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Fossil fuels have fueled the world economy for decades. However, given their limited nature, fluctuating prices and the escalating environmental concerns, there is an urgent need to develop and valorize cheaper, cleaner and sustainable alternative energy sources to curb these challenges. Biomass represents a valid alternative to fossil fuels, especially for fuel and chemical production as it represents the only natural organic renewable resource with vast abundance. A vast array of conversion technologies is used to process biomass from one form to another, to release energy, high-value products or chemical intermediates. This paper extensively reviews the thermochemical processing of biomass to fuels and high-value chemicals, with an emphasis on the process performance, conditions, and weaknesses. Technologies with great future prospects as well as those with possible linkage to CO2 capture and sequestration are highlighted. The important chemical compositions of biomass feedstock, their conversion technologies and most importantly, the role of catalysis in their conversion to fuels, fuel additives, based chemicals, and added-value chemicals are also discussed. Special attention is given to biofuel production for transportation as this sector is responsible for the highest global greenhouse gas emissions, and has an emerging market with promising future prospects for sustainable large-scale biomass processing. The processes involved in the purification and upgrading of biomass-derived products into higher-value products are equally discussed and reviewed.

This study is dedicated to present a reliable numerical methodology using Aspen Plus process simulator capable of performing a sensibility analysis of the downdraft gasification of Portuguese forest residues. Effects of critical parameters, including gasification temperature and steam-to-biomass ratio (SBR) on composition of the produced gas are discussed. The sensibility analysis is conducted using Aspen Plus simulator incorporating Fortran subroutines. The model is validated by experimental data and found to be in good agreement. The results of the sensibility analysis performed using air as gasification agent indicate that higher temperatures are favourable for a produced gas with higher hydrogen content and heating value. The simulation results also demonstrate that the use of steam as gasification agent allows increasing the hydrogen content and heating value of the produced gas in comparison to the use of air as gasifying agent. The knowledge of this data is decisive to the development of projects concerning the use of Portuguese forest residues as energy source.

Cameroon is blessed with a vast potential of renewable energy resources: solar, biomass, hydropower, wind and geothermal energies. These resources are currently poorly valorized. The country depends mainly on hydropower for its electricity supply and traditional biomass for its energy consumption. This dependency on hydropower causes acute power shortage nationwide, especially in remote communities where grid access is limited. Rural electrification is mainly conducted through costly grid extensions, small hydro and diesel plants with enormous health and environmental effects. This study assesses the feasibilities of hybrid renewable power systems for remote applications in Southern Cameroons using the climate data of Wum. HOMER was used to perform the comparative analysis. Nine hybrid systems were considered in this study based on the following components: PV module, wind turbine, micro-hydro turbine, diesel generator, battery, charge controllers, and inverters. Two energy sources and storage battery characterized each system. The PV/diesel/small hydro/battery was found to be the most viable economic system for Southern Cameroons with a 0.443$/kWh energy cost. The optimum system was proven to be very resilient to variations in streamflow, interest rate, fuel price, and PV cost. This outcome was found to be highly relevant to policy makers and investors in Cameroon.

Biomass combustion is an important pathway of energy generation from renewable resources. Even though biomass combustion is an established conversion process, there is a high potential for further optimization of the used technologies. In particular, the advantage of numerical methods for the improvement of biomass combustion systems is not used to its full extent, because the simulation of these complex systems requires various sub-models for the thermo-chemical conversion of the biomass and sufficient computational resources for the combustion simulation. However, simulations are a valuable tool to enhance the design and operating conditions of biomass combustion systems with regard to high efficiency, low emissions and high flexibility. In addition, comparison of experimental and numerical results leads to a better understanding of the processes involved in biomass combustion. The present study gives a comprehensive overview of simulations of biomass combustion systems based on computational fluid dynamics that are available in the literature. It focusses on systems with fixed bed and covers various technologies (moving bed, pellet boilers, wood log stoves) as well as a wide range of sizes from laboratory reactors to industry scale. Besides woody biomass, also alternative fuels such as straw or municipal solid waste are considered. All relevant sub-models for the thermo-chemical conversion of the fuel on the one hand and for the gas-phase combustion on the other hand are discussed in detail. The recent advances in the concerned research fields are described.

International audience; This paper aims at studying the combustion in a homemade combustion chamber of biofuel briquettes prepared from olive mill solid wastes. Two different samples of briquettes compressed under 150 MPa and 35 degrees C were investigated: with binder (BOMSW) by adding corn starch and without binder (OMSW). Three combustion regimes were considered: 18, 36, and 55 m(3)/h, respectively. Seven K-thermocouples were used for temperature measurements. Moreover, the gaseous emission analysis was undertaken. Results show that the flame front propagation velocity reaches maximum values of 11 mm s(-1) and 8.54 mm s(-1) for OMSW and BOMSW, respectively. We find that the NOx content was higher in the case of the OMSW. On the contrary, the percentages of methane, carbon monoxide, and sulfur dioxide were more significant with the BOMSW. The combustion efficiency was evaluated and gives a higher value with BOMSW. We concluded that the used binder affects seriously the combustion efficiency and the gaseous emissions.

