Johnson Matthey Technol. Rev., 2020, 64, (2), 119
Autothermal Fixed Bed Updraft Gasification of Olive Pomace Biomass and Renewable Energy Generation via Organic Rankine Cycle Turbine
Green energy generation from waste biomass in the Mediterranean region
Waste biomass, a renewable resource, is a reasonable choice for green clean power generation using advanced thermal treatment technologies such as gasification. In this research, dried-densified olive pomace residues from olive oil production have been applied as biomass feedstock in a new gasification process for synthesis gas (syngas) generation using a 500 kg h−1 throughput capacity autothermal modified updraft gasifier system. The product syngas generation rate is found to be approximately 2.5 Nm3 kg−1 of olive pomace, with a calorific value (CV) between 5.0 MJ Nm−3 and 7.0 MJ Nm−3. More than 85% of carbon in pomace is converted to produced syngas by the gasification system. The gasification reactor generates syngas which passes through a specially designed swirl hot gas burner and is then burned directly in a thermal oil boiler retrofitted to an organic Rankine cycle (ORC) turbine generator. As a result, the produced syngas at around 350°C is directly combusted with tars so that a great deal of chemical energy loss is prevented. The thermal oil heater has a thermal energy capacity of 1.77 MWh. The generated 1.6 MWh thermal energy from the thermal oil heater is transferred to the ORC turbine to generate 240 kW electrical power.
Modern energy sources, mainly fossil fuels, are being used inefficiently at a high rate with concern of exhaustion. At the same time, there is growing comprehension and recognition of greenhouse gas emissions, climate change and environmental pollution issues which have drawn worldwide attention to renewable power sources. Recent environmental and energy policies support investigations to increase the use of renewable sources to reduce fossil fuel use and decrease environmental impacts. As a renewable source, biomass is an attractive feedstock for decentralised power generation. The European Union (EU) is increasingly highlighting the objectives of decreasing emissions of greenhouse gases and enlarging the portion of renewable energy sources and hence using waste biomass as a valuable resource. A significant amount of renewable energy is derived from biomass feedstock. The renewable electricity provided from biomass feedstock is assumed to be around 14% of the entire renewable electricity production by 2030 in the Eurozone (1).
In general, fossil-based fuels are the primary feedstock for fuels and power sources on our planet. The use of biomass feedstock for energy production can reduce the consumption of fossil-based fuel and contribute to decreasing the emissions of greenhouse gas (2). Biomass materials constitute the most significant proportion of carbonaceous waste materials. As an alternative form of energy, the use of waste biomass feedstock to form fuel sources is most welcome and appreciated because of regulations and legislation. Biomass is abundant and widely available in nature. Biomass can provide constant power besides generating other types of products. Consequently, biomass waste is considered to be a clean energy source and one of the alternatives to fossil-based fuels for the future. Biomass feedstock residues mostly comprise wastes of forestry, agriculture and the food processing industry.
Olives are a significant agrarian product. The world’s largest producers are Spain, Italy, Greece, Turkey, Portugal, Tunisia, Morocco and Syria. 70% of the world capacity of olive pits is produced within the EU countries. The rest of the world produces the remaining 30% (3). From the olive industry, the most critical and massive wastes are olive pomace formed during oil production. A small volume of pelleted olive pomace residue is burnt; however, this feedstock can lead to several complications in combustion boilers such as slagging, agglomeration and formation of clinkers (4, 5). Many studies have been conducted to date on the combustion of olive wastes. In comparison, there are insufficient observations available on olive pomace use in gasifier systems.
Several thermochemical conversion technologies can be applied for power generation from waste biomass. However, gasification is a convenient choice because it supplies higher efficiencies compared to combustion or pyrolysis (6, 7). Biomass gasification is a thermochemical conversion process that uses limited oxygen at high temperature conditions to transform the solid form of biomass into gas, volatile organic compounds (such as tars) and a small volume of ash and char. The gas produced from the gasifying process agent (air, O2, steam, enriched air) is used to create the proper operating conditions. For instance, the lower heating value (LHV) of the produced gas must be between 4 MJ Nm−3 and 6 MJ Nm−3 when air is applied as the gasification agent and has a much higher value when O2 (40 MJ Nm−3) or steam (12 MJ Nm−3 to 18 MJ Nm−3) are applied (8). As compared to combustion, gasification processes are more efficient and effective at generating combined electrical power and thermal energy (9). However, some factors, such as reactor design parameters, feedstock properties (moisture content, particle size and ash) and gasifier reactor operating performance conditions (temperature, residence time and equivalence ratio) affect gasifier efficiency (10, 11).
