate and therefore in the superficial velocity, while the gas residence time would increase. In order to maintain a reasonable fluidising velocity and fuel/gas input ratio, the feed to the system was adjusted to obtain conditions that enabled steady operation. Constraints in superficial velocity and the range of solid feeding rates achievable in the system did not allow the same coal to gasification gas flow rate ratio to be maintained in the range from 1 to 10 bara. Therefore, the flow rates of coal and gasification gas were kept the same only for 5 and 10 bara experiments, while the mole ratio of inlet gasification gas to carbon in the coal was kept constant at 2. The same approach has been used in other studies [20,21]. Gas flowrates of Fin = 2.39 and 9.01 NL min−1 were used at atmospheric and high pressures (5 and 10 bara), respectively. Superficial velocities were Us = 0.25, 0.19 and 0.09 m s−1 for 1, 5 and 10 bara respectively.
This paper studies the influence of gasification agent, temperature and pressure on the process performance. Temperature is investigated independently from the fuel to oxidizer ratio by using the capability of the experimental set-up to heat the process externally to the desired temperature. The discussion of the results is separated into three sections. The first two show the results obtained at different temperatures using CO2 and steam/CO2 as gasification agents, at atmospheric pressure. The third part discusses the effect of operation at elevated pressure with CO2/steam mixtures. The main parameters used to judge the process performance in each set of tests are: fuel gas composition, heating value and carbon conversion. To gain an insight into the underlying reactions taking place, the gas product distribution, the steam and CO2 decomposition and the hydrogen product distribution (fraction of hydrogen in the fuel gas as H2, H2O and CH4) have been estimated. The conditions for the sets of experiments are summarised in Table 3. Fuel was gasified at 750 °C, 850 °C and 950 °C using different
Sample
T [°C]
13.2 4.1 44.0 38.7 0.23 0.02 57.39 4.03 20.42 0.61
3.1. Effect of CO2/C ratio
“As received” basis By difference Corrected for moisture
Increasing the CO2/C ratio increases the amount of reactive gas per unit fuel and therefore potentially affects the extent of char gasification. 3
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70 60
[vol., dry, %]
50 40
30 20 10
CO2/C Ratio:
0
0.9
1.1
1.4
1
1.4
950 C
1.8
0.9
1.1
850 C
1.3
750 C
Fig. 1. Gas composition obtained from the gasification of GL at different temperatures and CO2/C ratios (□ CO2, ♢CO, Δ H2, × CH4).
At 850 °C results show that the char-CO2 reaction seems to be fast enough to be influenced by the partial pressure of CO2. At a CO2/C ratio of 1.8 it reaches the maximum carbon conversion of 85%. At 750 °C the char-CO2 reaction is slow, and the majority of the carbon conversion was a result of pyrolysis. Therefore, increasing CO2/C ratio had only a limited effect on overall conversion, but the fuel gas analysis showed dilution by the increasing CO2 inlet flow. The carbon conversion was nearly equal to the carbon content of the volatiles, plus a small amount from a limited extent of gasification by CO2. Increasing the CO2/C ratio diluted the fuel gas and therefore decreased the heating value of the fuel gas at each temperature (Fig. 3). At 950 °C, the value decreased from 10 to 8.5 MJ m−3 as the CO2/C ratio was raised. These are reasonable values for a syngas, reflecting that a gas with medium heating value can be obtained. The heating values decreased at the lower temperatures, but the trends at each temperature were similar as the CO2/C ratio was increased. Another important parameter is the CO2 conversion, which is a guide to the efficiency of the process. CO2 conversion is an indicator of the fraction of CO2 used by gasification compared to the CO2 passing the reactor as a quasi-inert gas. Maximising the CO2 conversion is important for the process efficiency as the amount of unconverted CO2 increases the amount of energy required to heat the input gas stream to gasification temperature and dilutes the fuel gas. However, it is also noted that the amount of CO2 in a fluidised bed gasifier cannot be varied independently, as a certain amount is required to ensure
Carbon Conversion [%]
100
75
50
25
0 0.8
1.0
1.2
1.4
1.6
1.8
CO2/C Ratio Fig. 2. Carbon conversion at different temperatures and CO2/C ratios (♢950 °C, □ 850 °C, Δ 750 °C).
However, increasing the amount of CO2 entering the reactor increases the heat required to raise the input stream to gasification temperature, due to the high heat capacity of CO2. The fuel gas compositions obtained in tests performed using different CO2/C ratios are summarised in Fig. 1, while Fig. 2 shows the changes in carbon conversion. It can be seen that at 950 °C, the concentration of CO decreased with increasing CO2/C ratio (and fluidising velocity), which was accompanied by an increase in the CO2 output. These changes diminished with decreasing temperature. H2 was released mainly by pyrolysis in the CO2 atmosphere and the amount followed a similar trend to the CO output. The accompanying carbon conversions show that the value was nearly constant at 85% at 950 °C. At 850 °C, the carbon conversion increased from 60 to 85% with increasing CO2/C ratio. At 750 °C, a small increase in carbon conversion from 32 to 37% with increasing ratio was seen. The results reveal that at 950 °C a near maximum possible carbon conversion is reached. These experiments were carried out with a high superficial velocity (up to Us = 0.27 m s−1 at the highest CO2/C ratio). Examination of the bed material recovered after each test showed that this resulted in a larger number of char particles being ejected from the bed and collected in the tar trap. Therefore, the limit of 85% conversion is probably due to the carbon loss from the bed by entrainment during the experiment. Therefore, the sequence of tests at 950 °C, was limited by the availability of C. Even with the lowest CO2/C ratio, the input carbon was reacted efficiently. At this temperature, the amount of CO in the fuel gas decreased with increasing CO2/C ratio, because it was diluted by the higher input flow of CO2.
10
LHV [MJ/m3]
8
6 4 2 0 0.8
1.0
1.2
1.4
1.6
1.8
CO2/C Ratio Fig. 3. Fuel gas heating value (LHV) at different temperatures and CO2/C ratios (♢950 °C, □ 850 °C, Δ 750 °C). 4
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with temperature can be explained by an increase in the extent of pyrolysis of the lignite volatiles and by the shift of equilibrium of water–gas shift reaction towards higher percentage of H2 due to the increase of CO concentration. The carbon conversion increases with temperature and this will have increased the amount of carbon in the gas phase as CO. This is considered to be the main reason for the observation, although increases in the extent of the char-steam reaction and approach towards thermodynamic equilibrium may also have contributed. Steam in the fuel gas is produced from the fuel-moisture and steam release during pyrolysis. Steam is a reactive gasification agent, particularly at the higher temperatures used. Hydrogen to CH4 conversion was between 6 and 8% for all conditions investigated. These values are similar to the observed for oxy-fuel conditions with O2 (shown in previous work [4]). The comparison between the results obtained under pyrolysis (not shown) and gasification conditions indicates that CH4 is produced mainly during pyrolysis and reaches a maximum at 750 °C. Its gas phase concentration is controlled by the endothermic steam methane reforming reaction. Equilibrium predicts decreasing CH4 concentration with increasing temperature.
CO2 Conversion [%]
50 40
30 20 10
0 0.8
1.0
1.2
1.4
1.6
1.8
CO2/C Ratio Fig. 4. CO2 conversion at different CO2/C ratios and temperatures (♢ 950 °C, □ 850 °C, Δ 750 °C).
