Pilot Scale Downdraft Gasifier Research Articles

Hydrogen Production Enhancement and Tar Reduction using New Three-Layer Annular Downdraft Air-Steam Biomass Gasifier

This study presents experimental analysis of biomass air-steam gasification for the enhancement of H2 production in producer gas (PG). Wood pellets which are widely available in Malaysia are used in this investigation. A pilot scale downdraft gasifier based on new three-layers annular reactor design was implemented to produce hydrogen enriched PG. The reactor is initially heated using hot flue gas from LPG combustion which is switched to PG when the reactor reaches stable operation. The effect of steam to biomass (S/B) ratio from 1.1 to 1.5 on PG quality in terms of gas composition, HHV and tar contamination is studied. Maximum H2 production of 27.7% was achieved at optimum air flow rate of 75 LPM and S/B ratio of 1.2. At the optimum air flow rate, HHV and tar contamination in PG where in the range of 5.06 - 6.08 MJ/Nm3 and 187 - 30mg/m3, respectively.

Steam reforming of tar using biomass gasification char in a Pilot-scale gasifier

Gasification char is a residual material produced during the biomass gasification process. Considered as industrial waste, it is typically disposed of through incineration or landfilling, thereby incurring significant economic costs. Nonetheless, gasification char is characterized by its high carbon content and surface area making it an economical alternative to common catalysts and catalyst support materials. In this study, char from a pilot-scale downdraft gasifier was used to reform the tar generated from the same gasifier. Reforming was performed both in the presence and absence of steam. The aim is to convert condensable hydrocarbon derivatives (tars) into non-condensable lower molecular weight products such as H2 and CO. Reforming test conducted with 0.18 kg/h steam for 2 h at 750 °C and char bed weighing 600 g resulted in a reduction of tar concentration from 2407 mg/Nm3 to 20 mg/Nm3. The same test conditions were also responsible for an increase in H2 production from 15 to 26 vol%. The combined effect of steam and char bed suggests that both upgrading the producer gas as well as cleaning it can be made possible in a single process.

Olive mill solid waste gasification in a pilot-scale downdraft gasifier with three-stage air supply: performance, mass-energy balance and fate of inorganic elements

Olive mill solid waste (OMSW), also called olive pomace, is a biomass waste available in many Mediterranean countries, including developing countries. In this study, OMSW has been tested as a fuel in an original pilot-scale downdraft gasifier with three-stage air supply, and extensive metrology was implemented. After pelletizing and characterization of the biomass waste, gasification was investigated by operating a 2-day campaign using air as a gasifying agent. The evolution over time of operating parameters such as flowrates, temperatures and pressure drop were recorded. The syngas (including tars) and solid residues were analyzed. The fate of inorganic elements was investigated. Mass and energy balances were performed and the performance of the gasifier was evaluated. The syngas contained 14 % v/v H2 and 17 % v/v CO (dry and tar-free basis) and its lower heating value was 4.7 MJ/Nm3 (wet basis). The solid residues were rich in potassium and phosphorus and could be considered as an organic amendment. Cold gas efficiency was 68 % and carbon conversion efficiency was 89 %. The points of caution concerned the low melting point of the ash, the relatively high tar content in the syngas and the high NOx content of the flue gases after syngas combustion in a boiler. The energy potential of OMSW is substantial and could help reduce the pressure on fossil fuels.

Gasification of Solid Recovered Fuels with Variable Fractions of Polymeric Materials