In updraft gasification, biomass waste feedstock, which is delivered from the upper part of the gasifier reactor, is later conducted to drying zone, pyrolysis, reduction and oxidation processes, respectively. The updraft gasifier reactor as given in Figure 1 shows the gas generated proceeding upwards. Syngas, which is the main product of the gasification process, flows through the gas exit at the upper section of the gasifier. In a typical air-fed gasifier, syngas is a mixture of a flammable gas such as hydrogen, carbon monoxide (CO), methane (CH4), some types of tars and non-flammable inert gases like carbon dioxide (CO2) and nitrogen. The variety of syngas from gasification of biomass is influenced by aspects such as process parameters, biomass specifications and design of the gasifier reactor. The features of biomass that have to be dealt with in gasification are physical and chemical structures, such as density, elemental composition, fixed carbon (FC), volatile matter (VM), moisture and ash content. Controlling parameters in gasification are equivalence ratio, temperature conditions and feedstock throughput rate. The produced syngas can be directly burned as a fuel without cooling at atmospheric pressure in gas burners; there is no requirement for syngas treatment, reinforcing the efficiency of gasification. The syngas burner design is a critical part since the syngas has a high tar content. A properly designed burner helps the produced gas to burn efficiently.
Feedstock reliability is vital in gasification systems to accomplish sustained flow through the gasifier and to supply consistently produced gas composition and higher heating value (HHV) for the upstream power conversion. Additionally, densification of the feedstock reduces the gasifier size, while the size and shape of the intensified biomass reduce fluctuations in produced gas. Fuel flow also affects the subsequent quality of the products (12).
Updraft gasifiers are identified as counter-current reactors since oxidising agent passes upwards and the feedstock flows downwards under gravitational force. These types of gasifiers are considered appropriate for fuels with relatively high ash and water content, have a high thermal efficiency due to low exit gas temperature and have a low ash carryover due to the filtering effect of the fuel bed (13–15). In a fixed bed updraft gasifier, the entrance point for the gasifying agent is at the bottom section and for the feedstock, it is at the top section. Updraft gasifier reaction sections such as the drying section, pyrolysis section, flaming pyrolysis (partial oxidation) and gasification zones take place in a sequence in autothermal gasification systems. These zones reach different temperature conditions in the flow of the gaseous product. As a result of diversity in temperature and reaction zone sequence in the fixed bed updraft reactor, the performance of the gasifier is affected by design and operating parameters (16).
Biomass-based cogeneration processes are becoming increasingly prevalent and several studies summarise what has been accomplished in this field. Some researchers have investigated the usefulness of using biomass in combined heat and power (CHP) plants. Furthermore, most of these researchers have concentrated on methods that combine biomass incineration with ORC turbines and few researchers have considered the probability of combining biomass gasification processes. Comparing various biomass incineration and gasification systems, the gasification process was superior to incineration processes, both techno-economically and in terms of the performance of the power conversion process. Despite these advantages, CHP systems operating via ORC turbine based on biomass gasification have not been used so far and practically no references can be identified in that field (17–19). ORC turbines are advanced energy generation machinery, based on organic substances with favourable thermodynamic properties as working fluids: pressure and low critical temperature conditions, low viscosity, small specific volume, high thermal conductivity and surface tension. The main advantage in handling organic working fluid is less need for heat for fluid evaporation compared to water; thus, ORC turbines operate at lower pressures and temperatures than the conventional steam process (20–22). These techniques provide a performance of about 15% in electricity and 60–70% in heat (23).
1.2 Aims of the Study
The present pilot-scale research study was implemented in an autothermal updraft gasifier reactor with a throughput capacity of 500 kg h−1. The primary purpose of this study is to inspect and to analyse the experimental data obtained including gas concentration, temperature profiles, mass and energy balance. Syngas LHV, carbon conversion and energy output via the ORC turbine are further presented and discussed. Another goal of this work is to demonstrate the possibility of using pelletised olive pomace in cogeneration systems based on the gasification process.
The evaluation of state of the art affirms, therefore, that the combination of two unique technologies, i.e., biomass gasification and ORC turbine, which are both technologies in progress, can be considered as an original approach. Currently, the original combined biomass fixed bed autothermal updraft gasification and ORC turbine pilot-scale plant are both in operation.
In summary, this study was carried out using a pilot-scale (500 kg h−1) gasification plant consisting of an autothermal updraft gasifier, hot syngas burner where the raw produced syngas was not cooled or treated prior to the specially designed syngas burner on the thermal oil heater which runs in an ORC turbine at an output capacity of 240 kW electrical power. The excess 1.36 MWh of thermal heat is used for evaporation of the blackwater, a harmful byproduct of the olive oil production facility. The most important properties of pelleted olive pomace that are known to impact the gasification systems are water content, shape and size, bulk and total density, chemical composition (i.e., ultimate and proximate analysis) and the HHV. This paper will focus on the production of power and heat from the gasification of olive pomace in a pilot-scale autothermal fixed bed updraft gasifier.
The objectives of the study are to:
Evaluate the performance of the updraft gasifier using dried and pelleted olive pomace as a fuel for proof of concept
Determine the fuel and char rates, gas compositions and CV of the gas produced by the gasification of pelleted olive pomace in an autothermal updraft gasifier
Generate the fundamental energy and mass balance data and diagram for the gasifier and ORC turbine system
Assess the feasibility of operating an ORC turbine using the product gas in a thermal oil heater.