adequate fluidization of the bed. The actual input is therefore a balance between the needs for an efficient process and the needs of fluidisation. The CO2 conversion can be estimated from the CO2 balance for the reactor for each condition. The value includes effects of pyrolysis and water–gas shift reaction, together with the amount involved in gasification. However, the influence of pyrolysis is considered to be small in comparison with the total amount of CO2 in the fuel gas and there seems to be no significant effect from the water–gas shift reaction on the concentration of CO and CO2 (discussed below). Therefore, the overall balance of CO2 in the process is considered as a reasonable approach to calculate the CO2 conversion. The results are shown in Fig. 4. At 950 °C, the carbon conversion had already reached a maximum with the lowest ratio and no extra CO2 could be converted as the ratio was raised. The fraction of CO2 converted therefore decreased with increasing CO2/C ratio. A different situation can be observed at 750 °C, where CO2 conversion slightly increases, which reflects the small change in the carbon conversion, probably by gasification. The most interesting case is 850 °C. Under the conditions used, a maximum in CO2 conversion can be observed at a CO2/C rate of 1.4. At this point, carbon conversion versus CO2 conversion graph reached a maximum. For this set of experiments, 73% of the hydrogen enters the reactor as fuel-hydrogen, while the balance was from the fuel-moisture. The hydrogen distribution (fraction of hydrogen in the fuel gas as H2, H2O and CH4) in the different products is shown in Fig. 5. The concentration of H2 (shown in Fig. 1) increases with temperature (11–12% at 750 °C to 18–19% at 950 °C), but there seems to be no significant change with CO2/C ratio. The increased concentrations of H2
3.2. Effect of Steam/CO2 ratio 3.2.1. Effect of Steam/CO2 ratio at atmospheric pressure It is well known that the gasification with steam is faster than the reaction with CO2 [5,15,22,23]. To investigate the impact of this in the spouted bed reactor, CO2 was stepwise replaced with steam as the fluidising/gasification agent. Adding steam to a larger scale O2/CO2 blown process could be considered for two reasons: using the faster char-steam rate to increase carbon conversion and heating value of the fuel gas, and increasing the H2/CO ratio in the syngas, as required by downstream applications. Different CO2/steam ratios were obtained by replacing 25%, 50% and 75% of CO2 by steam (molar basis) from baseline experiments with 100% CO2 at each temperature where the total gas flow rate entering the reactor was kept constant: Fin = 2.6 NL min−1, Us = 0.24–0.27 m s−1 and CO2/C ratio = 0.4–1.4. As described above, these conditions led to incomplete carbon conversions at 750 °C and 850 °C, and maximum carbon conversion at 950 °C. The set of experiments carried out are presented in Table 3, and the resulting gas composition is shown in Fig. 6. The reaction chemistry is altered as the proportion of inlet steam is raised. Because of its higher rate, the steam-C reaction will tend to compete more favourably than CO2 for the available C. This will increase the proportion of H2 in the gas, which will impact on the water gas shift reaction. These changes can be clearly seen in the fuel gas composition in Fig. 6. At each temperature, replacing CO2 with steam increases the H2 concentration and decreases the CO concentration in the fuel gas. Fig. 5. Hydrogen product distribution at different temperatures and CO2/C ratios ( H2, H2O, CH4).
Hydrogen Product Distribtuion
100%
75%
50%
25%
0% CO2/C Ratio:
0.9
1.1 950 C
1.4
1
1.4
1.8
0.9
850 C
1.1 750 C
5
1.3
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70
[vol., dry, %]
60 50
40 30
20 10
0 Steam/CO2 Ratio:
0
0.33
1
0
0.3
950 C
1
2.3
0
0.3 750 C
850 C
Fig. 6. Gas composition obtained from the gasification of GL in steam/CO2 mixtures at different temperatures (♢CO, □ CO2, Δ H2, × CH4).
100
60
Steam Conversion [%]
Carbon Conversion [%]
40
75
50
25
20 0 -20 -40
-60 -80 -100
-120
0 0.00
0.33
1.00
0.00
2.33
0.33
1.00
2.33
Steam/CO2 Ratio
Steam/CO2 Ratio Fig. 7. Carbon conversion at different temperatures and steam/CO2 ratios (♢950 °C, □ 850 °C, Δ 750 °C).
Fig. 8. Steam conversion at different temperatures and steam/CO2 ratios (♢950 °C, □ 850 °C, Δ 750 °C).
Fig. 7 shows the change in carbon conversion with increasing steam/CO2 ratio. It can be seen that replacing 25% of CO2 by steam increased the carbon conversion at 750 °C and 850 °C while at 950 °C no change occurred (as expected). This result is similar to the one discussed in the previous section, showing a maximum possible carbon conversion of 85% at 950 °C, independent of CO2/C ratio and steam/ CO2 ratio and was limited by elutriation of fines from the fluidised bed. At 850 °C carbon conversion was increased by 25% steam addition to a level comparable with results from experiments carried out at 950 °C with no added steam. Further increasing steam/CO2 ratio did not change the carbon conversion at 850 and 950 °C. The results clearly show the effect of the faster steam-C reaction compared to the CO2-C reaction, which enables maximum conversion to be reached at lower temperatures. The steam conversion has also been calculated and the values are shown in Fig. 8. Steam input is defined as steam in the fluidising gas plus steam formed from moisture in the fuel. In the case of 750 °C and no steam addition a negative value is calculated, which indicates that more steam is generated during pyrolysis than is used in the char-steam reaction. The value ‘increased’ to zero when steam was added, which shows that some reaction did occur with steam, even at 750 °C. At higher temperatures, the steam conversion levelled off at around 40% for all levels of steam addition. This suggests the system was carbon limited, as was found for the experiments using CO2 alone at 950 °C. As the steam/CO2 ratio was increased, more steam reacted (on a mass
basis), but at the expense of the Boudouard reaction. Fig. 9 shows that the fraction of hydrogen, as H2, in the fuel gas decreased with decreasing temperature. This was due to the decreasing rate of the steam-C reaction and reduced extents of fuel pyrolysis. The value also decreased with increasing steam/CO2 ratio at 850 °C and 950 °C. This shows that although the hydrogen input increased with the increase in the steam input, the extent of H2 production could not keep pace, because of limitations in the amount of available C. However, as identified in Fig. 8, steam conversion under these conditions was constant at approximately 40% and was found to be independent of the amount of steam injected. This is consistent with the above result if the steam successfully competed with the remaining CO2 in the gasification process and the CO2 conversion fell with the increased steam input. CH4 (Fig. 6) is mainly a product of pyrolysis and its release is fairly constant in the range tested. Therefore, total hydrogen to CH4 conversion (Fig. 9) decreased due to the dilution by increased input of hydrogen with increasing steam/CO2 ratio. 3.2.2. Effect of Steam/CO2 ratio at elevated pressure It should be noted that different flow rates of coal and gasification agent were used at atmospheric pressure experiments compared with those at pressurised conditions, while (CO2 + steam)/C ratio were kept constant at 2 in the following series of tests. As discussed above, this is to enable comparison of results at 5 and 10 bara, because the superficial velocity is increased in inverse proportion to the pressure and there is a 6
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Fig. 9. Hydrogen distribution in the fuel gas at different temperatures and steam/CO2 ratios ( H2, H2O, CH4).
Hydrogen Product Distribution
100%
75%
50%
25%
0%
Stream/CO2 Ratio:
0
0.33 950 C
1
0
0.3
1
2.3
850 C
0
0.3 750 C
Fig. 10. Fuel gas composition in the gasification of GL in steam/CO2 mixtures at 850 °C at different pressures (♢CO, □ CO2, Δ H2, × CH4).
Fig. 11. Dry fuel gas yields/kg GL at 850 °C at different pressures and steam/CO2 ratios (♢CO, □ CO2, Δ H2, × CH4, ● total fuel gas volume on a dry basis).