Gasification is a promising thermochemical technology used to convert waste materials into energy with the introduction of low amounts of an oxidant agent, therefore producing an environmental impact that is lower when compared to incineration and landfilling. Moreover, gasification allows a sustainable management of wastes and reduces the use of fossil fuels responsible for the increment of greenhouse gases. This work aimed to perform gasification tests with solid recovered fuels (SRF) containing organic fractions mainly retrieved from construction and demolition wastes to assess the potential for energy conversion. Tests were conducted in a pilot-scale downdraft gasifier (maximum feedstock input of 22 kg/h) at c.a. 800 °C, using SRF samples containing different proportions of polymeric wastes ranging between 0 and 20 wt %. Gas and chars obtained as by-products were analysed to evaluate their properties and to establish valid pathways for their valorisation. The addition of polymeric wastes reduced char production but rose both tar and HCl concentrations in the gas. The SRF with 10 wt % of polymeric wastes generated the best results, producing the highest calorific value for the gas (3.5 MJ/Nm3) and the highest cold-gas efficiency (45%). Possible char applications include their use as catalysts for tar decomposition, or as an additive in construction materials. Gasification can therefore be considered a valid solution for the energetic valorisation of these SRFs.

A comprehensive pathway on the determination of the kinetic triplet and the reaction mechanism of brewer's spent grain and beech wood chips pyrolysis

This study paves a strategy to better understand the thermal conversion of biomass residues. In particular, a comprehensive study to determine the multi-step reaction model to describe the pyrolysis process of brewer's spent grain from Pilsner (PBSG) and Weizen (WBSG) malts, and beech wood chips (BWC) has been presented.The results of the kinetic triplet optimisation show that the process begins with a nth order reaction Fn (n = 1.74, 1.52, 1.54) for extractives, followed by Avrami-Erofeev type nucleation reactions An (n = 1.50, 1.50) for hemicellulose, with the exception of beech hemicellulose that follows the nth order mechanism Fn (n = 1.25), and An (n = 1.50, 1.50, 1.50) for cellulose, for PBSG, WBSG and BWC, respectively. Moreover, by virtue of the comprehensive approach proposed in this study, a single nth order mechanism Fn with n = 1.19, 1.23, 1.32 for PBSG, WBSG and BWC, respectively, has been obtained for the lignin.The developed model has been used to support the design of a pilot-scale down-draft gasifier for the thermal valorisation of brewer's spent grain.

A comprehensive experimental and modeling investigation of walnut shell gasification process in a pilot-scale downdraft gasifier integrated with an internal combustion engine

In the present study, a small-scale downdraft gasification system integrated with an internal combustion engine was investigated for the sustainable conversion of biomass into energy in the form of electricity within heat recovery stages using walnut shell with a lower heating value of 18.07 MJ/kg. Different measurements and analyses, including biomass characteristics, syngas composition, bottom ash assessment, and flue gas emission, were performed to monitor and evaluate the quality of feedstock and products. The results indicated that with increasing electrical loads from 4 to 12 kW, the lower heating value of syngas decreased from 5.07 to 4.72 MJ/Nm3, while the cold gas efficiency and carbon conversion efficiency increased from 66% to 71%, and 74% to 83%, respectively. The syngas composition was found to be 15.42–14.78 % H2, 17.31–16.87% CO, 6.93–7.29% CO2, 3.41–2.77% CH4, and 56.93–58.29% N2. From the bottom ash analyses results, it was observed that walnut shell biochar contained high concentrations of K2O and CaO that could have many effects on the gasification process, such as tar removal, CO2 reduction, and less agglomeration. In addition, a modified quasi-thermo-equilibrium model has been proposed to predict the composition of syngas, yields of tar (five common biomass tar compounds), and char released during the gasification process. To improve the accuracy of the model, non-equilibrium correction factors were estimated and validated with 40 experimental data sets, including 25 various biomass feedstocks in the literature for equivalence ratio and reduction temperature of 0.2 to 0.45 and 740 to 1300 K, respectively. After all modifications, the average RMSE value was reduced from 5.02 to 2.19.

