Journal of Fluid Flow, Heat and Mass Transfer (JFFHMT)

ISSN: 2368-6111

2.2.2 XRF (X-ray fluorescence) Analysis

A solid dolomite and lignite ash sample is irradiated with high-energy X-rays from a controlled X-ray tube. A standard instrument, Rigaku X-Ray Spectrometer: Model-ZSX mini-II, is used to analyze dolomite Lumps (22-25 mm). The identification of major, minor, and accessory minerals in oxide states and their percentage is tabulated in Table 2.

The main constituents present in the dolomite lumps are CaO (48.67%) and MgO (27.77%), as mentioned in Table 2. As a result, this is believed to be pure dolomite. Several elements with 13 % content also had Al₂O₃, SiO₂, Cr₂O₃, and Fe₂O₃.

2.2.3 XRD (X-ray diffraction) Analysis

The crystalline structure of dolomite lumps was characterized by the XRD technique in a Philips X’PERT MPD X-ray diffractometer (UC Santa Barbara), with a source of Cu Kα (1.5405 Å). After various treatments, the XRD patterns of the calcined sample were recorded in a 2θ range between 10⁰ and 80⁰, with a rate of 4⁰/min. In the XRD analysis, the data were collected from 0 to 80 2θ. The highest peak was seen at 2θ 30⁰of calcite (CaCO₃). These Sharp peaks of dolomite appear on 2θ 30.934⁰ and 41.108⁰ are resembled with rhombohedral crystal system, as shown in Figure 1. The obtained XRD of dolomite lumps was compared with the standard XRD of fresh dolomite. The weight % of calcium and magnesium oxide varies based on the dolomite source, and the weight % of calcium and magnesium oxide was a key determinant of dolomite effectiveness. The tar cracking efficiency was lowest in dolomites with lower CaO and MgO composition.

Brunauereemmetteteller (BET) and Barrett-Joyner-Halenda (BJH) analysis

The active site of catalysts before and after use was characterised with Micromeritics ASAP 2010 using nitrogen adsorption/desorption isotherms, and results are tabulated in Table 3. Prior to characterization, the catalyst samples were degassed at 170℃ for 1 h. The adsorption and absorption isotherms provide the specific surface area as a function of the sample's relative pressure (P/P0). The catalyst's surface was observed to comprise two types of pores, i.e., macro-porous and micro/mesoporous. The specific surface area includes both external and pore areas in m² g^-1. The specific surface area of macro-pores is determined under BET analysis, whereas the pore volume and pore diameter of micro- and mesopores are determined using BJH. Above 700 to 900℃, the mesopores and macropores area increases due to the desorption of some gases, which was observed in TGA analysis in this temperature range. Secondly, the calcined dolomite has more macropores in comparison with raw dolomite. As a result, the average pore diameter has also increased.

2.2.7 Scanning electron microscope (SEM)

SEM analysis for surface morphology is shown in Figure 2 (A). It depicts that the unused catalyst shows high porosity due to the presence of micro and macro pores over the surface. In the used catalyst of Figure 2 (B), the ash or tar is deposited on its surface, which covers some of the catalyst's active sites and thus decreases surface area, which is also confirmed from BET analysis.

3. Experimental and Thermodynamic analysis

3.1 Experimental setup

The experiments were conducted in a 10 kWe atmospheric pressure downdraft gasifier with wood and lignite as fuel and dolomite lump as a catalyst. A representation of the gasification system diagram is illustrated in Figure 3. A vibration apparatus was mounted at the top side of the gasifier reactor to prevent the fuels from channelling or bridging inside the reactor. The water circulating pump, water tank, and water jet (nozzle) arrangement provided negative pressure for smooth gas flow. A wet scrubber, surge tank (sawdust-filled), and fabric filter were kept downstream of the gasifier to get a dust-free gas. The gas flow was observed using a calibrated orifice meter (with a U-tube manometer), and temperatures were measured using calibrated thermocouples (K type, Chromel-Alumel). Thermocouples were used to measure temperatures in the vertical direction inside the gasifier reactor in this investigation. A hotwire anemometer (Amprobe TMA-21HW) with a data logger and a Shimadzu 2010 gas chromatograph was used to evaluate air flow rate and producer gas concentrations, respectively. A Shin Carbon ST 100/120 micro-packed column and micro-thermal conductivity detector were used in a gas chromatograph. After the cold start, a flame is attained at the gas burner (15-20 min). The temperatures in the gasifier did not change significantly after the flame propagated. The experiments were conducted three times to check the repeatability of the results.

