Small-scale Biomass Gasification Research Articles

Techno-economic analysis of small-scale ammonia production via sorption-enhanced gasification of biomass

Biomass gasification is a green and efficient pathway for lowering the carbon footprint of ammonia production. Decentralized, small-scale biomass gasification plants (<150 t NH3/day output) are more practical than large plants due to sustainable large-scale biomass supply constraints. Small-scale biomass gasification for ammonia production faces the challenges of high energy consumption and production costs. Process intensification through sorption-enhanced gasification (SEG) integrates steam gasification and in-situ CO2 separation using regenerable CaO sorbents, directly producing hydrogen-rich syngas (∼70 vol%), which streamlines syngas conditioning and increases cost-effectiveness. Two SEG process configurations for small-scale biomass-to-ammonia (∼45000 t of agricultural residues/year) were proposed: SEG process A, which produced low-carbon ammonia without air separation, and SEG process B, which employed an air separation unit to deliver carbon-negative ammonia by capturing CO2. The two processes were compared against a dual fluidized bed gasification (DFBG) process, which generated green ammonia without an air separation, like SEG process A. Of the three cases, SEG process B showed the lowest biomass (2.14 t biomass/t NH3) and water consumption (0.47 t H2O/t NH3) and avoided 2.8 t CO2/t NH3, making it carbon-negative. While the DFBG process and SEG process A had higher energy efficiencies, SEG process B posted the lowest net energy consumption (36.6 GJ/t NH3). Furthermore, SEG process B had the lowest ammonia production cost ($692/t NH3) and payback time (9.4 years), making it the most economically viable. Sensitivity analysis showed that favorable carbon credit policies could boost the economics of SEG process B to match large-scale plants.

Conventional and advanced exergy and exergoeconomic analysis of a biomass gasification based SOFC/GT cogeneration system

In this paper, a small scale biomass gasification based solid oxide fuel cell/gas turbine (SOFC/GT) combined heat and power (CHP) plant is investigated by means of both conventional and advanced exergy and exergoeconomic analysis. A one-dimensional model of an internal reforming planner SOFC is employed to account for the temperature gradient within the fuel cell solid structure, which is maintained at the maximum allowable temperature gradient (150 K) under different operating conditions. Two main parameters of the gasification process, namely, air-to-steam ratio and modified equivalence ratio, are investigated, and the key parameters of the cycle exergy and exergoeconomic study are analyzed. Moreover, a multi-objective optimization procedure is applied to determine the unavoidable gasifier conditions required for the advanced exergy analysis of the system. The results of the conventional exergy and exergoeconomic analysis reveal that the highest rate of exergy destruction occurs in the gasifier, followed by the afterburner (AB) with 41.87% and 21.98%, respectively. Also, the lowest exergoeconomic factor is related to AB by 5.34%, followed by heat recovery steam generator (HRSG), gasifier, air compressor, and SOFC, which implies that the priority is to improve these components to reduce the exergy destruction cost rate. The results obtained from the advanced exergy and exergoeconomic analysis indicate that the most of the total exergy destruction rate is unavoidably in the CHP plant. The AB shows the least improvement potential in terms of reduction of the exergy destruction by almost 2% avoidable part, followed by Heat Exchanger 3 (H.X.3), gasifier, and SOFC duo to their lowest avoidable exergy destruction parts of almost 5%, 10% and 13%f respectively. Furthermore, the unavoidable part of the investment cost rate for all the components of the cogeneration plant is larger than the avoidable part, which means that it is difficult to reduce the investment cost rate of the system components. Meanwhile, the endogenous/exogenous analysis shows that the exergy destruction is completely endogenous for all components of the integrated plant, except for HRSG, GT, and HX1. Compressors and turbines have the highest potential to reduce endogenous exergy destruction. This is due to their higher avoidable endogenous exergy destruction. Reducing the investment cost rate seems difficult, as the main investment cost rate was found to be an unavoidable endogenous part for all system components. Finally, some results obtained from the advanced analysis approach are the opposite to those of the conventional method. This fact emphasizes that the results of conventional exergy analysis alone are insufficient and unreliable. For example, based on the advanced analysis perspective, the gas turbine and H.X.2 by 8.9% and 8.46% modified exergoeconomic factor, respectively, should be considered for reducing investment cost rate, while the conventional method gives opposite results.