2. Experimental Setup
A pilot-scale autothermal updraft gasifier with a capacity of 500 kg h−1 has been specially designed and applied in this experimental pilot-scale setup. The thermal capacity of the gasifier designed is 2.20 MWh when the proper biomass is used in this unique system. The design of the system and operation conditions of the updraft gasifier require the understanding of biomass feedstock characteristics. Properties of biomass such as shape, size, composition and water content are significant parameters that need to be considered prior to the design of a gasifier. Operation parameters such as feeding rate, gasification temperature and air:fuel ratio need to be measured as well. All these parameters play a crucial role in the performance of the gasifier in terms of gasification efficiency and quality of the gas produced during operation.
2.1 Gasification Parameters
The autothermal fixed bed gasifier reactor gasifies a maximum of 500 kg h−1 biomass feedstock. This amount of gasified biomass approximately supplies 1250 Nm3 h−1 of produced gas to the thermal oil heater. After the gasification process, almost 10% of the gasified biomass comes out as char, a byproduct from the reactor. Gasification of the pelleted olive pomace is carried out in an air-blown (680 Nm3 h−1 at the maximum load) updraft gasifier system operating slightly under atmospheric pressure.
The gasification reactor was built using a 6 m long reactor made of SUS 306 stainless steel with a 1.5 m diameter. The reactor body is well insulated to prevent any significant heat losses. A basic plan of the autothermal updraft gasification system is shown in Figure 2.
The updraft reactor is made of a cylindrical-conical reaction vessel. The fixed bed gasifier structure is cylindrical with a feed rate of about 500 kg h−1. The biomass is conveyed from the main hopper to the upper part of the reactor using a motorised elevator and screw feeder. Fuel is admitted at the top with a screw conveyor and proceeds by gravity down through inside the unit.
For the start-up, primary air is used to light the biomass. Then, primary blower air is adjusted to maintain the desired temperatures. Once the preferred temperature of the reactor is achieved (about 900°C in that case), the moving grate is activated and frequency of the feeder is regulated to stabilise the feeding rate required for olive pomace. The produced syngas exits the reactor at around 350°C through the channel. Typically, it requires around 1 h or 2 h to stabilise the operating conditions with respect to gasifier temperatures. All parameters are kept constant for at least an hour for the analysis of the produced gas.
In the gasification (reduction) zone, with a high amount of thermal energy from the oxidation region below, a number of endothermic reactions take place between the gases and the char including steam, obtaining a large amount of H2 and CO, along with CH4 gases. For instance, the incandescent char in the gasification region reacts with CO2 gas that should be in the temperature range of 700°C to 850°C and the char volume shrinks as it delivers C atoms to the CO2 to convert CO.
In the partial oxidation region, the gasifying agent is provided at the bottom of the reactor and is dispersed via movable grates to the pyrolysed char. Not only the incandescent char but also pyrolytic products such as partially oxidised heavy hydrocarbons (tars) enter that region. The pyrolytic molecules oxidise in the gas phase to form CO2 and H2O. The thermal energy, which is transferred to and used in other regions, is supplied by the exothermic reactions in this oxidation zone. The temperature of the partial oxidation zone is between 900°C and 1100°C. The basic gasification process is described by the simplified chemical formulas in Table I (Equations (i)–(vii)) (24).
|Heterogeneous gas-solid reactions|
|Water gas reaction||(ii)|
|Homogeneous gas-gas reactions|
|Water gas shift reaction||(iv)|
The moving grate inside the reactor is shown in Figure 3. Transferring the char is possible by agitating the grill. Char movement causes a loss in pressure over the char bed at this stage. When the pressure drop across the oxidation zone in the reactor exceeds a threshold, the system activates the moving grate. The ash and char are removed at the bottom of the reactor by a screw conveyor.
The gas generated in the reactor is then taken out from the top of the gasifier by repulse and pressure force of the induced draft (ID) fan and the forced draft (FD) fan. As the solid feedstock is transformed into gas, it conducts the remaining material to move through the reactor under a gravitational effect. The char residues formed during the process are automatically discharged into the char box by intermittently rotating the screw conveyor. The produced gas leaves the reactor at a temperature range of 250°C to 350°C. The produced gas is then flared at the well-designed burner and fed to a thermal oil boiler to generate 1.77 MWh of thermal energy. This thermal energy is transferred to the ORC turbine to generate 240 kW (15%) of electrical power. The excess 1.35 MWh useable thermal energy of waste heat from the system is used in the blackwater evaporation unit.
The whole system used in this study consists of an updraft gasifier reactor, hot gas cyclone, syngas burner, thermal oil heater and ORC turbine, ID stack fan, FD air fans and a stack which is illustrated in Figure 2.