Total Particulate Matter (PM) and tar were measured as per the standard of the Ministry of New and Renewable Energy (MNRE), Government of India [13]. A setup was developed for measuring these contaminants: a PM holder with mica heater, shell and tube type glass condenser, chilling water arrangement with circulating pump and vacuum pump. Axiva makes glass fibre filter paper was used for collecting total particulate matter. The producer gas stream was taken from the mainline and diverted the producer gas in this system through a vacuum pump. The detailed working of the gasification system and tar-PM measurement can be found in the authors’ previous work [2], [6], [8], [14].

3.2 Thermodynamic Analysis

3.2.1 Mass Balance

This study aimed to conduct a mass balance analysis to determine the reliability of all the experiments conducted on the gasification of various feedstocks in the 10kWe gasifier. Mass conservation must hold for a control volume, i.e., the difference between input and output masses must equal zero. The feedstock and atmospheric air comprise the entire input mass, while the dry producer gas, char, tar, ash, and water vapours comprise the total output mass. Tar production was found insignificant in contrast to all other masses; thus, it was removed from the analysis [4].

For the gasification process, the following mass balance equation was used:

(1)

The m_feed and m_air in the above equation represents the mass consumption rate of feedstock and mass flow rate of oxidizer in gasifier respectively while m_gas, m_ash, m_char, m_tar and m_water represent the mass flow rate of producer gas, ash, char, tar, and water formed in the process, respectively.

3.2.2 Energy Balance

For any thermal system, studying energy balance is critical since it can assist in reducing system losses. It is a crucial thermodynamic study that aids in improving the system's performance. The equation was used to calculate the energy balance.

(2)

where the energy rate of the feed (E_feed) and the energy content of the air entering E_air define the input energy. While the energy rates of producer gas (E_gas), char (E_char), tar (E_tar), ash (E_ash), water (E_water), and losses in the gasifier E_losses represent the output of the gasifier system.

3.2.3 Exergy Balance

The current study also included an exergy analysis of gasification with lignite as a fuel. The system's thermodynamic performance is revealed through an exergy study. The gasification energy balance can be expressed as (Eq. (3)):

=

(3)

Where ∅_(out ) and ∅_(in )are the output and input exergy, respectively, and I is the irreversibility that occurred during the conversion process. The change in enthalpy of a specific gas component from the reference state to the specified pressure and temperature is referred to as thermo-physical exergy. The normal chemical exergy mixing of all ingredients and the loss in entropy owing to the blending of different species of gases is referred to as the chemical exergy of the mixture. The ratio of the exergy of the producer gas to the sum of the exergy of the lignite and air is defined as exergy efficiency (Eq. (4)):

(4)

Detailed procedures for calculating mass, energy, and exergy analysis can be found in the authors’ previous work [6], [15], [16].

4. Results and Discussion

4.1 Temperature

Figure 4 indicates the drying, pyrolysis, combustion, and reduction zone temperatures for different feedstock. The temperatures range in pyrolysis and the drying zone were observed between 355°C-452°C and 150°C-178°C, respectively. Moreover, the temperatures range in reduction and the combustion zone were observed between 570°C-675°C and 840°C-973°C, respectively. It was observed that the temperature was found higher when dolomite was added with feedstock. The thermal decomposition of dolomite is an endothermic process; however, thermal energy used by dolomite catalyst was released afterwards and boosted the Water Gas Shift reaction [17]. It is responsible for increasing the temperature in the combustion zone. It was also determined that wood feedstock offered higher temperatures than lignite feedstock. It may be because the wood has higher volatile matters and lower moisture–ash contents which would help to boost the temperature.