Experimental investigation of wastewater microalgae in a pilot-scale downdraft gasifier

Microalgae are potential feedstock for biofuels due to higher heating value and rapid growth rate. It can be cultivated in sewage, avoiding costs associated with clean water and nutrients, making the possibility of energy generation from wastewater treatment plants appealing. Compact microgeneration systems involving biomass gasifiers are already commercialized for small-scale projects and could be adapted for use with microalgae in wastewater. In this study, an experimental investigation of a commercial downdraft gasifier was conducted using microalgae produced in a wastewater treatment plants. The effects of the air-fuel equivalence ratio on syngas composition, higher heating value, and production rates were evaluated. An increasing and then decreasing trend in equivalence ratio with a peak was observed, indicating that the optimum equivalence ratio for the best performance is 0.23. The cold gas efficiency was 87%, higher heating value was 6.2 MJ/Nm3, and the production rate was 2.8 Nm3/kg dry biomass. The syngas composition was 11.9% H2, 19.5% CO, 8.5% CxHy, and 9.8% CO2. The H2/CO ratio observed in the syngas was 0.61, which is very close to the 0.60 recommended for synthetic fuel production (Fischer-Tropsch gasoline and diesel).

Co-gasification of oil palm biomass in a pilot scale downdraft gasifier

The present study focused on co-gasification of empty fruit bunch (EFB), mesocarp fibre (MF) and palm kernel shell (PKS) in a 75 kWth pilot scale downdraft gasifier for possible synergic reactions between the biomass. A series of experiments was carried out using equal blend of EFB, MF, and PKS (particle sizes of 14 and 6.7 mm) and equal blend of MF and PKS. Advanced infrared multi-gas analyser, and thermal conductivity gas analyser were employed to measure the produced gases. The elemental compositions of the raw biomass, ash and slag generated were determined using Scanning Field Emission Gun Scanning Electron Microscopy with accelerating voltage 20.0 kV and working distance 6 mm and the measurements processed using AztecEnergy V2.2 software. The co-gasification of blend of EFB, MF, and PKS, compared to the blend of MF and PKS led to higher gas yield (4.82 and 3.47 m3/kg_biomass), cold gas efficiency (16.2 and 13.37%), and carbon conversion efficiency (56.3 and 34.18%), respectively. When compared to particle size of 14 mm, the PKS of particle size of 6.7 mm in the EFB/MF/PKS blend increased the lower heating value and the higher heating value of the producer gas by 20% and 20.3%, respectively, and the residue yield was 18.6% less. The overall result has provided evidence on the importance of co-gasification of biomass especially EFB, MF and PKS, which will result in increased utilization of EFB.

Experimental and Modeling Analysis of Brewers´ Spent Grains Gasification in a Downdraft Reactor

The first part of the current reported work presents experimental results of brewers’ spent grains gasification in a pilot-scale downdraft gasifier. The gasification procedure is assessed through various process characteristics such as gas yield, lower heating value, carbon conversion efficiency, and cold gas efficiency. Power production was varied from 3.0 to 5.0 kWh during the gasification experiments. The produced gas was supplied to an internal combustion engine coupled to a synchronous generator to produce electricity. Here, 1.0 kWh of electricity was obtained for about 1.3 kg of brewers’ spent grains pellets gasified, with an average electrical efficiency of 16.5%. The second part of the current reported work is dedicated to the development of a modified thermodynamic equilibrium model of the downdraft gasification to assess the potential applications of the main Portuguese biomasses through produced gas quality indices. The Portuguese biomasses selected are the main representative forest residues (pine, eucalyptus, and cork) and agricultural residues (vine prunings and olive bagasse). A conclusion can be drawn that, using air as a gasifying agent, the biomass gasification provides a produced gas with enough quality to be used for energy production in boilers or turbines.

Analysis of pollutants in the product gas of a pilot scale downdraft gasifier fed with wood, or mixtures of wood and waste materials