Data obtained in gasification system experiments include the flow rates of feedstock and produced gas, produced gas compositions, temperatures, pressure throughout the operation line and electrical power generated in the ORC system. Every 15 s, a programmable logic controller (PLC) records all temperatures for air inlet, oxidation zone, reduction zone, pyrolytic zone, drying region, cyclone outlet and thermal oil boiler. The pressure drop is recorded at the top of the reactor and the cyclone outlet. The air flow rate is measured after the primary ID fan. The generated gas flow rate is calculated from the gas exit channel of the gasifier.
The produced gas exits the gasifier at around 250°C to 350°C. From one exit located at the top of the gasifier, product gas passes through the cyclone and then the thermal oil boiler. The produced gas exiting from the reactor includes some fine particulate matter passing through the cyclone which is used as a dedusting unit to separate these particles. The cyclone eliminates most of the fine particles and dust from the hot gas produced. The produced gas channels and cyclone are well insulated to prevent tar condensation. Afterwards, the produced hot gas is transferred to the syngas burner and combusted in a well-insulated thermal oil boiler. The pumps circulate thermal oil in the heated coils through the ORC turbine to generate 240 kW of electricity and excess heat is transferred to the evaporator units which vaporise the blackwater produced from the olive oil facility.
2.2 Biomass Feeding System Units
The main feedstock hopper and the screw conveyor are shown in Figure 2. The main biomass hopper, which has a volume of 3 m3 at the top of the reactor, is packed with the pelleted feedstock. A screw conveyor intermittently feeds pelleted olive pomace into the reactor at the upper part of the gasifier. A frequency converter can convert the needed amount of feedstock. The fuel flow out of the hopper interconnects with the entire reactor and the rotation speed of the drive motor.
A container with an elevator in the basement feeds the pelleted olive pomace to the main hopper. After this, the feedstock is fed into the main hopper where a screw conveyor feeds the biomass to the reactor. The main hopper system with a screw conveyor not only prevents air leakage to the reactor but also avoids gas leakage from the gasifier.
2.3 The Gas Analyser
The gas sampling port is located at the syngas exit point between the cyclone and the gas burner of the system as shown in Figure 2. A portable Vario Plus (MRU, Germany) syngas analyser is used to measure the volumetric fractions of the main product gas components. After attaining a steady state condition, the product gas analyser is switched on. A heated probe is sucked into a small stream of the produced gas; then the gas is passed through a filter box filled with glass wool. The gas flows from bottles filled with water which act as a cooler; the cooled and clean product gases are analysed by the MRU syngas analyser. The volumetric fractions of the gas components (H2, CO, CO2, O2, CH4, ethylene (C2H4) and ethane (C2H6)) are measured on a data acquisition for a definite period during 24 h of process operation. During the plant operation, the composition of the gas is analysed and data are collected for about half an hour.
2.4 Control System
The gasification system is controlled by a PLC. The entire control strategy used in that research aims to generate a continuous syngas flow for the thermal oil boiler to produce thermal heat and transfer it to the ORC turbine. To achieve these tasks, the ID suction fan is functioned at a constant rate after reaching steady state. The algorithmic system is equipped with automatic security controllers and can be operated remotely.
Temperatures are recorded with an analogue-to-digital (ATD) converter. Four thermocouples are fixed to the internal refractory wall inside the gasifier, to prevent probable issues with the flow of feedstock while it is consumed. The thermocouples are located at corresponding positions along the vertical axis of the gasifier as shown in Figure 2. The temperature of the gas generated in the reactor is calculated at the outlet channel of the gasifier. Three digital manometers are used to calculate the pressure at different locations and to measure the pressure changes over the system. These fixed locations are the top of the gasifier and the channel between gasifier reactor and dust gas cyclone. The heat exchanger is used for the assessment of the waste heat remaining from the gas combusted in the thermal oil heater. The hot air generated from the heat exchanger is used as a gasifying agent and combustion air in the thermal oil heater. The flow rates of the gasifying agent, produced gas and combustion air used in the syngas burner flow rates are calculated by flow meters. The values are recorded every five seconds. These digital indicators are connected to the PLC system and a supervisory control and data acquisition (SCADA) computer for data retrieval.
FD and ID fans are placed in the system. One of these supplies gasifying agent from the bottom of the reactor through the gasification system generating the updraft effect. The other is located near the stack for a suction effect into the system so that the produced gas is pulled over from the gasifier, resulting in a slight pressure drop. Negative pressure is provided at the top of the reactor for safety reasons. The gasifying agent flow rate is controlled to keep the temperature of the oxidation zone between 900°C and 1200°C. As mentioned earlier, four thermocouples are located in the different reaction zones of the reactor to measure the temperature. In addition, there are nine thermocouples located at the cyclone gas inlet and outlet channels, the syngas burner, the thermal oil heater, the ORC turbine inlet and outlet, the combustion air inlet channel and the stack.
2.5 Experimental Procedure
In the primary phase of the start-up, the gasifier was ignited with charcoal to reach the desired temperature for gasification. First, charcoal was ignited using a natural gas burner through the ignition point. The optimum amount of air supplied to the oxidation zone was regulated by FD and ID fans located at the inlet of the gasifier reactor and at the stack respectively.