maximum superficial velocity limitation to prevent the entrainment of the feedstock from the gasifier. The influence of adding steam to the fluidising gas on the fuel gas composition and gas yields, as the pressure was raised to 10 bara is shown in Figs. 10 and 11. The trends in the fuel gas concentrations (Fig. 10) and fuel gas yields (Fig. 11) against the inlet steam/CO2 ratio are similar at each tested pressure. With increasing steam input, the H2 concentration increased, CH4 showed a small increase and the CO and CO2 concentrations decreased at each pressure. The total dry fuel gas yield was reduced by the increase of
steam concentration, due to the impact of the undecomposed steam in the outlet gas, which was removed before the fuel gas flow was measured, by the dry gas meter. This effect will also have caused an apparent increase in the concentration of the gases in the fuel gas on a dry basis, as the steam input was raised. Therefore, the gas concentrations have been discussed (below) in terms of their yield per unit mass of fuel input. The data from Fig. 11 suggests that the CH4 yield increased by a small amount when steam was added and the pressure was raised. 7
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Fig. 12. CO2 conversion at 850 °C at different pressures and steam/CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
Fig. 13. Dry fuel gas heating value (LHV) at 850 °C at different pressures and steam/CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
However, the effect is minor and not of technological significance. The H2 yield increased from 0.2 to 0.5 m3 kg−1 lignite at atmospheric pressure, and from 0.1 to 0.4 m3 kg−1 at both 5 and 10 bara, when the inlet steam concentration was increased. The increase in the H2 production with steam addition was mainly via the steam/C gasification reaction. The increase in the H2 concentration with steam input (0.3 m3 kg−1) was the same at each pressure. The CO yield decreased from 1.3 to 0.9 m3 kg−1 at 1 bara and from 0.9 to 0.7 m3 kg−1 at 5 and 10 bara with the increasing input steam concentration. This is because the Boudouard reaction was increasingly replaced by the steam-char reaction. In addition, the water gas shift reaction will have been affected by the increase in steam and decrease in CO2 inputs, as the equilibrium concentrations would be altered. This could also have enhanced the H2 concentration. CO2 conversion as a function of steam/CO2 ratio and pressure is shown in Fig. 12. CO2 was consumed through the Boudouard reaction, and it could be produced through the water gas shift reaction and pyrolysis, so the overall effect is a balance between these opposing influences. The extent of pyrolysis was constant at the different ratios of gasification agents, because the coal feed rate was not changed. The CO2 conversion decreased with increases in the inlet steam concentration. There are two possible reasons for this observation: (1) the extent of Boudouard reaction was reduced as more steam was injected and this competed successfully for the char active sites, (2) CO2 was increased by the equilibrium shift of water gas shift reaction. In the experiment using 60% CO2 and 40% steam (the highest steam input used) as gasification medium at 1 bara, the CO2 conversion was negative. This indicates more CO2 was produced through pyrolysis and water gas shift reaction than was consumed by the Boudouard reaction. This shows that the steam/C reaction was dominant. Fig. 13 and Fig. 14 show the data for the lower heating value and chemical energy output in the dry fuel gas during steam/CO2 gasification. The heating value was in the range 4.5 to 6.3 MJ Nm−3, which compares with literature values between 4 and 12 MJ Nm−3 for syngas [24–28]. The heating value of the fuel gas from pure CO2 gasification at atmospheric pressure was about 5.5 MJ Nm−3, which was lower than the values shown in Fig. 3 (6 to 8 MJ Nm−3 for 850 °C). This was because higher CO2/C ratio (2) was used than that (1–1.8) for previous set of experiments, resulting a dilution of CO, H2 and CH4. At each tested pressure, there is an increasing trend in the dry gas heating value with the rise in the inlet steam concentration. This is mainly due to the increase in the hydrogen concentration and the decrease in the dry fuel gas flow rate. However, the energy output in the dry fuel gas was relatively constant as steam input concentration was increased, which was due to decreases in the dry gas yield and increases in the
Fig. 14. Energy output of dry fuel gas at 850 °C at different pressures and steam/CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
Fig. 15. CO/H2 ratio of the fuel gas at 850 °C at different pressures and steam/ CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
percentage of the combustible components. At pressures of 5 and 10 bara, the heating value and energy output followed similar trends. The CO/H2 ratio in the gas from steam/CO2 gasification is shown in 8
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showing that it was mostly due to pyrolysis. The LHV of the fuel gas reached values of up to 10 MJ m−3. It increased with temperature and decreased with the CO2/C ratio at all temperatures. The H2 in the fuel gas in this part of the work was produced mainly by pyrolysis and by decomposition (by gasification) of the moisture in the fuel. The amount increased with temperature (as expected), but decreased slightly with increasing CO2/C ratio, which reflects the impact of higher CO2 concentrations on the water gas shift equilibrium. As the inlet CO2 was replaced by steam (up to 70%, by vol) during tests at atmospheric pressure, the limiting carbon conversion was reached at 850 °C. Tests have also been done with steam/CO2 mixtures at elevated pressure (up to 10 bara) at 850 °C. The H2 output increased and the CO decreased by similar amounts at 5 and 10 bara, as the steam/CO2 ratio was raised. However, when the pressure was raised to 5 and 10 bara, the carbon conversions were lower than at atmospheric pressure. Conversions increased from 55 to 65%, over the range of steam inputs and at both pressures. The drop in carbon conversion at higher pressures is consistent with deactivation of char by intra and inter particle deposition of unreactive carbon, produced from the lignite volatiles. Overall, the results show that the performance of the gasifier can be varied by altering the composition and amounts of the various input flows and temperature. The results will also be equipment dependent, as this will influence the nature of the circulation and fluidisation in the gasifier. This means that any larger scale developments of the concept will need to have a built-in flexibility in the range of conditions that can be tested to optimise the various performance factors which influence the efficiency and economics of operation.
Fig. 16. Carbon conversion at 850 °C at different pressures and steam/CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
Fig. 15. When the steam concentration increased, the CO/H2 ratio dropped due to the increase of the H2 concentration and reduction of the CO concentration. It can be seen that the CO/H2 ratio can be adjusted within the experimental range studied by changing the ratio of inlet steam/CO2. The carbon conversion is an important parameter in assessing the performance of gasification processes. It indicates the efficiency of utilisation of the solid fuel, which influences the energy output per unit size and the capital and operating costs. The carbon conversion measured from these tests with steam/CO2 at different pressures is shown in Fig. 16. This shows that the carbon conversion increased when the steam input was increased from 0 to 40% (steam/CO2 molar ratio: 0–0.67). At atmospheric pressure, the carbon conversion increased from 83 to 96%. These results are higher than those presented in section 3.2.1 (maximum around 85%) at the same pressure (1 bara) and temperature (850 °C). This is because the flow rate of gasification agent and fuel feed rate were lower, which resulted in less elutriation from the bed. When the pressure was raised from 5 to 10 bara, the carbon conversions were lower than at atmospheric pressure, but increased from 55 to 65%, over the range of steam inputs and at both pressures. The increase in carbon conversion, at a given pressure, reflects the higher reactivity of steam than CO2. The drop in carbon conversion from atmospheric to higher pressures is consistent with deactivation of char by intra and inter particle deposition of unreactive carbon produced from the lignite volatiles. The detailed discussion of this can be found in previous work [29].
Acknowledgements The research leading to these results has received funding from the European Union’s Research Fund for Coal and Steel (RFCS) research programme under grant agreements RFCR-CT-2007-0005 and RFCRCT-2010-0009. References [1] Collot A-G. Matching gasifiers to coals. International Energy Agency; 2002. [2] Bressan L. 2005. IGCC plants: a practical pathway for combined production of hydrogen and power from fossil fuels. In: International Hydrogen Energy Congress and Exhibition IHEC. Istanbul, Turkey. [3] Davison J, Bressan L, Domenichini RM. 2003. Coal Power Plants with CO2 capture: the IGCC option. In: Gasification Technologies Conference. San Francisco, USA. [4] Spiegl N, et al. Investigation of the Oxy-fuel Gasification of Coal in a LaboratoryScale Spouted-Bed Reactor: Reactor Modifications and Initial Results. Energy Fuels 2010;24(9):5281–8. [5] Higman C, Van der Burgt M. 2011. Gasification. 2011: Gulf professional publishing. [6] Gross M, Wolff J. 2000. Gasification of Residue as a Source of Hydrogen for Refining Industry in India. In: Gasification Technologies Conference. San Francisco, USA. [7] Romano MC, Lozza GG. Long-term coal gasification-based power plants with nearzero emissions. Part A: Zecomix cycle. Int J Greenhouse Gas Control 2010;4(3):459–68. [8] Romano MC, Lozza GG. Long-term coal gasification-based power with near-zero emissions. Part B: Zecomag and oxy-fuel IGCC cycles. Int J Greenhouse Gas Control 2010;4(3):469–77. [9] Jillson KR, Chapalamadugu V, Erik Ydstie B. Inventory and flow control of the IGCC process with CO2 recycles. J Process Control 2009;19(9):1470–85. [10] Andries J, Becht JGM. Pressurized fluidized bed gasification of coal using flue gas recirculation and oxygen injection. Energy Convers Manage 1996;37(6):855–60. [11] Andries J, Becht JGM, Hoppesteyn PDJ. Pressurized fluidized bed combustion and gasification of coal using flue gas recirculation and oxygen injection. Energy Convers Manage 1997;38:S117–22. [12] Lath E, Herbert P. Make CO from Coke, CO2 and O2. Hydrocarbon Processing 1986;65(8):55–6. [13] Shao Y, Golomb D. Power plants with CO2 capture using integrated air separation and flue gas recycling. Energy Convers Manage 1996;37(6):903–8. [14] Normann F, Thunman H, Johnsson F. Process analysis of an oxygen lean oxy-fuel power plant with co-production of synthesis gas. Energy Convers Manage 2009;50(2):279–86. [15] Williams A, et al. Combustion and Gasification of Coal. New York: Taylor and Francis; 2000. [16] Ergun S. Kinetics of the reactions of carbon dioxide and steam with coke. Technical
4. Conclusions A laboratory scale, a spouted bed gasifier has been used to investigate simulated oxy-fuel gasification. The aim has been to study the impact of fluidising gas composition (CO2/C and steam/CO2 ratios) at different temperatures and pressures. It was found that during tests with neat CO2, at atmospheric pressure, a limiting carbon conversion of 85% was reached at 950 °C, and all studied CO2/C ratios. This limit is thought to be due to elutriation from the bed under these conditions. However, it does show that high carbon conversions are achievable. Tests with a lower feed rate and therefore lower CO2 input flow rate (and lower fluidising velocity), achieved a higher carbon conversion because elutriation was reduced. By contrast, the carbon conversion is sensitive to the CO2/C ratio at 850 °C, under conditions which were not carbon limited. At higher CO2/C ratio it is possible to match the complete carbon conversion achieved at 950 °C (to the level limited by elutriation). Conversion at 750 °C was limited and did not show changes with CO2/C ratio, 9
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N. Spiegl, et al. Report Archive & Image. Library 1961. [17] Walker PL. Reaction of Carbon with Carbon Dioxide and Steam. New York, US: Marcel Dekker; 1965. [18] Cousins A, et al. Development of a bench-scale high-pressure fluidized bed reactor and its sequential modification for studying diverse aspects of pyrolysis and gasification of coal and biomass. Energy Fuels 2008;22(4):2491–503. [19] Spiegl N. Oxy-fuel gasification in fluidised beds, in PhD Thesis Imperial College London; 2010. [20] Valin S, et al. Comprehensive study of the influence of total pressure on products yields in fluidized bed gasification of wood sawdust. Fuel Process Technol 2010;91(10):1222–8. [21] Mayerhofer M, et al. Influence of pressure, temperature and steam on tar and gas in allothermal fluidized bed gasification. Fuel 2012;99:204–9. [22] Kristiansen A. Understanding Coal Gasification. International Energy Agency; 1998. [23] Yurum Y. Kinetics in Coal gasification. New trends in coal science. Dordrecht,
Netherlands: Kluwer Academic Publishers; 1988. [24] Kang T-J, et al. A study on the direct catalytic steam gasification of coal for the bench-scale system. Korean J Chem Eng 2017. [25] Herdel P, et al. Experimental investigations in a demonstration plant for fluidized bed gasification of multiple feedstock’s in 0.5MWth scale. Fuel 2017;205(Supplement C):286–96. [26] Xiang W-G, Zhao C-S, Pang K-L. Experimental Investigation of Natural Coke Steam Gasification in a Bench-Scale Fluidized Bed: Influences of Temperature and Oxygen Flow Rate. Energy Fuels 2009;23(1):805–10. [27] Lv PM, et al. An experimental study on biomass air-steam gasification in a fluidized bed. Bioresour Technol 2004;95(1):95–101. [28] Karmakar MK, Datta AB. Generation of hydrogen rich gas through fluidized bed gasification of biomass. Bioresour Technol 2011;102(2):1907–13. [29] Long X. 2018. Advanced fluidised bed gasification for carbon capture and fuel cell applications, in PhD Thesis. Imperial College London.