Figure 4 indicates the drying, pyrolysis, combustion, and reduction zone temperatures for different feedstock. The temperatures range in pyrolysis and the drying zone were observed between 355°C-452°C and 150°C-178°C, respectively. Moreover, the temperatures range in reduction and the combustion zone were observed between 570°C-675°C and 840°C-973°C, respectively. It was observed that the temperature was found higher when dolomite was added with feedstock. The thermal decomposition of dolomite is an endothermic process; however, thermal energy used by dolomite catalyst was released afterwards and boosted the Water Gas Shift reaction [17]. It is responsible for increasing the temperature in the combustion zone. It was also determined that wood feedstock offered higher temperatures than lignite feedstock. It may be because the wood has higher volatile matters and lower moisture–ash contents which would help to boost the temperature.

Clinker formation was observed to be very common with lignite feedstock. Due to the same, the gasifier couldn’t operate for a long time. It was found that after the addition of catalyst in the lignite, no clinkers were found during the 180 minutes of the experimental run. For wood feedstock also, no clinkers were observed.

4.2 Fuel consumption rate and producer gas flow rate

Figure 5 includes each feedstock's fuel consumption, specific fuel consumption, and gas flow rate. Fuel and catalyst were weighed and blended before experiments started. The remaining fuel inside the gasifier reactor was measured after 3 hours of experimentation. The weight of the catalyst was found to be the same before and after the experiment. For selected feedstock, it was observed that fuel consumption was found in the range of 10.01 kg h^-1 to 11.56 kg h^-1. Adding the catalyst into feedstock increases the fuel consumption rate, as shown in Table 4. It may be due to the catalyst activity and rate of reactions [18]. The catalyst would boost the reaction rate in the gasifier reactor due to thermodynamically promoting the active phase [19]. The dolomite catalyst would also enhance conversion efficiency by offering a large reaction contact area, potentially leading to higher fuel consumption. By employing a catalyst with feedstock, carbon conversion efficiency improves, resulting in higher fuel consumption and gas flow rates.

As a result, heavy tar was converted into a gas, possibly increasing gas yields [20]. It is possible because the gas yield increases when a catalyst is fed to the feedstock. The specific fuel consumption (SFC) was evaluated, as mentioned by Karagiannidis [21]. SFC was calculated and found in the range of 1.79 kg kWh^-1 - 1.92 kg kWh^-1 for all selected feedstock, which aligns with the literature [21]. Catalysts offer lower SFC, hich shows their positive effect on the gasification system. Airflow and gas flow rates were also found to be incremental when the catalyst was employed in the reactor. The same trend aligns with fuel consumption, as mentioned in Figure 5.

4.3 Gas composition and LHV of producer gas

The combustible gases, dust, soot, tar, particulates, and other non-combustible are present in the producer gas. The gasifier generates combustible gases, including CO, H₂, CH₄, and other hydrocarbons and non-combustible gases, such as CO₂ and N₂. In contrast to H₂ and CO, the concentration of CH₄ in producer gas is substantially lower. Figure 6 shows the concentrations of the producer gas compositions determined by gas chromatography, with lignite and wood as feedstock, total combustible gas content (CO, H₂, CH₄) 25.07% & 25.96% and 29.35% & 29.69% for with and without using a catalyst, respectively. The maximum combustible gas concentrations were observed with the addition of a catalyst in a feedstock. It is because the boudouard reaction occurred at higher temperatures and a favourable catalyst reactively at a higher temperature. With the use of catalysts, the concentration of CO and H₂ in the producer gas was found to be higher

Furthermore, the tar reforming reaction on the catalyst surface was enhanced by the extraction of carbon, which resulted in the production of a wide range of CO and H2 [22], [23]. With a catalyst, CO yield increased by 7.29% and 7.98%, respectively, when the catalyst was added to lignite and wood feedstock. The favourable Water Gas Shift (WGS) reaction at higher combustion zone temperatures resulted in a high H₂/CO in the producer gas [24]. It was observed that the ratio of H₂/CO in the producing gas increased with a catalyst, as mentioned in Figure 7. CO₂ concentrations decreased significantly when the experiment was carried out with a catalyst. However, the CH₄ content from all the feedstocks was almost constant.