Small scale gasification of Solid Recovered Fuels (SRF) in downdraft reactors could be an alternative to large scale waste-to-energy schemes. In this perspective, the assessment of the pollutant emissions at pilot scale is necessary. This work compares pollutant emissions from wood and SRF air gasification in a downdraft fixed bed gasifier. Five fuels have been studied: Poplar wood, SRF wood, and three different mixtures containing mass fractions of 80% SRF wood with 20% of either tire, plastic waste or sewage sludge. Air gasification was performed in a pilot scale reactor in fed-batch mode using a fuel mass ranging from 5 to 8 kg and an air inlet flow of 170–180 L min−1 (at 0 °C and 101 325 Pa). Depending on the fuel, Equivalence Ratios (ER) ranged from 0.22 to 0.29 and gasification temperatures from 690 to 850 °C. Emissions analyses were performed on product gas, condensable species and remaining chars, with a particular focus on sulfur, nitrogen and heavy metals. Regarding sulfur, wood and SRF wood led to low H2S contents (6–8 μmol mol−1), when SRF mixes led to higher concentrations (44–96 μmol mol−1) in addition to of up to 11 heavier sulfur compounds. Regarding nitrogen, SRF produced higher ammonia concentrations (2.1–7.6 mmol mol−1) than Wood (619 μmol mol−1), as a result of initial nitrogen mass fractions17 to 27 times higher in the SRF mixes. Heavy metals analyzed in remaining solids after gasification showed low recovery rates (<0.4%), and tend to accumulate in fine particles, as a result of their volatility.

Pilot scale autothermal gasification of coconut shell with CO2-O2 mixture

This paper explored the feasibility and benefit of CO2 utilization as gasifying agent in the autothermal gasification process. The effects of CO2 injection on reaction temperature and producer gas composition were examined in a pilot scale downdraft gasifier by varying the CO2/C ratio from 0.6 to 1.6. O2 was injected at an equivalence ratio of approximately 0.33–0.38 for supplying heat through partial combustion. The results were also compared with those of air gasification. In general, the increase in CO2 injection resulted in the shift of combustion zone to the downstream of the gasifier. However, compared with that of air gasification, the long and distributed high temperature zones were obtained in CO2-O2 gasification with a CO2/C ratio of 0.6–1.2. The progress of the expected CO2 to CO conversion can be implied from the relatively insignificant decrease in CO fraction as the CO2/C ratio increased. The producer gas heating value of CO2-O2 gasification was consistently higher than that of air gasification. These results show the potential of CO2-O2 gasification for producing high quality producer gas in an efficient manner, and the necessity for more work to deeply imply the observation.

Using quadrupole mass spectrometry for on-line gas analysis – Gasification of biomass and refuse derived fuel

A quantitative method of on-line gas analysis was developed, using quadrupole mass spectrometry to measure gas composition when charcoal, wood, straw, and refuse derived fuel were gasified. Despite overlapping between some of the species, the method was demonstrated on small-scale laboratory gasifiers, recording the main components (e.g. N2∼54–67vol.%; H2∼5–13vol.%; CO2∼15–17vol.%; CO∼10–17vol.%), and lower levels of CH4 (∼1–2vol.%) and O2 (∼0.1–1vol.%) in the gas. Trace levels of H2S (∼100–300ppmv) and COS (∼10–30ppmv) were also measured (important for gas clean-up strategies). On-line measurements were performed on a commercial pilot-scale down-draft gasifier (using waste wood), and the concentrations of H2S varied from 200 to 700ppmv, and COS from 7 to 17ppmv. The ratio of H2S: COS was higher than in the laboratory trials – probably because of COS hydrolysis reactions taking place in the wet scrubbing systems.

Quantification of Tars, Particulates, and Higher Heating Values in Gases Produced from a Biomass Gasifier

Syngas from biomass gasifiers contains impurities such as tars and particulates, which can create difficulties for the downstream processes (e.g., internal combustion engines and the Fischer-Tropsch process). To design an efficient and effective gas cleaning system, it is important to accurately quantify the tars and particulates. The absence of an ASTM procedure for tars and particulates produced from a gasifier led to the development and testing of the protocol presented in this study. Syngas was generated from woodchips using a pilot-scale downdraft gasifier, which was designed and constructed in-house. The sampled impurities were analyzed using mass gravimetry, solvent evaporation, and weight differential methods. The higher heating value of the exiting gases was estimated from the syngas composition. The average tar and particulate concentrations of the sample runs were 1.8 to 3.1 g/m3 and 5.2 to 6.4 g/m3, respectively. The higher heating values of the syngas ranged between 4.38 and 4.55 MJ/m3.