The experimental conditions, energy and mass balance data are presented in Figure 4 for autothermal updraft air gasification. After layer embers in the gasifier were attained, feeding of the olive pomace pellets was started in the gasifier. From the bottom of the gasifier, at around 680 m3 h−1, the gasifying agent air at maximum load was supplied to obtain the updraft effect in the reactor. When the temperature of the gasification region reached between 300°C and 400°C, biomass pellets were fed at 8.35 kg min−1. During this period, 500 kg h−1 feedstock was fed to the gasifier reactor forming around 5 m bed height. This was provided to reach the maximum, to keep the bed height steady during the operation modes of the gasifier. Air permitted to form 0.25 equivalence ratio was preserved throughout the updraft process. The gasifier temperature was stabilised by achieving steady-state conditions; then gas sampling was carried out to analyse the gas composition. The temperature and gas composition were measured during the gasification experiments until all of the material in the bed was gasified.
2.6 Test Procedure and Power Generation
The pelleted olive pomace was gasified with the method described above. The method was repeated several times to attain reliable results. For power generation, the produced gas was passed through the combustion chamber of the syngas burner, which is placed at the top of the thermal oil boiler. Then, the burner increased the temperature of the thermal oil, the heated fluid was transferred to the ORC turbine to generate electricity and excess heat was passed through the blackwater evaporation units to vaporise the blackwater.
The operation of the gasification system could be portioned into three parts as described below.
For the initial application, an amount of charcoal is ignited by the natural gas burner from the oxidation region to warm up the system. Pre-weighed olive pomace biomass in the form of densified logs (pelleted) is charged through the main hopper from the top of the reactor. The maximum bed height level of the gasifier is determined by a mixer; then, the ID stack fan and FD fans are adjusted for the updraft gasification process. The start-up period comprises all operations needed until a steady state whereby the gas quality for the thermal oil boiler is reached.
The gasification system generally attains a steady state about an hour after the initial ignition. Afterward, the temperature of the oxidation region reaches between 900°C and 1200°C and the generated gas is ignited at the syngas burner. When the produced gas steadily burns in the syngas burner and thermal oil reaches 290°C, then the ORC turbine is started up to generate 240 kWh electrical power. The data collected during the steady operation of the gasification system are temperature and pressure, fuel-flow rate, gas composition and char rate. Temperatures were measured with an ATD converter every 15 s for oxidation zones, pyrolysis zone, drying zone, gasifier gas outlet, cyclone outlet, thermal oil boiler, thermal oil inlet and outlet and stack. Pressure data were collected at the gasifier gas outlet pipe, cyclone outlet and thermal oil boiler outlet channel. The flow rate of the produced gas was measured by carefully calibrated gas flow meters placed before the gas burner and cyclone outlet to measure the air flow from the inlet channels.
Lastly, the shutdown procedure refers to all operations needed to seal the gasification system safely. Gas suction ID and FD fans are turned off; gasifier inlet valves, outlet and stack gas valves are switched off in a systematic arrangement. The off-gas burner remains on as a secondary natural gas burner until no more product gas is generated.
3. Results and Discussion
The experimental tests were performed in Marmarabirlik’s pilot gasification facility at Bursa, Turkey (Figure 5). The gasifier reactor was designed and built to implement experimental tests of olive pomace gasification at high temperature with air as the gasifying agent.
3.1 Feedstock Characteristics
The quality of syngas is affected by feedstock characteristics (water content, particle size and composition). Proper homogeneous feedstock size is an essential factor in generating better quality gas. Compared with small size feedstock, larger sizes produce lower quality syngas. However, feedstock which contains fine particles has low porosity in the reactor and as a result, tends to lead to higher pressure loss in the gasifier. Gasification of small size feedstock could lead to high pressure drop as well as excessive fine particle content in the produced gas. Also, inconvenient build-up issues arise in the reduction region of the gasification bed with small size and low-density feedstock.
Conversely, larger particle size feedstock decreases the reactivity of the fuel and triggers bridging and channelling obstacles that reduce the amount of gas produced. Feedstock size homogeneity also influences the operation performance of the reactor. The gasifier efficiency rises with increasing feedstock size homogeneity. For all these reasons, as shown in Figure 6, feedstock fuel is prepared by pelleting to fractions of the preferred particle diameter (dp) (10 mm < dp < 12 mm) with a bulk density of 589 kg m−3.
The water content in the feedstock also affects the quality of produced gas. Feedstock with lower water content produces better-quality product gas than that with a higher moisture content. The heating value of the produced gas can be influenced by the feedstock water content. Feedstock with high water content generates produced gas with low CV. Feedstock with higher than 30% water content reduces the CV of the produced gas due to low heat transfer to the endothermic pyrolysis zone reactions during the gasification process. More of the heat is absorbed by biomass to evaporate water in the drying process. Thus, the heat required in the pyrolysis zone for reactions is insufficient.