Non-Thermal Air Arc Plasma Assisted Biomass Gasification

A novel small-scale biomass gasification system assisted by a non-thermal air arc plasma is introduced in this paper and the gasification experiment setup, procedure and gasification results are described in detail. The results show that the production of syngas (CO and H2) is in the range of 1.14 Nm3/h to 1.46 Nm3/h during the system normal operation at plasma power consumption 120 W and biomass feed rate 3360 g/h, and the volume fraction of syngas in produced gas is in the range of 20.2% to 23.89%. The maximum cold gasification efficiency is 44.56% and the minimum specific energy consumption (defined as the ratio of plasma power consumption to the heat power content of the produced gas) of the gasification system is 2.18%, which is much lower than that of gasification system with thermal plasma.

Simulations and experimental study to compare the behavior of a genset running on gasoline or syngas for small scale power generation

Syngas produced from small-scale biomass gasification plants (tens of kW) can generate power from biomass resources. However, due to the syngas gensets demand is marginal, compared to gasoline gensets, it is not easy to find a genset specially designed to run on syngas. In this paper, an experimental study of a genset running on two kinds of fuel has been carried out. The genset used is a 10 kW ICE initially designed to run on gasoline, in parallel, simulations based on CHEMKIN-PRO software were made. The study focuses on the comparison of the ICE operation independently working on gasoline and syngas. Modifications were required in the genset to adapt the intake system to run on syngas. The syngas is obtained from a gasifier. According to the test, it is possible to convert a gasoline engine to a 100% syngas engine through straightforward modifications in the inlet manifold but reducing the efficiency and power. The electrical power running the genset on gasoline was 6.7 kW, whereas, with syngas, it is 4.8 kW (31.4% of reduction). The efficiency running the ICE on gasoline was 13.7% at its rated power, whereas for the genset fuelled on syngas, it was 10.5% (23.4% reduction).

Evaluation and extension of the load and fuel flexibility limits of a stratified downdraft gasifier

The small-scale biomass gasifiers' application for CHP in conventional households is still inhibited by their inflexible operating conditions, which do often not meet the user's desires of yielding varying loads and switching between any available fuels. Thus, the present work aims to identify the gasifier's flexibility limits by investigating sudden steps in load and fuel water content in a woodchips-driven, stratified downdraft gasifier with a 90-kW nominal fuel load. In contrast to former work, the focus was set on the transient processes, during which the product gas composition and the gasifier bed temperatures were continuously monitored. The evaluations revealed that the gasifier produces a continuously high and almost load independent product gas quality in the range of ±20% of the nominal load, even throughout sudden load changes. Steps of 8 m% fuel water content from dry to wet fuel turned out to be the maximum manageable step size without additional measures. Switches from wet to dry fuel required the previously introduced methods of exhaust gas recirculation and water injection. These methods successfully enlarged the allowable step to over 8 m% fuel water content. These promising results will allow a future more versatile and user-optimized operation of stratified downdraft gasifiers.

Experimental study of combustion of low-calorific producer gas from small scale biomass gasification within porous burner

The aim of this experimental study is to investigate the combustion low-calorific producer gas within porous burner. The small scale downdraft biomass gasification for rural area was performed to produce gaseous fuel. Three types of wood in Thailand were used as raw materials to produce producer gas, i.e. Acacia-mangium, White popinac and Eucalyptus. The low heating values of producer gas were in the range of 3800-4232 kJ.kg-1 that are difficult to burn in conventional burner. Tapered and bilayer porous burners were used to overcome this limitation. The effects of air preheating modes, equivalence ratio and firing rate on thermal structure and pollutant emission were revealed. The results showed that the complete combustion with low emission of low-calorific producer gas was accomplished with low firing rate in the range of 2.8 – 3 kW. Both CO and NOx emission were less than 160 ppm for all of tests. The combustion within tapered porous burner emitted small CO emission nearly zero for all of equivalence ratios. The tar reduction was 99.5% by combustion within porous burner.

Mini-Review on Hot Gas Filtration in Biomass Gasification: Focusing on Ceramic Filter Candles

Biomass gasification is increasingly attracting interest in the research of biorenewable energy all over the world, as a carbon-neutral supplement to the conventional fossil energy. It can convert biomass to the burnable producer gas for heat and power production or synthesis of fuels and chemicals. However, its commercial applications are seriously interfered by the minor but unavoidable contaminants or impurities in the raw product gas, which cause severe problems in the downstream equipment. This paper reviews the recent progress on the hot gas filtration technologies for removing particulate matters (PMs) and tars from the biomass-derived product gas, focusing on ceramic filter candles, which are widely applied in the biomass gasification systems. Developments of PM characterization, hot gas catalytic filtration, and oxidative filtration as well as the numerical simulation of computational fluid dynamics in hot gas filtration are summarized in detail. It also critically discusses the major challenges and future opportunities in hot gas filtration and concludes that the combined oxidative filtration and catalytic filtration with highly efficient catalyst at moderate temperatures (<600 °C) should be the most economical option for the widespread commercial small-scale biomass gasification systems.