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Full Length Article
Production of a fuel gas by fluidised bed coal gasification compatible with CO2 capture
T
Nicolas Spiegl, Cesar Berrueco, Xiangyi Long, Nigel Paterson, Marcos Millan
⁎
Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK
ARTICLE INFO
ABSTRACT
Keywords: Coal gasification Fluidised bed reactor Spouted bed Oxy-fuel Steam/CO2 gasification
A continuously fed, laboratory scale spouted bed gasifier has been used to study oxy-fuel gasification of German lignite. In this paper, the influence of different gasification agents and bed temperature on the process performance, during tests at atmospheric and elevated pressure are studied. Two gasification agents have been used, CO2 (with different CO2/C ratios) and mixtures of CO2/steam. The results show that despite the relatively slow CO2-char reaction, good gasification performance could be achieved with German lignite by adjusting the operating conditions at atmospheric pressure: complete carbon conversion, high energy conversion and a medium heating value fuel gas (8–10 MJ m−3). The CO2/C ratio was found to have a large effect on the gasification performance. Increasing the ratio increased the carbon conversion, but the CO2 conversion decreased. At 950 °C, maximum carbon conversion was already achieved with pure CO2, therefore using steam at this temperature could not increase the conversion, but did increase the H2/CO ratio in the fuel gas. At 850 °C, replacing 25% of CO2 with steam increased the carbon conversion to the level achieved at 950 °C without steam. Replacing more than 25% of CO2 with steam increased the H2/CO ratio further. Therefore, with the addition of steam, the operating temperature could be reduced from 950 °C to 850 °C while maintaining the gasification performance. The changes of gasification performance with steam addition at pressures up to 10 bara followed the same trends achieved at atmospheric pressure.
1. Introduction The higher efficiency, lower emission of greenhouse gases and market flexibility mean that gasification technology is a promising option to improve the efficiency of power generation compared to conventional coal fired pulverised fuel power plants [1]. This can involve Integrated Gasification Combined Cycle (IGCC) technology at large scales or Integrated Gasification Fuel Cell concept at smaller scales. A list of the principal reactions that may occur during gasification is shown in Table 1. Oxygen blown gasification is considered the most promising route for reducing carbon emissions, as it can be combined with carbon capture and storage (CCS), avoiding energy and cost intensive processing for downstream CO2-N2 separation [2,3]. Fluidised bed gasification has potential for this technology option, due to fuel versatility, including the possibility to use low-grade fuels. To combine the advantages of gasification, fluidised bed operation and oxygen blown processes, a novel process configuration proposed in a previous article [4] involves operation of a fluidised bed gasifier with O2 and recycled CO2. In combination with an IGCC power plant, this process concept
⁎
has the potential to produce electricity with a higher efficiency than a combustion based process and to use both fossil and biomass based fuels. It would produce a stream of CO2 ready for sequestration. This process option can be regarded as a further development of the concept of oxy-fuel combustion, where O2 can be supplied together with CO2 to the gasifier or to a separate combustion bed in dual fluidised bed systems. The actual use of recycled CO2 in large scale gasification processes is not very common. It is occasionally used to replace nitrogen for pneumatic fuel transportation - especially when it is desirable to avoid nitrogen contamination of the syngas [5]. More frequently, recycled CO2 is used in syngas production by reforming, to adjust the CO/H2 ratio [6]. The concept of operating a fluidised bed gasifier with pure O2 and recycled CO2 is a novel process configuration. Nevertheless, using recycled CO2 for different purposes was considered by a number of authors proposing new process options, as discussed below. Romano et al. [7,8] used thermodynamic equilibrium calculations to assess the performance of three novel plant configurations for nearzero emissions power generation from coal. One of the plants consisted of an entrained flow gasifier operating at 70 bar. The gasifier was O2/
Corresponding author. E-mail address: [email protected] (M. Millan).
https://doi.org/10.1016/j.fuel.2019.116242 Received 15 July 2019; Received in revised form 17 September 2019; Accepted 18 September 2019 Available online 27 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Normann et al. [14] proposed an oxy-fuel process where fuel would be burned in a slightly sub stoichiometric atmosphere. Producing heat and additional flue gas for subsequent chemical synthesis (dimethyl ether production) would reduce the production cost. The process included co-processing of up to 20% biomass. The study was based on an Aspen Plus simulation. The key issue identified was the oxygen lean combustion or gasification and the related issues in coal conversion and high temperature corrosion. From these references, two driving forces behind the development of gasification processes with CO2 recycle can be identified, production of pure CO and an increase in plant efficiency by using the CO2 available from the CCS integration. The second reason is certainly the dominating one. It also seems that the majority of the studies are theoretical and do not address issues arising from the high partial pressure of CO2 in the gasifier. The reactive nature of CO2 in gasification reactions compared to the use of an inert gas, such as nitrogen, will cause a change in the chemistry of the system [15]. CO2 gasification is inhibited by its product CO and requires temperatures higher than 700 °C for the reaction rate to be significant [16,17]. This paper studies the influence of different gasification agents and bed temperature on the process performance in atmospheres that do not contain N2 and are rich in CO2. The first objective is to obtain data about the overall process performance and to judge the potential of a process based on CO2 gasification. The second objective is to gain a better understanding of the mechanism involved in the fuel conversion when using CO2 and mixtures of steam/CO2 as gasification agents and to determine how this can be used to develop strategies to optimise the process performance.