For this reason, biomass with high moisture content (>30%) must be dried during feedstock fuel preparation before the gasification process. Prior to the gasification experimental tests, raw olive pomace with an original moisture content of 60% by weight was dried and then pelleted during the feedstock preparation process at 105°C for 6 h. Proximate, final analysis and gross CV (GCV) of olive pomace sample results are compared with oak woodchips and presented in Table II. Proximate analysis supplies the composition of a substance in terms of FC, moisture, ash and VM as well as GCV. The ultimate analysis provides elemental compositions containing C, H, sulfur, N, O and moisture. Absolute and bulk densities of both olive pomace and woodchips are shown in Table II for comparison.
The HHV of olive pomace is theoretically calculated as 17.65 MJ kg−1 (IGT method formula). In wt%: C = carbon; N = nitrogen; H = hydrogen; O = oxygen; S = sulfur; A = ash and M = moisture content of olive pomace.
To generate thermochemical conversion systems such as gasification reactors, determination of the LHV rather than the HHV of fuel in the calculation is more effective. The water heat of vaporisation and the moisture content of the feedstock can be overlooked as these do not contribute any CV to the biomass.
where the LHV of olive pomace is theoretically calculated as 17.48 MJ kg−1.
Standard biomass feedstock for gasification has LHVs of around 15–17 MJ kg−1; woody feedstock that has been the conventional fuel for gasification systems has HHV in the range of 17–21 MJ kg−1. The LHV that was calculated as 17.48 MJ kg−1 for olive pomace demonstrated that this feedstock is suitable for gasification in terms of CV equivalent to woodchips.
The feedstock moisture content greatly affects both the quality of the produced gas and the operational parameters of the gasifier. Excessive water in the feedstock drops the operational temperature of the reactor and that leads to long chain hydrocarbons in the form of heavy tars in the produced gas leaving the reactor. The water content of the feedstock specifies the type of gasifier design that is used. Higher moisture contents of biomass feedstocks are accepted for updraft reactors.
The absolute and bulk density of biomass is essential for process design, handling and storage. Biomass with lower bulk densities frequently causes deficient current under the gravitational force that leads to insufficient gas CV and char burnouts in the gasification region. However, biomass with higher bulk densities requires lesser reactor vessels for a definite refuelling time. The bulk density of olive pomace is higher than that of woodchips (589 kg m−3) and the experiments verified that there was minimum char burnout that appeared in the reduction region. Due to minimised reactor dimensions and the feeding charge capability of the gasifier, the feedstock is compressed in the form of pellets. Figure 7 presents the pelleted feedstock (10–12 mm diameter): olive pomace obtained from Marmarabirlik’s olive oil facility was used in this pilot-scale system. Commonly, pellets are produced by pressing the pomace under high pressures using standard compress equipment. Intensification of fuel could decrease the space occupied by the feedstock in the reactor. Fuel intensification has some advantages such as reduction of gasifier dimensions, convenience of feedstock management and inhibiting dust exposure. Pellets that have uniform dimensions enable identical flow by gravitational force and homogeneous pellets create a uniform void field in the gasifier which avoids channelling throughout the gasification section.
3.2 Gasification Characteristics
Gasification characteristics of pelleted olive pomace obtained during three runs are presented in Table III and compared with oak woodchips. Table III also illustrates the different flow rates that cause different characteristics of produced gas. The equivalence ratio and air intake of the gasifier are also shown.
During Run 1, the gasifier reactor operated at the lowest quantity of gasifying agent. Therefore, the reaction slowed and feed consumption decreased. In Run 2, the gasifier operated at 300 kg h−1 (half the capacity) and the quality of the gas slightly improved. However, in Run 3, the gasifier operated at maximum capacity (500 kg h−1) and the syngas CV was highest. Therefore, it is understood that the gasifier reached its maximum efficiency at the highest load.
During the pilot-scale updraft reactor operations, produced gas was taken by a probe and analysed externally. The analysis results during steady state conditions are given in Table III and graphical results are illustrated in Figure 8. Measured compositions show CO in the range of 23 ± 1%, H2 7 ± 2%, CH4 3.5 ± 0.8%, CO2 10 ± 2% and the balance N2. During steady state operating conditions, power generation at 240 kW was continuously observed via the ORC turbine with the pelleted olive pomace whose moisture content was around 25 wt%. Gas with a typical LHV of 5.0–6.5 MJ Nm−3 was generated in the reactor. The characteristics of the syngas composition are presented in Table III.
where XH2, XCO and XCH4 are the mole fractions of the main combustible gases, H2, CO and CH4 respectively.