The Effect of Fuel Type on Temperature Profile within Pilot-Scale Downdraft-Gasifier for the LPG Replacement in Electricity Production

This research presents the effect of fuel type on temperature profile within the pyrolysis-, combustion-and reduction-zone of the pilot-scale downdraft gasifier, and the replacement potential of LPG with syngas for electricity production. The experimental results showed that gasification of different fuels have the distinctions of temperature level in each zone. More stable syngas production was obtained either from wood chips (WC) or plastic carrier bag (PCB). In contrast, gasification of aluminum coated plastic bag (ACPB) showed the unsatisfied syngas. From the operation with the dual mode of LPG and syngas for electric production, LPG consumption was reduced about 27.08%, 14.58% and 8.33% when PCB-, WC-and ACPB-syngas were used, respectively.

Studies of Biochars Generated from Pilot-Scale Downdraft Gasification

<abstract><title><italic>Abstract.</italic></title> Biochars were generated with a pilot-scale downdraft gasifier using wood chips as the raw material. Biochars were collected from the bottom of the gasifier (coarse char) and from the filter (fine char). They were characterized by scanning electron microscopy (SEM), elemental analysis, thermogravimetric analysis (TGA), and higher heating value (HHV) and compared with the raw material and commercial activated carbon. Coarse char showed a larger average particle size than fine char. Coarse char also had higher carbon content (47.2%) than fine char (32.4%). Both biochars had much lower carbon content but higher mineral content than commercial activated carbon (Filtersorb 200; 88.1%). Both coarse char and fine char retained considerable HHV (14.5 and 10.0 MJ kg^-1 respectively), but their HHVs were lower than that of wood chips (17.7 MJ kg^-1) and much lower than that of Filtersorb 200 (30.7 MJ kg^-1). Fine char showed higher thermal stability than coarse char. Both biochars were more thermally stable than wood chips but less stable than activated carbon.

Analysis of the fly ash from the processing of wood chips in a pilot-scale downdraft gasifier: Comparison of inorganic constituents determined by PIXE and ICP-AES

Gasification of biomass ultimately generates at least one solid byproduct in which the inorganic constituents of the biomass are concentrated. Given the potential for utilization, or issues with disposal, facile methods are needed for determining the compositions of the fly ashes from recently-available gasifier-based bioenergy systems. Proton induced x-ray emission spectroscopy (PIXE) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were used to characterize the fly ash recovered from a pilot-scale (25 kW) modular bioenergy system operated with wood chips as the feedstock. The composition of the fly ash from the downdraft gasifier showed some similarities to compositions reported for boiler wood ashes, apart from one half of the material being unburned carbon. Although ICP-AES showed greater sensitivity for the analysis of the fly ash, especially for small amounts of heavy metal contaminants, PIXE proved to be a powerful analytical tool for screening of elements from sodium to uranium. Such broad spectrum screenings could prevent the inadvertent land application of unsuspected pollutant elements. Fly ashes from biomass gasification appear to be suitable for use as ash-based fertilizers for forest lands; however, combustion to remove unburned carbon may be advisable.

Simulation of High Temperature Air – Steam Biomass Gasification in a Downdraft Gasifier Using ASPEN PLUS

A comprehensive process model is developed for high temperature air – steam biomass gasification in a downdraft gasifier using the ASPEN PLUS simulator. The simulation results are compared with the experimental data obtained through pilot scale downdraft gasifier. In this study, the model is used to investigate the effects of gasifying agent preheating, equivalence ratio (ER), and steam/biomass (S/B) on producer gas composition, high heating value (HHV), and cold gas efficiency (CGE). Results indicate that H2 and CO contents have increased when gasifying agent preheating is used, while gasifying agent preheating has no effect with H2 and CO at high ER. At high level of S/B, the concentrations of H2 and CO are related with water-gas shift reaction in significant. HHV and CGE depend on the concentrations of H2 and CO in producer gas, which can increase by preheated gasifying agent. However, gasifying agent preheating should apply with waste heat from the process because there is no additional cost of energy price.