A novel low-cost tar removal technology for small-scale biomass gasification to power

Small-scale decentralized biomass gasification could be an effective way to reduce air pollution and improve living quality through electrification in rural areas. Tar removal is a challenge at the small-scale and its high cost limits the application of small-scale gasification-to-power systems. A simple, compact, and low-cost tar removal technology for small-scale application is investigated. Hot raw producer gas is partially oxidized under fuel rich condition in an internal combustion engine. Tars carried by the producer gas are destroyed under high temperature and pressure from compression and partial oxidation. High tar cleanup efficiency with low energy consumption is achieved. Economic analysis of the technology is evaluated on a 20 kWe gasification-to-power system. The capital expenditures (CAPEX) and the operating expenditures (OPEX) reduction are 40% and 17%, respectively. The OPEX can be further reduced. The large cost reduction is mainly attributed to the high tar toleration of the proposed technology.

State-of-the-art of small-scale biomass gasification systems: An extensive and unique monitoring review

During the last few years, there has been an increasing interest in small-scale biomass gasification in Central and Northern Europe. Since 2011, almost fifty small-scale biomass gasification plants have been authorized and built in the South Tyrolean region (Italy), and most of them are currently operating. Within this framework, an extensive survey was performed by means of questionnaires to the plant owners for assessing the biomass and char flows in the region. Moreover, a comprehensive monitoring campaign was carried out onsite on representative plants of almost all the available operating technologies. For each of the monitored plants, the feedstock and the gasification products were characterized and their fluxes quantified, leading to energy and mass balances. This allowed collecting an extended set of data and drawing up a unique overview of the reference values for the ranges of operation of small-scale biomass gasification systems currently available in the European market, in terms of equivalence ratio, dry gas composition and heating value, specific electricity production, and conversion efficiencies. Moreover, samples of chars were deeply characterized, providing an insight into possible utilization pathways for the valorization of this by-product that is currently disposed of, representing an economical and environmental burden.

A Novel Low-Cost Tar Removal Technology for Small-Scale Biomass Gasification to Power

A Novel Low-Cost Tar Removal Technology for Small-Scale Biomass Gasification to Power

A biomass-based small-scale power generation system with energy/exergy recuperation

Small-scale biomass power generation will play an important role in ensuring the regional power supply by using the local biomass resource and protecting the environment in the future. In this study, a small-scale high-efficient combined heat and power generation system with a separated-type biomass gasification process combining the energy/exergy recuperation is proposed for the first time. The spatial subdivision of the processes for the biomass pyrolysis, char combustion, tar reforming and catalyst regeneration is adopted by using a separated-type biomass gasifier design to realize the optimization of each conversion step and improve the whole system performance. To obtain the maximum power generation efficiency, the energy flow and exergy flow in the system are analyzed in details and the operating condition of the gasification system is optimized. The results demonstrate that the relatively low temperature as well as low steam/carbon ratio in the tar reformer should be conducive to the improvement of energy and exergy efficiencies. In the optimum operation condition, the biomass input of 548.86 kW (higher heating value) could generate 263.65 kW of electrical power with the total energy and exergy efficiencies of 37.9% and 43.2%, respectively, in which 153.44 kW of energy could be recuperated back to the gasification process by air and steam with 136.56 kW of energy obtained from gas turbine exhaust to enhance the whole power generation efficiency. It is expected to provide a new design concept for the development of high-efficient small-scale biomass gasification system for the combined heat and power generation.

Experimental Investigation of Bioenergy Production from Small-Scale Gasification of Landfill-Diverted Wood Wastes

Small-scale biomass gasification systems integrated with internal combustion engines can be a solution to the dual challenges of reducing greenhouse gas emissions from electricity production and reducing waste sent to landfills; however, these systems are typically designed for high-value, virgin wood chip feedstocks. This investigation aims to improve the feasibility of this technology by exploring the use of landfill diverted wood wastes (construction and demolition waste, used pallets and oriented strand board) in place of costly wood chips for the production of bioenergy in a pilot scale gasifier combined with an internal combustion engine generator operating at 10 kWe. The effect of feedstock bulk properties on gasifier operational stability and process parameters, including magnitude and variability of temperature and pressure drop, are investigated. Low bulk density ( 55°) increased system instability through the formation and collapse of bridges and channels in the gasifier. Tar and particulate matter in the producer gas are quantified, as well as removal efficiencies of the gas clean-up for different feedstocks. Tar concentration in the syngas was in the range of 1.2 to 5.2 g/m3 downstream of gas clean-up, and was found to increase with system instability parameters and feedstock moisture. Finally, over the range of feedstocks examined in this study, a novel empirical correlation was developed to describe tar production from the gasifier as a function of feedstock bulk properties. The relationship between feedstock bulk properties and tar production is a unique finding and can enable to development of reliable waste-to-energy gasification systems.