Reed et al. [25] suggested a method for calculating the producer gas Lower Heating Value (LHV). For calculating the LHV of the producer gas, combustible constituents such as H₂, CO, and CH₄ gases were used. Heavy hydrocarbons were ignored due to their least significant compared to the other gases mentioned above. The LHV was calculated and found to be higher when fuel was blended with a dolomite catalyst, which is consistent with studies by Appell et al. [26]. It has a similar trend of H₂ and CO gases. The LHV remained between 4.63 and 5.11 MJ Nm^-3 for all selected feedstock. The LHV of producer gas with different feedstock is shown in Figure 6. The LHV was alleged in an incremental direction as dolomite was added in the lignite.

4.4 Mass balance

Table 4 shows the mass balance for various feeds and catalysts. In the gasification process, the mass of fferent feed and air mixes was used as an input, while the mass generated dry gas, char, water, and ash was used as an output. The amount of tar content in producer gas is usually overlooked because of the low concentration of tar compared to other constituents in the output. A hydrometer was used to assess the total moisture present in the product gas with all feedstock. The ash pit was used to collect ash and char. The amount of ash in the output was solely affected by the volume of fuel used.

The char content was computed as a result of this. Because of the higher reaction temperatures, the char content decreased with the addition of the catalyst. Depending on output mass versus input mass, the Mass Balance Closure (MBC) was computed and found to be in the range of 1.02-1.04. The variability in MBC could be attributable to physical or measurement mistakes and uncertainty during the experiments.

4.5 Energy balance

Table 5 shows the input and output energies for various feedstock and the energy balance closure (EBC). Fuel energy is determined entirely by multiplying the LHV of fuel and the fuel consumption rate. Because the LHV per kg of feed remained constant, the energy rate of fuel is only dependent on fuel consumption. The airflow rate and temperature of air impact the energy of air. As multiple trials were conducted on different days, variations in air temperature were observed. The energy derived from ash was shown to be higher for lignite feedstock than wood feedstock due to the higher amount of ash content available in raw material. The trend of ash, char, and water in the energy balance is similar to the same mass balance for different feedstock. The ECB ratio was determined to be between 0.776 and 0.816.

4.6 Cold gas efficiency and Exergy analysis

Figure 8 shows cold gas efficiency and exergy efficiency for various feedstock. Both the terms are measured as mentioned in literature [6], [27], [28]. It is observed that the addition of catalysts would be favourable in terms of cold gas efficiency and exergy efficiency. It may be due to the catalyst's higher fuel conversion efficiency and reactivity. Lignite offered higher cold gas efficiency compared to wood. It is because lignite had lower fuel consumption and offered relatively good quality producer gas.

Exergy analysis was carried out as mentioned in section 3.2. The composition of the fuel determines exergy. Because the exergy of fuel is independent of the fuel composition and stayed relatively stable throughout the experimental run, the exergy was increased with different feedstock. It can be observed from Figure 8 that the exergy efficiency keeps on increasing with the catalyst. Exergy efficiency was calculated and found in the 56.10% – 64.92% range for selected feedstock.

4.7 PM and tar in the producer gas

The PM and tar content harm the downstream gasifier process equipment in the producer gas. For gas turbines and internal combustion engines, experts have established a tolerable range for tar and PM [28] [29]. The PM was measured after the proper cleaning, while tar was measured from the gasifier system at two points: 1) just after the gasifier reactor and 2) after the cleaning system (before the gas burner). Figure 9 depicts the amount of PM and tar in producer gas.

PM concentrations were measured after the cleaning system (before the burner) and found to be between   48.11 and 71.12 mg Nm^-3. The use of a catalyst was shown to result in decreased PM levels in the producing gas. It's because, in the presence of a catalyst, a more significant temperature would transform huge PM into small particles. The concentrations of tar in producer gas immediately after the gasifier reactor and after the cleaning system were measured and found in the range of 1091 mg Nm^-3- 4150 mg Nm^-3 and 145 mg Nm^-3- 625 mg Nm^-3 for all selected (lignite and wood) feedstock. The dolomite played a significant role in destroying the tar in the producer gas. Reduction in tar is mainly due to catalyst activities inside the reactor and higher temperatures. Reduction in tar concentrations may increase the amount of combustible gases, improving the gasifier's overall performance. It was also observed that almost 85%-87% of tar is captured in the cleaning system compared to tar in raw gas measured immediately after the gasifier reactor.