Table 1 Principal gasification reactions. Reaction type
Reaction
ΔH, MJ kmol−1
Combustion reactions
C + 0.5 O2 → CO CO + 0.5 O2 → CO2 H2 + 0.5 O2 → H2O C + CO2 ↔ 2 CO C + H2O ↔ CO + H2 C + 2 H2 ↔ CH4 CO + H2O ↔ CO2 + H2 CH4 + H2O ↔ CO + 3 H2
−111 −283 −242 +172 +131 −75 −41 +206
Boudouard reaction Water gas reaction Methanation reaction Water gas shift reaction Methane steam reforming reaction
steam blown and recycled CO2 was used to replace N2 for dry fuel feeding. The fuel gas stream was cooled to 850 °C in a syngas cooler, cleaned via hot gas particle cleaning technology and combusted in a gas turbine using pure O2 and recycled CO2. The technology required for the hot fuel gas cleaning process and the development of a gas turbine operating with CO2 based stream was seen as the main obstacle for the development of such a process. The overall electrical efficiency of the plant was calculated to be to be around 45% (based on the lower heating value, LHV), including the impact of carbon capture and storage. A different proposal from Jillon et al. [9] presented an IGCC plant with two CO2 recycle streams. In the model, the first stream was used to control the fuel gas composition in a Conoco/Phillips type of gasifier by injecting recycled CO2 together with the oxygen into the gasifier. The second CO2 stream was used to replace N2 as temperature moderator in the gas turbine. Only a simplified model of the gasification process was used to simulate the effect of operating conditions on gasification performance. The CFD model showed two effects of the recycled CO2 on the gasification performance: a change in temperature profile as the rate of the endothermic CO2 gasification of coal increased by increasing CO2 recycling rate and the possibility of controlling the syngas composition with the CO2 recycling rate. No information was given whether a real gas turbine was used as a basis for the model. The emphasis of the work was on process control and therefore no further information about the feasibility of the project or possible technical and financial challenges were given. Andries et al. [10] reported plans to modify a pressurised fluidised bed reactor with 1.6 MW thermal capacity for oxy-fuel gasification and combustion with flue gas recycling. The fluidised bed was connected to a combustor, where fuel gas could be combusted with additional oxygen. The objective of the project was to study the effect of operational parameters on fuel gas composition, conversion, in-situ sulphur removal and formation of nitrogen compounds. A second paper published by the same group [11] reported primary results of oxy-fuel fluidised bed combustion at 850 °C with 24% and 27% O2. However, no results were published on the oxy-fuel gasification concept. A new process for the production of CO was proposed by Lath et al. [12]. The key component would be a slagging fixed bed gasifier, which would be oxygen blown and CO2 used as a temperature moderator. The proposed process would operate under slightly elevated pressure and would produce a flue gas high in CO (90–92%). Furthermore, it was claimed that running a gasifier under such conditions would be economic. In another theoretical study, Shao et al. [13] described a power plant concept based on a combined cycle powered either by natural gas or fuel gas from a gasifier. Pure oxygen and steam were used as gasification agent, whereas the gas turbine was operated with pure O2 and recycled CO2 to improve the integration with CCS. The main focus of the paper was the possible integration of the air separation unit with the CO2 capture unit. The problem to operate a gas turbine with a CO2 based stream was not addressed in this report. The performance of the gasifier was calculated based on thermodynamic equilibrium calculations.
2. Experimental 2.1. Spouted-Bed gasifier A bench-scale fluidised-bed reactor was modified to allow continuous operation under oxy-fuel gasification conditions, i.e. in CO2 rich environments with temperatures up to 1000 °C and pressures up to 2 MPa. O2 was not used in this study, as the main objective was to investigate the influence of CO2 rich gas, as gasification agent, on its performance. The heat for gasification was provided by electrical resistance heating. This reactor has been successfully operated in several configurations [18], and the details of the modifications to the reactor to enable operation under CO2 rich oxy-fuel conditions and the experimental procedure are described in an earlier paper [4]. Briefly, the reactor consists of a 504-mm-long (34 mm internal diameter) Incolloy 800 HT column, which is heated by a high-current, low-voltage alternating current (AC) with two copper electrodes connected to the reactor body. A fluidised sand bed is held inside a quartz liner (28 mm i.d.), with a spout jet inlet at the apex of the inverted cone shaped base. The solid fuel is continuously injected into the bed through the spout jet, which provides effective mixing with the bed material and avoids agglomeration at the base of the gasifier, with feed rates from 0.5 to 6 g min−1. 2.2. Fuel gas analysis Online gas analyzers (Servomex Xentra 4200 for O2 and CO2; ADC analyzer for CO and CH4 and Hitech K1500 for H2) were used to quantify the major components in the fuel gas produced during the experiments. The analyzers were calibrated before each experiment using a certified calibration gas mixture (BOC Ltd.). All gas concentrations are presented on a dry basis. The volumetric flow rate of the fuel gas was measured using a dry gas meter (Model G6, Meters UK Ltd). After measuring the fuel gas flow rate and fuel gas concentrations, the CO2 conversion (conv.CO2, %) was calculated from the inlet and outlet CO2 flowrates by Eq. (1). 2
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conv .CO2 =
CO2 in CO2 out × 100% CO2 in
Table 3 Operating Conditions for Gasification of GL with Different Temperatures, Pressures and Gasification Agents.
(1)
The LHV of the fuel gas (LHVfuel gas, MJ m−3) was calculated from the contribution of each gas by Eq. (2), which involves the heating value of the gas components (LHVCO, LHVH2, LHVCH4, MJ m−3) and their volumetric concentration (CCO, CH2, CCH4, %vol.).
LHVfuel gas = LHVCO × cCO + LHVH2 × c H2 + LHVCH4 × cCH4
(2)
The C conversion (conv.C, %) to gas products excluding tars was calculated from the mol flowrates of C in the coal and inlet and outlet gas by Eq. (3).
conv .C =
C outlet gas C inlet gas × 100% C in the coal
(3)
To gain an insight into the underlying reactions taking place, the hydrogen product distribution (fraction of hydrogen in the fuel gas as H2, H2O and CH4) was estimated. The fraction of H2O was determined by difference, performing a balance between the hydrogen input in the coal and steam and the ouput in H2, CH4 and steam. 2.3. Feedstock German lignite coal (GL) was used for all the experiments in this study (Table 2). The fuel sample was ground using a swinging hammer mill and then sieved to recover fuel in the size range of 200–300 µm. Acid-washed silica sand (particle size of 100–300 µm, VWR) was used as the initial bed material in the FBR. 3. Results and discussion
Table 2 Feedstock Analysis. German lignite 1
Prox. Analysis Moisture % Ash % Volatile Matter % Fixed Carbon %2 Ult. Analysis1 Sulphur % Chlorine % Carbon % Hydrogen %3 Oxygen %2, 3 Nitrogen % 1 2 3
Pressure [bara]
Gas
CO2/C or steam/ CO2 ratio
Fuel Feed Rate [g min−1]
Flow [NL min−1]
750 750 750 850 850 850 950 950 950 750 750 850 850 850 850 950 950 950 850 850 850 850 850 850 850 850 850
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5 5 5 10 10 10
CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/ Steam/
0.9 1.1 1.3 1.0 1.4 1.8 0.9 1.1 1.4 0 0.3 0 0.3 1.0 2.3 0 0.3 1.0 0 0.25 0.67 0 0.25 0.67 0 0.25 0.67
1.7 1.7 1.8 1.6 1.7 1.0 1.8 2.3 1.7 1.8 1.6 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.0 1.0 1.0 3.9 3.9 3.9 3.9 3.9 3.9
1.8 2.0 2.6 1.8 2.6 2.0 1.8 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.7 2.6 2.6 2.6 2.4 2.4 2.4 9.0 9.0 9.0 9.0 9.0 9.0
CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2
CO2/C ratios for the first set of experiments and several CO2/steam ratios (0, 0.3, 1 and 2.3) for the second set of experiments. Different CO2/C ratios were achieved by altering the amount of CO2 injected into the reactor and keeping the fuel feeding rate constant. This changed the superficial velocity (Us), but maintained the contribution of pyrolysis to the overall performance due to the constant fuel feeding rate. Additional tests were performed to test this assumption and these showed that the performance was not affected by the change in fluidising velocity over the range studied [19]. The different steam/CO2 ratios were obtained by replacing between 20 and 75% of CO2 by steam (molar basis). The total flow rate (Fin) entering the reactor was kept constant: Fin = 2.6 NL min−1 and Us = 0.24–0.27 m s−1 for the second set of experiments. In the third set of experiments the pressures tested were 1, 5 and 10 bara. As gas density increases with pressure, keeping constant coal and gas (steam and CO2) mole flow rates would cause a decrease in the gas volumetric flow rate and therefore in the superficial velocity, while the gas residence time would increase. In order to maintain a reasonable fluidising velocity and fuel/gas input ratio, the feed to the system was adjusted to obtain conditions that enabled steady operation. Constraints in superficial velocity and the range of solid feeding rates achievable in the system did not allow the same coal to gasification gas flow rate ratio to be maintained in the range from 1 to 10 bara. Therefore, the flow rates of coal and gasification gas were kept the same only for 5 and 10 bara experiments, while the mole ratio of inlet gasification gas to carbon in the coal was kept constant at 2. The same approach has been used in other studies [20,21]. Gas flowrates of Fin = 2.39 and 9.01 NL min−1 were used at atmospheric and high pressures (5 and 10 bara), respectively. Superficial velocities were Us = 0.25, 0.19 and 0.09 m s−1 for 1, 5 and 10 bara respectively.