Analysis of the performance of an integrated small-scale biomass gasification system in a Canadian context

Small-scale downdraft biomass gasification is a promising technology for off-grid combined heat and power generation utilizing Canada’s vast biomass resources. As small-scale gasifiers are currently emerging into the Canadian market, the current study investigates the required feedstock characteristics for stable operation using Canadian softwood, along with the quality of the biochar produced as a byproduct. It was found that reducing feedstock moisture from 20 to 9% increased overall system efficiency from 8.2 to 12%. Equilibrium simulations were utilized to evaluate the deviation from ideal conversion when using feedstock outside the gasifier feedstock specifications. These showed the gasifier was operating close to equilibrium, with the lower heating value of the actual producer gas deviating by an average of 9% from simulated results. This study provides a novel methodology for the quantification of reliability and efficiency of these systems which can be used to evaluate the economic benefit of these systems in Canada.

Studies for removal of tar from producer gas in small scale biomass gasifiers using biodiesel

Biomass gasification based electricity generation systems are emerging as an important component of the decentralised energy supply systems in rural India. Each type of gasifier has different reaction conditions, temperature, residence time, pressure, feedstock, reactor design, and therefore the tar and particulate matter (PM) compositions and concentrations are found to vary. A field study was conducted on a 35 kWe downdraft gasifier to measure and characterize the tar in producer gas using GC-MS, for rice husk and pine needles as the two biomass feeds. Use of water-based scrubbers for removal of tar and PM is prevalent, however it is often the case that such clean-up is not adequate for meeting the engine manufacturers’ requirements for the quality of intake gas. Limited attempts have been reported for the use of organic solvent based gas cleaning in small scale downdraft gasifiers in the range 15–50 kWe. In the present work, toluene, naphthalene and phenol were selected as representative compounds of tar, and methyl oleate was selected to represent biodiesel as an organic solvent. A bench scale packed bed scrubber was designed for 95% removal of toluene. An 86–97% removal of toluene from the gas stream was achieved, and similar results were obtained for phenol and naphthalene. Further experiments were carried out with actual producer gas from a 1 kWe downdraft wood gasifier. Pongamia pinnata based biodiesel was used as the solvent, and 88–92% of the tar removal from the producer gas stream was achievable.

Emissions factors from distributed, small-scale biomass gasification power generation: Comparison to open burning and large-scale biomass power generation

Waste woody biomass is one of the most readily available yet underutilized renewable energy sources. Small quantities of wood waste in remote locations are often openly burned when they are not economical to transport and burn in large-scale facilities. Small-scale (10–100 kW) distributed gasifier-generator systems offer an alternative use for this biomass. In this work, we present emissions factors experimentally measured from a commercially available gasifier-generator system. Average emissions factors per mass of dry biomass with high quality producer gas were found to be 1.53 kg-CO2, 11.3 g-CO, 8.7 g-CH4, 2.4 g-NOx, and 0.01 g-soot per kg of feedstock. On an energy basis, emissions were 2.04 kg-CO2, 30.3 g-CO, 18.6 g-CH4, 5.0 g-NOx, and 0.02-g soot per kWh of electricity produced at a maximum load of 15 kW. Our research found that producer gas heating value is inversely proportional to both power output and biomass moisture content. Low quality producer gas was found to significantly increase CO, CH4 and soot emissions from the system but decrease NOx emissions. Small-scale biomass gasification emits less CO2, CO, and soot than open burning, but further improvements are required before emissions from small-scale systems are competitive with emissions from large-scale biomass power generation.

Modeling the emissions of a dual fuel engine coupled with a biomass gasifier-supplementing the Wiebe function.

There is a growing market demand for small-scale biomass gasifiers that is driven by the economic incentives and the legislative framework. Small-scale gasifiers produce a gaseous fuel, commonly referred to as producer gas, with relatively low heating value. Thus, the most common energy conversion systems that are coupled with small-scale gasifiers are internal combustion engines. In order to increase the electrical efficiency, the operators choose dual fuel engines and mix the producer gas with diesel. The Wiebe function has been a valuable tool for assessing the efficiency of dual fuel internal combustion engines. This study introduces a thermodynamic model that works in parallel with the Wiebe function and calculates the emissions of the engines. This "vis-à-vis" approach takes into consideration the actual conditions inside the cylinders-as they are returned by the Wiebe function-and calculates the final thermodynamic equilibrium of the flue gases mixture. This approach aims to enhance the operation of the dual fuel internal combustion engines by identifying the optimal operating conditions and-at the same time-advance pollution control and minimize the environmental impact.