4.8 Cost of energy and payback period

The current study considers lignite and wood as feedstock for energy generation via air gasification (catalytic). Here, only lignite calculation is presented. Similarly, the wood calculation can also be carried out. The actual cost of the material and cleaning system was mentioned for this calculation. However, it is a fact that the cost is reduced at an extended level if someone upscaled the gasification unit due to lower running costs. For raw material treatment, lignite cost and dolomite (5% wt. than lignite) cost were considered. As lignite and dolomite are available directly from the mines as a huge lump, sizing is required. The sizing of feedstock is considered in the electricity cost. Apart from that, commercial electricity cost is considered ₹8 per unit as per Gujarat State Electricity Corporation Limited. The gas cleaning system is majorly a combination of three units, 1) wet scrubber, 2) sawdust-filled filter (surge tank), and 3) bag-type fabric filter. A wet scrubber utilizes the water circulation pump, so the energy consumption of a centrifugal pump is considered. The remaining two systems' electricity consumption is not mentioned as per the reason mentioned in Table 6. Overhead changes and maintenance charges are also considered here. It was observed from Table 6 that electricity cost by the gasification system is more than which government supplied. Due to the same, this laboratory scale system could not be profitable to install where such electricity is available. However, for the interior part of the country, where the power grid is not available, or the cost of raw (feedstock) material is almost free of cost, this system is ideal to operate. An increment in auxiliary power consumption and reduction in running cost is generally expected for commercial gasification systems instead of this lab-scale system.

Total cost (as per Table 7) per year = Total cost per day × 300 = 3,00,000

Total Investment = Gasifier unit with Generator Cost + Total cost per year = 8,70,000

Total revenue generated = Electricity generated × Unit price = 3,67,301

Payback period = Total investment/ Total revenue

The payback period was calculated to inspect the economic feasibility of a gasification-based powerplant production system. This is the one of the most basic approaches for evaluating an investment proposal. It is basically the period during which the project's initial expenditure is recovered. The payback period is premeditated considering the upscaling of the gasification system of 10 kWe. The Table 7 demonstrates the values and parameters considered for the payback period. As per calculation, total investment and total revenue generated are ₹8,70,000 and ₹3,67,301, respectively. As per that, the payback period is around 2.37 years for an existing catalytic lignite gasification system, which is in line with the literature [30].

5. Conclusions:

Experiments were conducted with two feedstocks (lignite and wood feedstock) and a catalyst to check its feasibility and performance on a 10 kWe downdraft gasifier. Lump dolomite was used as a catalyst (5%, W/W). The employment of a catalyst was responsible for all zone temperatures for both fuels. The CO and H₂ gas concentration was increased, whereas the CO₂ gas concentration was decreased for catalytic gasification. The CH₄ gas concentration was found to be almost constant. The fuel consumption, gas flow rate, and LHV of producer gas increased, whereas SFC decreased with the addition of a catalyst with fuels. Cold gas efficiency and exergy efficiency were calculated and found in the range of 62.60%-74.93% and 56.1%-64.9%. Mass balance closure (MBC) and Energy balance closure (EBC) were found in the range of 1.02 – 1.04 and 0.75 – 0.83, respectively for selected feedstock. 56.64% (L) and 49.83% (W) tar and 12.68% (L) and 10.42% (W) PM in the producer gas (collected after the cleaning system) were reduced when the catalyst was added in lignite and wood fuels, respectively. The major conclusion of this study is that adding a dolomite catalyst in lignite/wood feedstock improves the overall performance of the gasifier.

Acknowledgement:

The authors express sincere appreciation to the Nirma University, Ahmedabad (Project Grant No: NU/Ph.D./MRP/IT-ME/ 16e17/851) and Department of Science and Technology, New Delhi (Project Grant No: SR/S3/MERC-0114/2010) for the financial support. Dr Darshit S. Upadhyay expresses gratitude towards International Travel Support (File Number: ITS/2023/000740) from the Science and Engineering Research Board (Govt. of India) for giving financial aid to attend the 10th International Conference of Fluid Flow, Heat and Mass Transfer, Canada on 07 - 09 June 2023.

Disclosure statement

No potential conflict of interest was reported by the authors.

References