This paper studies the influence of gasification agent, temperature and pressure on the process performance. Temperature is investigated independently from the fuel to oxidizer ratio by using the capability of the experimental set-up to heat the process externally to the desired temperature. The discussion of the results is separated into three sections. The first two show the results obtained at different temperatures using CO2 and steam/CO2 as gasification agents, at atmospheric pressure. The third part discusses the effect of operation at elevated pressure with CO2/steam mixtures. The main parameters used to judge the process performance in each set of tests are: fuel gas composition, heating value and carbon conversion. To gain an insight into the underlying reactions taking place, the gas product distribution, the steam and CO2 decomposition and the hydrogen product distribution (fraction of hydrogen in the fuel gas as H2, H2O and CH4) have been estimated. The conditions for the sets of experiments are summarised in Table 3. Fuel was gasified at 750 °C, 850 °C and 950 °C using different
Sample
T [°C]
13.2 4.1 44.0 38.7 0.23 0.02 57.39 4.03 20.42 0.61
3.1. Effect of CO2/C ratio
“As received” basis By difference Corrected for moisture
Increasing the CO2/C ratio increases the amount of reactive gas per unit fuel and therefore potentially affects the extent of char gasification. 3
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70 60
[vol., dry, %]
50 40
30 20 10
CO2/C Ratio:
0
0.9
1.1
1.4
1
1.4
950 C
1.8
0.9
1.1
850 C
1.3
750 C
Fig. 1. Gas composition obtained from the gasification of GL at different temperatures and CO2/C ratios (□ CO2, ♢CO, Δ H2, × CH4).
At 850 °C results show that the char-CO2 reaction seems to be fast enough to be influenced by the partial pressure of CO2. At a CO2/C ratio of 1.8 it reaches the maximum carbon conversion of 85%. At 750 °C the char-CO2 reaction is slow, and the majority of the carbon conversion was a result of pyrolysis. Therefore, increasing CO2/C ratio had only a limited effect on overall conversion, but the fuel gas analysis showed dilution by the increasing CO2 inlet flow. The carbon conversion was nearly equal to the carbon content of the volatiles, plus a small amount from a limited extent of gasification by CO2. Increasing the CO2/C ratio diluted the fuel gas and therefore decreased the heating value of the fuel gas at each temperature (Fig. 3). At 950 °C, the value decreased from 10 to 8.5 MJ m−3 as the CO2/C ratio was raised. These are reasonable values for a syngas, reflecting that a gas with medium heating value can be obtained. The heating values decreased at the lower temperatures, but the trends at each temperature were similar as the CO2/C ratio was increased. Another important parameter is the CO2 conversion, which is a guide to the efficiency of the process. CO2 conversion is an indicator of the fraction of CO2 used by gasification compared to the CO2 passing the reactor as a quasi-inert gas. Maximising the CO2 conversion is important for the process efficiency as the amount of unconverted CO2 increases the amount of energy required to heat the input gas stream to gasification temperature and dilutes the fuel gas. However, it is also noted that the amount of CO2 in a fluidised bed gasifier cannot be varied independently, as a certain amount is required to ensure
Carbon Conversion [%]
100
75
50
25
0 0.8
1.0
1.2
1.4
1.6
1.8
CO2/C Ratio Fig. 2. Carbon conversion at different temperatures and CO2/C ratios (♢950 °C, □ 850 °C, Δ 750 °C).
However, increasing the amount of CO2 entering the reactor increases the heat required to raise the input stream to gasification temperature, due to the high heat capacity of CO2. The fuel gas compositions obtained in tests performed using different CO2/C ratios are summarised in Fig. 1, while Fig. 2 shows the changes in carbon conversion. It can be seen that at 950 °C, the concentration of CO decreased with increasing CO2/C ratio (and fluidising velocity), which was accompanied by an increase in the CO2 output. These changes diminished with decreasing temperature. H2 was released mainly by pyrolysis in the CO2 atmosphere and the amount followed a similar trend to the CO output. The accompanying carbon conversions show that the value was nearly constant at 85% at 950 °C. At 850 °C, the carbon conversion increased from 60 to 85% with increasing CO2/C ratio. At 750 °C, a small increase in carbon conversion from 32 to 37% with increasing ratio was seen. The results reveal that at 950 °C a near maximum possible carbon conversion is reached. These experiments were carried out with a high superficial velocity (up to Us = 0.27 m s−1 at the highest CO2/C ratio). Examination of the bed material recovered after each test showed that this resulted in a larger number of char particles being ejected from the bed and collected in the tar trap. Therefore, the limit of 85% conversion is probably due to the carbon loss from the bed by entrainment during the experiment. Therefore, the sequence of tests at 950 °C, was limited by the availability of C. Even with the lowest CO2/C ratio, the input carbon was reacted efficiently. At this temperature, the amount of CO in the fuel gas decreased with increasing CO2/C ratio, because it was diluted by the higher input flow of CO2.
10
LHV [MJ/m3]
8
6 4 2 0 0.8
1.0
1.2
1.4
1.6
1.8
CO2/C Ratio Fig. 3. Fuel gas heating value (LHV) at different temperatures and CO2/C ratios (♢950 °C, □ 850 °C, Δ 750 °C). 4
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with temperature can be explained by an increase in the extent of pyrolysis of the lignite volatiles and by the shift of equilibrium of water–gas shift reaction towards higher percentage of H2 due to the increase of CO concentration. The carbon conversion increases with temperature and this will have increased the amount of carbon in the gas phase as CO. This is considered to be the main reason for the observation, although increases in the extent of the char-steam reaction and approach towards thermodynamic equilibrium may also have contributed. Steam in the fuel gas is produced from the fuel-moisture and steam release during pyrolysis. Steam is a reactive gasification agent, particularly at the higher temperatures used. Hydrogen to CH4 conversion was between 6 and 8% for all conditions investigated. These values are similar to the observed for oxy-fuel conditions with O2 (shown in previous work [4]). The comparison between the results obtained under pyrolysis (not shown) and gasification conditions indicates that CH4 is produced mainly during pyrolysis and reaches a maximum at 750 °C. Its gas phase concentration is controlled by the endothermic steam methane reforming reaction. Equilibrium predicts decreasing CH4 concentration with increasing temperature.
CO2 Conversion [%]
50 40
30 20 10
0 0.8
1.0
1.2
1.4
1.6
1.8
CO2/C Ratio Fig. 4. CO2 conversion at different CO2/C ratios and temperatures (♢ 950 °C, □ 850 °C, Δ 750 °C).
adequate fluidization of the bed. The actual input is therefore a balance between the needs for an efficient process and the needs of fluidisation. The CO2 conversion can be estimated from the CO2 balance for the reactor for each condition. The value includes effects of pyrolysis and water–gas shift reaction, together with the amount involved in gasification. However, the influence of pyrolysis is considered to be small in comparison with the total amount of CO2 in the fuel gas and there seems to be no significant effect from the water–gas shift reaction on the concentration of CO and CO2 (discussed below). Therefore, the overall balance of CO2 in the process is considered as a reasonable approach to calculate the CO2 conversion. The results are shown in Fig. 4. At 950 °C, the carbon conversion had already reached a maximum with the lowest ratio and no extra CO2 could be converted as the ratio was raised. The fraction of CO2 converted therefore decreased with increasing CO2/C ratio. A different situation can be observed at 750 °C, where CO2 conversion slightly increases, which reflects the small change in the carbon conversion, probably by gasification. The most interesting case is 850 °C. Under the conditions used, a maximum in CO2 conversion can be observed at a CO2/C rate of 1.4. At this point, carbon conversion versus CO2 conversion graph reached a maximum. For this set of experiments, 73% of the hydrogen enters the reactor as fuel-hydrogen, while the balance was from the fuel-moisture. The hydrogen distribution (fraction of hydrogen in the fuel gas as H2, H2O and CH4) in the different products is shown in Fig. 5. The concentration of H2 (shown in Fig. 1) increases with temperature (11–12% at 750 °C to 18–19% at 950 °C), but there seems to be no significant change with CO2/C ratio. The increased concentrations of H2
3.2. Effect of Steam/CO2 ratio 3.2.1. Effect of Steam/CO2 ratio at atmospheric pressure It is well known that the gasification with steam is faster than the reaction with CO2 [5,15,22,23]. To investigate the impact of this in the spouted bed reactor, CO2 was stepwise replaced with steam as the fluidising/gasification agent. Adding steam to a larger scale O2/CO2 blown process could be considered for two reasons: using the faster char-steam rate to increase carbon conversion and heating value of the fuel gas, and increasing the H2/CO ratio in the syngas, as required by downstream applications. Different CO2/steam ratios were obtained by replacing 25%, 50% and 75% of CO2 by steam (molar basis) from baseline experiments with 100% CO2 at each temperature where the total gas flow rate entering the reactor was kept constant: Fin = 2.6 NL min−1, Us = 0.24–0.27 m s−1 and CO2/C ratio = 0.4–1.4. As described above, these conditions led to incomplete carbon conversions at 750 °C and 850 °C, and maximum carbon conversion at 950 °C. The set of experiments carried out are presented in Table 3, and the resulting gas composition is shown in Fig. 6. The reaction chemistry is altered as the proportion of inlet steam is raised. Because of its higher rate, the steam-C reaction will tend to compete more favourably than CO2 for the available C. This will increase the proportion of H2 in the gas, which will impact on the water gas shift reaction. These changes can be clearly seen in the fuel gas composition in Fig. 6. At each temperature, replacing CO2 with steam increases the H2 concentration and decreases the CO concentration in the fuel gas. Fig. 5. Hydrogen product distribution at different temperatures and CO2/C ratios ( H2, H2O, CH4).