Development of a two-phase model for the calculation of potassium, gaseous products and char yields in the after-burner of a small-scale biomass gasifier

In small scale biomass gasifiers, 2–5% of the input by mass is converted into char which is a carbonaceous product with a wide range of heavy metals composition that depend on the operating conditions of the gasifier and the biomass input. Char is usually treated as waste and is being disposed because the scale of operation is such that does not allow the economic viability of centralized char valorization facilities. The after-burner is a secondary reactor where the mass of char is reduced by means of sub-stoichiometric reaction with air under high temperature and is utilized as an onsite management solution. Commercial software are able to model the equilibrium of metals but have not focused so far on the behavior of char or the final composition of gases. After-burners are air-tight units where measurements cannot be easily obtained. This study introduces a model that utilizes the concentration of potassium as a control variable in order to assess the temperature of the after-burner and then proceeds in calculating realistic yields of char, carbon oxides and the reactive air. The model was validated with experimental measurements of an after-burner and it successfully correlated the operating conditions with verifiable char yields. The results show that the model can be applied for the design and optimization of after-burners where the scope is the simultaneous minimization of the char yields and the heavy metals concentrations in the solid phase.

Thermodynamic modelling of an onsite methanation reactor for upgrading producer gas from commercial small scale biomass gasifiers

Small scale biomass gasifiers have the advantage of having higher electrical efficiency in comparison to other conventional small scale energy systems. Nonetheless, a major drawback of small scale biomass gasifiers is the relatively poor quality of the producer gas. In addition, several EU Member States are seeking ways to store the excess energy that is produced from renewables like wind power and hydropower. A recent development is the storage of energy by electrolysis of water and the production of hydrogen in a process that is commonly known as “power-to-gas”. The present manuscript proposes an onsite secondary reactor for upgrading producer gas by mixing it with hydrogen in order to initiate methanation reactions. A thermodynamic model has been developed for assessing the potential of the proposed methanation process. The model utilized input parameters from a representative small scale biomass gasifier and molar ratios of hydrogen from 1:0 to 1:4.1. The Villar-Cruise-Smith algorithm was used for minimizing the Gibbs free energy. The model returned the molar fractions of the permanent gases, the heating values and the Wobbe Index. For mixtures of hydrogen and producer gas on a 1:0.9 ratio the increase of the heating value is maximized with an increase of 78%. For ratios higher than 1:3, the Wobbe index increases significantly and surpasses the value of 30 MJ/Nm3.

Impact assessment of biomass-based district heating systems in densely populated communities. Part II: Would the replacement of fossil fuels improve ambient air quality and human health?

To determine if replacing fossil fuel combustion with biomass gasification would impact air quality, we evaluated the impact of a small-scale biomass gasification plant (BRDF) at a university campus over 5 scenarios. The overall incremental contribution of fine particles (PM2.5) is found to be at least one order of magnitude lower than the provincial air quality objectives. The maximum PM2.5 emission from the natural gas fueled power house (PH) could adversely add to the already high background concentration levels. Nitrogen dioxide (NO2) emissions from the BRDF with no engineered pollution controls for NOx in place exceeded the provincial objective in all seasons except during summer. The impact score, IS, was the highest for NO2 (677 Disability Adjusted Life Years, DALY) when biomass entirely replaced fossil fuels, and the highest for PM2.5 (64 DALY) and CO (3 DALY) if all energy was produced by natural gas at PH. Complete replacement of fossil fuels by one biomass plant can result in almost 28% higher health impacts (708 DALY) compared to 513 DALY when both the current BRDF and the PH are operational mostly due to uncontrolled NO2 emissions. Observations from this study inform academic community, city planners, policy makers and technology developers on the impacts of community district heating systems and possible mitigation strategies: a) community energy demand could be met either by splitting emissions into more than one source at different locations and different fuel types or by a single source with the least-impact-based location selection criteria with biomass as a fuel; b) advanced high-efficiency pollution control devices are essential to lower emissions for emission sources located in a densely populated community; c) a spatial and temporal impact assessment should be performed in developing bioenergy-based district heating systems, in which the capital and operational costs should be balanced with not only the benefit to greenhouse gas emission reduction but also the health impact to the local community.

Small Scale Biomass Gasification Challenges For Power Generation In Rural Area: A Review

Small Scale Biomass Gasification Challenges For Power Generation In Rural Area: A Review