A comparison between Miscanthus and bioethanol waste pellets and their performance in a downdraft gasifier

Pelletised biomass has been found to have excellent potential for their utilisation in small to medium sized energy systems because of its advantages over loose feedstock. The energy density is increased and so the space occupied in transportation is decreased and the amount of problematic dust or fines is also decreased. Furthermore, pellets provide a more uniform fuel, allowing easier feeding and improved performance in thermal conversion processes. The pellet manufacturing process, or pelletisation process, plays a major role on the quality of pellets produced. Changes to pelletisation parameters such as feedstock moisture content, die diameter, particle size (or screen size), addition of lubricants or binders can significantly alter the quality of the pellets and therefore the ease with which the pellets can be gasified in a downdraft gasification process. One important quality parameter that greatly affects the downdraft gasification process is the strength or durability of pellets. Durability can be defined as the ability of pellets to resist mechanical breakdown during transport or during feeding into an energy plant. Other important parameters that affect downdraft gasification are the ash content and composition of the pellets. The ash is derived from the minerals in the feedstock, the addition of binders or lubricants and also the pellet production method. Furthermore, gasification efficiency can be also affected by the process parameters such as air-to-fuel ratio, air or biomass feed rate and operating temperature. The current article compares the properties of three different types of pellets and their gasification performance. Two types of Miscanthusand a bioethanol production reside (distiller’s dried grains with solubles (DDGS)) were used to make the pellets. The pellets made were of similar size (6–8mm) and ultimate analysis, so the paper focuseson the most important differences; these were durability, ash content and gasification parameters, expressed through the equivalence ratio which relates the actual air-to-fuel ratio with the calculated stoichiometric value. A series of experiments were conducted in a 50kWth pilot scale downdraft gasifier with the equivalence ratio varied in the range 0.2–0.3. The quality of the gas produced and the gasifier performance were assessed in terms of the gas composition, yield, heating value, cold gas efficiency and carbon conversion efficiency.

Gasification of pelletized biomass in a pilot scale downdraft gasifier

This work presents a pilot-scale investigation aimed at assessing the feasibility and reliability of biomass pellet gasification. Wood sawdust and sunflower seeds pellets were tested in a 200kW downdraft gasifier operating with air as gasifying agent. The gasification of pelletized biomass led to rather high and unstable pressure drops, reducing the gasifier productivity and stability. Furthermore the generation of fine residues compromised the operation of wet ash removal systems. On the other hand, good syngas compositions (H2 17.2%, N2 46.0%, CH4 2.5%, CO 21.2%, CO2 12.6%, and C2H4 0.4%), specific gas production (2.2–2.4Nm3kg−1) and cold gas efficiency (67.7–70.0%) were achieved. For these reasons pelletized biomass should be considered only as complementary fuel in co-gasification with other feedstock.

Elemental analyses of chars isolated from a biomass gasifier fly ash

Processing of pine wood chips in a pilot-scale downdraft gasifier generated a carbon-rich fly ash as a byproduct. Studies on biomass-derived fly ashes have generally focused on their direct characterisation, as generated, to develop options for utilisation or disposal. Analogous to studies on coal-derived fly ashes, strategies were applied here to assess the feasibility of isolating chars for value-added applications and to determine their elemental compositions relative to that for the feedstock. The inorganic ash content of the biomass-derived fly ash was 48.4%. Sieving followed by washing with deionized water afforded large char particles (>10 mesh) with an ash content of only 3.59%. A subsequent acid treatment reduced the ash content to 1.55%; further demineralization treatments to obtain lower ash contents were deemed impractical. Elemental analyses showed calcium was the most abundant inorganic element in the feedstock, fly ash, and water-washed char. The apparent removal of calcium carbonate by the acid treatment afforded a char in which potassium was the most abundant element. Results suggested that the chars surviving gasification may function as adsorbents in situ for elements that are volatile under the conditions of gasification.