Hydrogen Product Distribtuion
100%
75%
50%
25%
0% CO2/C Ratio:
0.9
1.1 950 C
1.4
1
1.4
1.8
0.9
850 C
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5
1.3
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70
[vol., dry, %]
60 50
40 30
20 10
0 Steam/CO2 Ratio:
0
0.33
1
0
0.3
950 C
1
2.3
0
0.3 750 C
850 C
Fig. 6. Gas composition obtained from the gasification of GL in steam/CO2 mixtures at different temperatures (♢CO, □ CO2, Δ H2, × CH4).
100
60
Steam Conversion [%]
Carbon Conversion [%]
40
75
50
25
20 0 -20 -40
-60 -80 -100
-120
0 0.00
0.33
1.00
0.00
2.33
0.33
1.00
2.33
Steam/CO2 Ratio
Steam/CO2 Ratio Fig. 7. Carbon conversion at different temperatures and steam/CO2 ratios (♢950 °C, □ 850 °C, Δ 750 °C).
Fig. 8. Steam conversion at different temperatures and steam/CO2 ratios (♢950 °C, □ 850 °C, Δ 750 °C).
Fig. 7 shows the change in carbon conversion with increasing steam/CO2 ratio. It can be seen that replacing 25% of CO2 by steam increased the carbon conversion at 750 °C and 850 °C while at 950 °C no change occurred (as expected). This result is similar to the one discussed in the previous section, showing a maximum possible carbon conversion of 85% at 950 °C, independent of CO2/C ratio and steam/ CO2 ratio and was limited by elutriation of fines from the fluidised bed. At 850 °C carbon conversion was increased by 25% steam addition to a level comparable with results from experiments carried out at 950 °C with no added steam. Further increasing steam/CO2 ratio did not change the carbon conversion at 850 and 950 °C. The results clearly show the effect of the faster steam-C reaction compared to the CO2-C reaction, which enables maximum conversion to be reached at lower temperatures. The steam conversion has also been calculated and the values are shown in Fig. 8. Steam input is defined as steam in the fluidising gas plus steam formed from moisture in the fuel. In the case of 750 °C and no steam addition a negative value is calculated, which indicates that more steam is generated during pyrolysis than is used in the char-steam reaction. The value ‘increased’ to zero when steam was added, which shows that some reaction did occur with steam, even at 750 °C. At higher temperatures, the steam conversion levelled off at around 40% for all levels of steam addition. This suggests the system was carbon limited, as was found for the experiments using CO2 alone at 950 °C. As the steam/CO2 ratio was increased, more steam reacted (on a mass
basis), but at the expense of the Boudouard reaction. Fig. 9 shows that the fraction of hydrogen, as H2, in the fuel gas decreased with decreasing temperature. This was due to the decreasing rate of the steam-C reaction and reduced extents of fuel pyrolysis. The value also decreased with increasing steam/CO2 ratio at 850 °C and 950 °C. This shows that although the hydrogen input increased with the increase in the steam input, the extent of H2 production could not keep pace, because of limitations in the amount of available C. However, as identified in Fig. 8, steam conversion under these conditions was constant at approximately 40% and was found to be independent of the amount of steam injected. This is consistent with the above result if the steam successfully competed with the remaining CO2 in the gasification process and the CO2 conversion fell with the increased steam input. CH4 (Fig. 6) is mainly a product of pyrolysis and its release is fairly constant in the range tested. Therefore, total hydrogen to CH4 conversion (Fig. 9) decreased due to the dilution by increased input of hydrogen with increasing steam/CO2 ratio. 3.2.2. Effect of Steam/CO2 ratio at elevated pressure It should be noted that different flow rates of coal and gasification agent were used at atmospheric pressure experiments compared with those at pressurised conditions, while (CO2 + steam)/C ratio were kept constant at 2 in the following series of tests. As discussed above, this is to enable comparison of results at 5 and 10 bara, because the superficial velocity is increased in inverse proportion to the pressure and there is a 6
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Fig. 9. Hydrogen distribution in the fuel gas at different temperatures and steam/CO2 ratios ( H2, H2O, CH4).
Hydrogen Product Distribution
100%
75%
50%
25%
0%
Stream/CO2 Ratio:
0
0.33 950 C
1
0
0.3
1
2.3
850 C
0
0.3 750 C
Fig. 10. Fuel gas composition in the gasification of GL in steam/CO2 mixtures at 850 °C at different pressures (♢CO, □ CO2, Δ H2, × CH4).
Fig. 11. Dry fuel gas yields/kg GL at 850 °C at different pressures and steam/CO2 ratios (♢CO, □ CO2, Δ H2, × CH4, ● total fuel gas volume on a dry basis).
maximum superficial velocity limitation to prevent the entrainment of the feedstock from the gasifier. The influence of adding steam to the fluidising gas on the fuel gas composition and gas yields, as the pressure was raised to 10 bara is shown in Figs. 10 and 11. The trends in the fuel gas concentrations (Fig. 10) and fuel gas yields (Fig. 11) against the inlet steam/CO2 ratio are similar at each tested pressure. With increasing steam input, the H2 concentration increased, CH4 showed a small increase and the CO and CO2 concentrations decreased at each pressure. The total dry fuel gas yield was reduced by the increase of
steam concentration, due to the impact of the undecomposed steam in the outlet gas, which was removed before the fuel gas flow was measured, by the dry gas meter. This effect will also have caused an apparent increase in the concentration of the gases in the fuel gas on a dry basis, as the steam input was raised. Therefore, the gas concentrations have been discussed (below) in terms of their yield per unit mass of fuel input. The data from Fig. 11 suggests that the CH4 yield increased by a small amount when steam was added and the pressure was raised. 7
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Fig. 12. CO2 conversion at 850 °C at different pressures and steam/CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
Fig. 13. Dry fuel gas heating value (LHV) at 850 °C at different pressures and steam/CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
However, the effect is minor and not of technological significance. The H2 yield increased from 0.2 to 0.5 m3 kg−1 lignite at atmospheric pressure, and from 0.1 to 0.4 m3 kg−1 at both 5 and 10 bara, when the inlet steam concentration was increased. The increase in the H2 production with steam addition was mainly via the steam/C gasification reaction. The increase in the H2 concentration with steam input (0.3 m3 kg−1) was the same at each pressure. The CO yield decreased from 1.3 to 0.9 m3 kg−1 at 1 bara and from 0.9 to 0.7 m3 kg−1 at 5 and 10 bara with the increasing input steam concentration. This is because the Boudouard reaction was increasingly replaced by the steam-char reaction. In addition, the water gas shift reaction will have been affected by the increase in steam and decrease in CO2 inputs, as the equilibrium concentrations would be altered. This could also have enhanced the H2 concentration. CO2 conversion as a function of steam/CO2 ratio and pressure is shown in Fig. 12. CO2 was consumed through the Boudouard reaction, and it could be produced through the water gas shift reaction and pyrolysis, so the overall effect is a balance between these opposing influences. The extent of pyrolysis was constant at the different ratios of gasification agents, because the coal feed rate was not changed. The CO2 conversion decreased with increases in the inlet steam concentration. There are two possible reasons for this observation: (1) the extent of Boudouard reaction was reduced as more steam was injected and this competed successfully for the char active sites, (2) CO2 was increased by the equilibrium shift of water gas shift reaction. In the experiment using 60% CO2 and 40% steam (the highest steam input used) as gasification medium at 1 bara, the CO2 conversion was negative. This indicates more CO2 was produced through pyrolysis and water gas shift reaction than was consumed by the Boudouard reaction. This shows that the steam/C reaction was dominant. Fig. 13 and Fig. 14 show the data for the lower heating value and chemical energy output in the dry fuel gas during steam/CO2 gasification. The heating value was in the range 4.5 to 6.3 MJ Nm−3, which compares with literature values between 4 and 12 MJ Nm−3 for syngas [24–28]. The heating value of the fuel gas from pure CO2 gasification at atmospheric pressure was about 5.5 MJ Nm−3, which was lower than the values shown in Fig. 3 (6 to 8 MJ Nm−3 for 850 °C). This was because higher CO2/C ratio (2) was used than that (1–1.8) for previous set of experiments, resulting a dilution of CO, H2 and CH4. At each tested pressure, there is an increasing trend in the dry gas heating value with the rise in the inlet steam concentration. This is mainly due to the increase in the hydrogen concentration and the decrease in the dry fuel gas flow rate. However, the energy output in the dry fuel gas was relatively constant as steam input concentration was increased, which was due to decreases in the dry gas yield and increases in the
Fig. 14. Energy output of dry fuel gas at 850 °C at different pressures and steam/CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
Fig. 15. CO/H2 ratio of the fuel gas at 850 °C at different pressures and steam/ CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
percentage of the combustible components. At pressures of 5 and 10 bara, the heating value and energy output followed similar trends. The CO/H2 ratio in the gas from steam/CO2 gasification is shown in 8
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showing that it was mostly due to pyrolysis. The LHV of the fuel gas reached values of up to 10 MJ m−3. It increased with temperature and decreased with the CO2/C ratio at all temperatures. The H2 in the fuel gas in this part of the work was produced mainly by pyrolysis and by decomposition (by gasification) of the moisture in the fuel. The amount increased with temperature (as expected), but decreased slightly with increasing CO2/C ratio, which reflects the impact of higher CO2 concentrations on the water gas shift equilibrium. As the inlet CO2 was replaced by steam (up to 70%, by vol) during tests at atmospheric pressure, the limiting carbon conversion was reached at 850 °C. Tests have also been done with steam/CO2 mixtures at elevated pressure (up to 10 bara) at 850 °C. The H2 output increased and the CO decreased by similar amounts at 5 and 10 bara, as the steam/CO2 ratio was raised. However, when the pressure was raised to 5 and 10 bara, the carbon conversions were lower than at atmospheric pressure. Conversions increased from 55 to 65%, over the range of steam inputs and at both pressures. The drop in carbon conversion at higher pressures is consistent with deactivation of char by intra and inter particle deposition of unreactive carbon, produced from the lignite volatiles. Overall, the results show that the performance of the gasifier can be varied by altering the composition and amounts of the various input flows and temperature. The results will also be equipment dependent, as this will influence the nature of the circulation and fluidisation in the gasifier. This means that any larger scale developments of the concept will need to have a built-in flexibility in the range of conditions that can be tested to optimise the various performance factors which influence the efficiency and economics of operation.