Figure 8 illustrates the composition of the syngas generated by the gasification reactor in Run 3. During that run, the composition of produced gas was quite stable, so the ORC turbine operated smoothly and stably. The flow rate of the gas produced by the gasifier in steady state operation is between 270 Nm3 h−1 and 1251 Nm3 h−1. In the operational run, the hot gas generated had an average LHV of 6.30 MJ Nm−3 and the gas was subsequently used in a thermal oil boiler to run the ORC turbine. The turbine is designed for the conversion of 1.36 MWh thermal energy input to 240 kW electricity power output, which means 15% efficiency of electricity generation. The gasifier was operated with a turndown ratio of around 5:1 and syngas generation was stable enough to operate the ORC turbine. Water was used for the turbine cooling system; the input temperature of the 50 m3 circulating water was 60°C and the output temperature was 90°C. The hot water obtained from the waste heat of the ORC turbine was used within the facility for the blackwater evaporation unit.
The performed runs indicated that the particle size and shape of the pelleted olive pomace significantly affect gasifier operation. Therefore, pelleted feedstock of size 12 mm × 50 mm was selected and used in the reactor. Referring to the size of the gasifier, it is assumed that there is an upper limit for the particle size of 12 mm. This feedstock size is optimised for smooth movement and to prevent bridge formation inside the gasifier.
The design and actuation system of the grate is important to discharge the char in the gasification operation. The amount of char byproduct from olive pomace feedstock gasification was observed to be quite low. Hence, it can be discharged from the gasifier less often, without interfering with the continuous production of high-quality syngas. A small amount of ash in olive pomace is beneficial to forestall probable clinker agglomeration in the gasifier owing to higher operation temperatures in the reactor. Clinker agglomeration could cause bridging and channelling problems on the grate just below the oxidation region. This can block the grate operation and high pressure drops can occur in the zone. Consequently, the feedstock characteristics, fuel preparation and sizing, gasifier design and operation parameters are all critical and interdependent factors and need to be carefully evaluated to avoid these problems. In this pilot-scale system, all these features were evaluated and the operation was terminated without any problems.
3.3 Energy and Mass Balance Analyses and Results
Determination and evaluation of the energy and mass balance of the system are essential to reveal the energy production potential of the autothermal updraft gasifier from pelleted olive pomace feedstock. The calculation of the energy and mass balance for the gasification system constitutes a significant factor in establishing the efficiency of conversion of feedstock to the product gas and the determination of energy production. The determination of the energy and mass balance varies according to the type and characterisation of the feedstock and the differences between the thermodynamic equilibrium and reaction kinetics and the three-reaction equilibrium that is essential in the gasification as specified in the introduction section. It may also be changed according to the type and operation of the gasifier reactor.
The energy and mass balance calculations on the process need an assessment of the inputs to and outputs from the reactor. To verify the mass and energy balance outputs, the results obtained from the olive pomace analyses, the fixed bed updraft gasifier capacity, the thermal oil boiler and the ORC turbine efficiency were determined and calculated. There are difficulties in getting 100% closure and obtaining these data. Nevertheless, the average energy balance closeness for three experimental runs was detected to be 96%, indicating a reasonable figure for the initial demonstration of olive waste gasification. The schematic diagram shown in Figure 4 is the energy and mass balance of the pelleted olive pomace as biomass feedstock in the gasification process. According to the energy and mass balance diagram, 500 kg h−1 of olive pomace is used. It has 17.65 MJ kg−1 chemical energy according to the fuel characteristic analysis. Pelleted olive pomace has 25% moisture. The net energy value of the 500 kg h−1 fuel fed to the reactor is 7881 MJ (2189 kWh). As stated in the literature for the updraft gasifier, the air:fuel ratio is determined to be approximately 1 kg of fuel to 1.6 kg air flow rate (1:1.6) (10). For the autothermal gasifier, 1% heat loss can be estimated (79 MJ). Depending on the feedstock, gasifier output char is about 17% of the fuel input. Thus, in this process, 89 kg h−1 of biochar is produced, the equivalent heat is 1043 MJ (290 kWh).
In the updraft gasifier, the ratio by mass of the feedstock and produced gas after gasification is approximately 1:2.5 and the volumetric flow rate of the product gas is 1251 Nm3 h−1. When the density of the syngas is about 1.18 kg Nm−3, the production of hot gas is 1475 kg h−1. Assuming that the temperature of produced gas is 350°C, the volumetric flow rate of syngas at this temperature is calculated as 2660 m3 h−1. If the heat losses are calculated, the energy of produced gas at 350°C is 1878 kWh (6760 MJ). The hot product gas is transferred to the syngas burner when gas is combusted in the thermal oil heater; the boiler thermal energy is calculated as 1596 kWh with 10% heat loss. This thermal energy produced is transmitted to the ORC turbine with thermal oil circulation; heat loss is not calculated because it has sufficient insulation. Since the ORC turbine efficiency is 15%, the turbine generates 240 kW gross, 221 kW net electrical power as 8% parasitic load is internally consumed. The ORC turbine also produces 1356 kWh of thermal energy in the form of waste heat. After the gasification of the gas produced in the boiler and the heated thermal oil in the ORC turbine is transformed into electricity and waste heat energy, thermal power can operate the blackwater evaporation system.