Fig. 16. Carbon conversion at 850 °C at different pressures and steam/CO2 ratios (♢1 bara, □ 5 bara, Δ 10 bara).
Fig. 15. When the steam concentration increased, the CO/H2 ratio dropped due to the increase of the H2 concentration and reduction of the CO concentration. It can be seen that the CO/H2 ratio can be adjusted within the experimental range studied by changing the ratio of inlet steam/CO2. The carbon conversion is an important parameter in assessing the performance of gasification processes. It indicates the efficiency of utilisation of the solid fuel, which influences the energy output per unit size and the capital and operating costs. The carbon conversion measured from these tests with steam/CO2 at different pressures is shown in Fig. 16. This shows that the carbon conversion increased when the steam input was increased from 0 to 40% (steam/CO2 molar ratio: 0–0.67). At atmospheric pressure, the carbon conversion increased from 83 to 96%. These results are higher than those presented in section 3.2.1 (maximum around 85%) at the same pressure (1 bara) and temperature (850 °C). This is because the flow rate of gasification agent and fuel feed rate were lower, which resulted in less elutriation from the bed. When the pressure was raised from 5 to 10 bara, the carbon conversions were lower than at atmospheric pressure, but increased from 55 to 65%, over the range of steam inputs and at both pressures. The increase in carbon conversion, at a given pressure, reflects the higher reactivity of steam than CO2. The drop in carbon conversion from atmospheric to higher pressures is consistent with deactivation of char by intra and inter particle deposition of unreactive carbon produced from the lignite volatiles. The detailed discussion of this can be found in previous work [29].
Acknowledgements The research leading to these results has received funding from the European Union’s Research Fund for Coal and Steel (RFCS) research programme under grant agreements RFCR-CT-2007-0005 and RFCRCT-2010-0009. References [1] Collot A-G. Matching gasifiers to coals. International Energy Agency; 2002. [2] Bressan L. 2005. IGCC plants: a practical pathway for combined production of hydrogen and power from fossil fuels. In: International Hydrogen Energy Congress and Exhibition IHEC. Istanbul, Turkey. [3] Davison J, Bressan L, Domenichini RM. 2003. Coal Power Plants with CO2 capture: the IGCC option. In: Gasification Technologies Conference. San Francisco, USA. [4] Spiegl N, et al. Investigation of the Oxy-fuel Gasification of Coal in a LaboratoryScale Spouted-Bed Reactor: Reactor Modifications and Initial Results. Energy Fuels 2010;24(9):5281–8. [5] Higman C, Van der Burgt M. 2011. Gasification. 2011: Gulf professional publishing. [6] Gross M, Wolff J. 2000. Gasification of Residue as a Source of Hydrogen for Refining Industry in India. In: Gasification Technologies Conference. San Francisco, USA. [7] Romano MC, Lozza GG. Long-term coal gasification-based power plants with nearzero emissions. Part A: Zecomix cycle. Int J Greenhouse Gas Control 2010;4(3):459–68. [8] Romano MC, Lozza GG. Long-term coal gasification-based power with near-zero emissions. Part B: Zecomag and oxy-fuel IGCC cycles. Int J Greenhouse Gas Control 2010;4(3):469–77. [9] Jillson KR, Chapalamadugu V, Erik Ydstie B. Inventory and flow control of the IGCC process with CO2 recycles. J Process Control 2009;19(9):1470–85. [10] Andries J, Becht JGM. Pressurized fluidized bed gasification of coal using flue gas recirculation and oxygen injection. Energy Convers Manage 1996;37(6):855–60. [11] Andries J, Becht JGM, Hoppesteyn PDJ. Pressurized fluidized bed combustion and gasification of coal using flue gas recirculation and oxygen injection. Energy Convers Manage 1997;38:S117–22. [12] Lath E, Herbert P. Make CO from Coke, CO2 and O2. Hydrocarbon Processing 1986;65(8):55–6. [13] Shao Y, Golomb D. Power plants with CO2 capture using integrated air separation and flue gas recycling. Energy Convers Manage 1996;37(6):903–8. [14] Normann F, Thunman H, Johnsson F. Process analysis of an oxygen lean oxy-fuel power plant with co-production of synthesis gas. Energy Convers Manage 2009;50(2):279–86. [15] Williams A, et al. Combustion and Gasification of Coal. New York: Taylor and Francis; 2000. [16] Ergun S. Kinetics of the reactions of carbon dioxide and steam with coke. Technical
4. Conclusions A laboratory scale, a spouted bed gasifier has been used to investigate simulated oxy-fuel gasification. The aim has been to study the impact of fluidising gas composition (CO2/C and steam/CO2 ratios) at different temperatures and pressures. It was found that during tests with neat CO2, at atmospheric pressure, a limiting carbon conversion of 85% was reached at 950 °C, and all studied CO2/C ratios. This limit is thought to be due to elutriation from the bed under these conditions. However, it does show that high carbon conversions are achievable. Tests with a lower feed rate and therefore lower CO2 input flow rate (and lower fluidising velocity), achieved a higher carbon conversion because elutriation was reduced. By contrast, the carbon conversion is sensitive to the CO2/C ratio at 850 °C, under conditions which were not carbon limited. At higher CO2/C ratio it is possible to match the complete carbon conversion achieved at 950 °C (to the level limited by elutriation). Conversion at 750 °C was limited and did not show changes with CO2/C ratio, 9
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Netherlands: Kluwer Academic Publishers; 1988. [24] Kang T-J, et al. A study on the direct catalytic steam gasification of coal for the bench-scale system. Korean J Chem Eng 2017. [25] Herdel P, et al. Experimental investigations in a demonstration plant for fluidized bed gasification of multiple feedstock’s in 0.5MWth scale. Fuel 2017;205(Supplement C):286–96. [26] Xiang W-G, Zhao C-S, Pang K-L. Experimental Investigation of Natural Coke Steam Gasification in a Bench-Scale Fluidized Bed: Influences of Temperature and Oxygen Flow Rate. Energy Fuels 2009;23(1):805–10. [27] Lv PM, et al. An experimental study on biomass air-steam gasification in a fluidized bed. Bioresour Technol 2004;95(1):95–101. [28] Karmakar MK, Datta AB. Generation of hydrogen rich gas through fluidized bed gasification of biomass. Bioresour Technol 2011;102(2):1907–13. [29] Long X. 2018. Advanced fluidised bed gasification for carbon capture and fuel cell applications, in PhD Thesis. Imperial College London.
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