Blackwater produced in the production of olive oil is an environmental problem. Work continues on the vaporisation of this blackwater using excess heat with an evaporation system. In this study, the remaining solid substance from blackwater vaporisation will be used in the gasification system as a feedstock by mixing with olive pomace biomass. In future studies, the produced steam will be converted to a superheated gasification agent in the reactor. Thus, the produced thermal power can also be evaluated efficiently within the facility.
3.4 Gasifier Temperature Profile
Figure 9 shows the temperature profiles of the oxidation, reduction, pyrolysis and drying zones in the updraft gasifier observed during 8 h of continuous operation in the test runs. In general, there is only a small dependence on the feed rate. However, the variation is more pronounced at the gasifier outlet, which could be due to variations in the aeration rates, especially at higher throughputs. However, as the air to fuel ratio increases, the zone temperatures increase. Because of very high temperatures around the moving grate zone (>1000°C), some forms of clinker were observed over the grate during the clean-up. In the literature, studies seem to have reached a consensus about the temperature (>1100°C) in the oxidation zone of an updraft gasifier (8). Reasonable residence time is necessary to destroy the refractory unsubstituted aromatics (tars) in the product gas, without catalytic tar cracking.
Therefore, the optimum operating temperature should be adjusted for each different fuel used in the reactor by considering tar cracking versus clinker formation. Obviously, ash fusion temperatures of the fuels are decisive in selecting operating temperatures of an updraft gasifier. Clinker formation has a more significant impact than tar formation, which can be easily treated by improving the clean-up of the system. Although tar formation above 900°C is small, the benefit of reducing clinker is substantial for the operation of the gasifier.
3.5 Thermal Oil Boiler and Organic Rankine Cycle Turbine Operation Results
The operation parameters of the process are 1600 kWh energy generated in the thermal boiler as a result of burning syngas is transferred to 60 m3 h−1 Therminol® 66 (Eastman, USA) fluids. Therminol® 66 is a high performance, highly stable synthetic heat transfer fluid. The chemical composition of this fluid was carefully selected to minimise the formation of low boilers and eliminate the risk of insoluble high boiler formation and fouling, provided proper attention is given to system design and operation is within the maximum bulk (345°C) and film (375°C) temperatures. To calculate the physical properties of the fluid such as density, heat capacity, thermal conductivity, kinematic viscosity and vapour pressure, formulas are given below (Equations (xi)–(xv)) (26):
According to these formulas, at 280°C physical properties of the fluid are 824.6 kg m−3 (density), 0.097 W m−1 K−1 (thermal conductivity), 2.492 kJ kg−1 K−1 (heat capacity), 0.56 mm2 s−1 (kinematic viscosity) and 19.46 kPa (absolute vapour pressure).
In this study, a pilot-scale gasification system based on CHP plant, its operation and energy production efficiency were investigated. Dried and pelleted olive pomace biomass from olive oil production facilities was used as fuel in the autothermal fixed bed updraft gasifier. It was seen that the feedstock characteristics and the design of the updraft gasifier play a significant role in process operation. Appropriate design of the gasifier diminishes drawbacks, thus improving the performance of the system. Olive pomace, which is used as feedstock, needs to be dried and pelleted before being gasified to prevent blockage and pressure drops in the gasifier.
The thermal oil boiler has a thermal capacity of 1.60 MWh. Most of the generated thermal energy is transferred via thermal oil to the ORC turbine to generate an electric output of 240 kW transferred to the rural grid at the maximum load. The excess useable waste heat energy of 1.35 MWh from the gasification system is used in the blackwater evaporation unit. Therefore, it was proved that this innovative, unique system is an important source of renewable energy for rural areas. Also, this type of process offers the possibility of converting feedstock biomass into power and heat with high efficiency. C conversion during gasification was around 85%, which is thought to be reasonable.
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Murat Dogru received his Doctorate degree (1996–2000) from the Chemical and Process Engineering department at Newcastle University, UK. He researched and lectured for eight years as a member of academic staff at Newcastle University from 2000 until 2008. He then worked as an associate consultant in the Adapt Low Carbon Group at the University of East Anglia, UK, between 2008 and 2012 and in 2013 he obtained his associate professorship in Environmental Engineering at Gebze Technical University in Istanbul, Turkey. Dogru is also Executive R&D Director at Beltran Technologies, Inc, New York, USA for the industrial development of gasification systems using a variety of biomass and wastes in order to develop renewable energy generation systems for local governments and industries. He has published more than 100 international publications in journals and international symposiums which all have direct relevance to renewable energy generation technologies, in particular biomass and waste gasification.
Ahmet Erdem received his Master’s degree (2013–2015) in Environmental Engineering from Gebze Technical University, Turkey. He has been a member of academic staff at Gebze Technical University since 2015. He worked as a commissioning engineer at a waste gasification research and development project in South Korea in 2014. The project was sponsored by the Korean Environmental Ministry and managed by Beltran Korea as subcontractor. He is in the last period of his doctoral study and his PhD thesis is on activated carbon production from biochar produced from gasification.