International audience; This paper aims at investigating the production of high quality briquettes from olive mill solid waste (OMSW) mixed with corn starch as a binder for energy production. For this purpose, different mass percentages of OMSW and binder were considered; 100%-0%, 90%-10%, 85%-15%, and 70%-30%, respectively. The briquetting process of the raw mixtures was carried out based on high pressures. Physico-chemical and mechanical characterizations were performed in order to select the best conditions for the briquettes production. It was observed that during the densification process, the optimal applied pressure increases notably the unit density, the bulk density, and the compressive strength. Mechanical characterization shows that the prepared sample with 15% of corn starch shows the best mechanical properties. Moreover, the corn starch binder affects quietly the high heating value (HHV) which increases from 16.36 MJ/Kg for the 100%-0% sample to 16.92 MJ/Kg for the 85%-15% sample. In addition, the kinetic study shows that the binder agent does not affect negatively the thermal degradation of the briquettes. Finally, the briquettes characterization shows that the studied samples with particles size less than 100 μm and blended with 15% of corn starch binder are promising biofuels either for household or industrial plants use.

MILD combustion of biogas takes its importance firstly from the combustion process that diminishes significantly fuel consumption and reduces emissions and secondly from the use of biogas which is a renewable fuel. In this paper, the influence of several operating conditions (namely biogas composition, hydrogen enrichment and oxidizer dilution) is studied on flame structure and emissions. The investigation is conducted in MILD regime with a special focus on chemical effects of CO 2 in the oxidizer. Opposed jet diffusion combustion configuration is adopted. The combustion kinetics is described by the Gri 3.0 mechanism and the Chemkin code is used to solve the problem. It is found that oxygen reduction has a significant effect on flame temperature and emissions while less sensitivity corresponds to hydrogen enrichment in MILD combustion regime. Temperature and species are considerably reduced by oxygen decrease in the oxidizer and augmented by hydrogen addition to the fuel. The maximum values of temperature and species are not influenced by the composition of the biogas in MILD regime. Blending biogas with hydrogen can be used to sustain MILD combustion at very low oxygen concentration in the fuel. In MILD combustion regime, the chemical effect of CO 2 in the oxidizer stream reduces considerably the flame temperature and species production, except CO which is enhanced. For high amounts of CO 2 in the oxidizer, the chemical effect of CO 2 becomes negligible.

Cameroon has an abundant reserve of energy resources, such as crude oil, natural gas, hydropower, biomass, solar, wind and geothermal energies. However, these resources are still weakly valorised. The country relies mainly on hydropower energy for electricity generation (73%) with persistent power outages throughout the country especially in the dry seasons when water levels are low. Electricity access is about 65–88% in urban areas and around 14% for rural populations. Cameroon has experienced a strong economic growth (growth rate of 5.9% in 2015), accompanied by a rapid increase in electricity demand (1455 MW in 2014). Electricity needs are expected to continue rising over the next decade to reach 5000 MW by 2020 and 6000 MW by 2030. This paper seeks to address energy issues (reliability, accessibility and security) in Cameroon and brings to light the potential and meaningful contributions of renewables in solving energy concern. In order to achieve this objective, an overview, of the energy sector (energy source, access, demand, supply and distribution) is given followed by a review of renewable energy potential, policy, benefits and barriers. In addition, this paper introduces the energy roadmap to achieve a universal access to electricity, which will pave the way for the country emergence by 2035. It is found that energy sector of Cameroon holds promising possibilities of development and diversification given the country's energy potential. With adequate policy, standards, regulations, awareness, capacity building and off-grid renewable energy investments measures, it is possible for Cameroon to meet the future energy targets and ensure meaningful development throughout the country.

Poultry farms with floor-standing poultry generate large amounts of poultry litter waste. The direct application of this waste as an organic fertilizer does not ensure sustainable and cost-efficient utilization of all waste fractions, and can also be linked to environmental hazards. Therefore, the development of new technologies is required for processing poultry litter into a safe product with higher added value. In this work, the characteristics of activated carbon derived from hydrochar, along with the liquid products obtained from hydrothermal carbonization (HTC) and the wet torrefaction (WT) of poultry litter, were investigated. Poultry litter (PL) was applied in a mixture with sawdust (SD) in the following ratios: 1:0 (PL/SD 1:0), 1:1 (PL/SD 1:1), 1:2 (PL/SD 1:2), and 2:1 (PL/SD 2:1). WT processing took place in an innovative fluidized bed system in a superheated steam medium with low overpressure (less than 0.07 MPa) at 300 °C and 350 °C for 30–45 min. Conventional HTC processing was performed in a water medium at 220 °C for 1–4 h. The hydrochar produced in the experiments was activated with steam for 1 h at 450–750 °C. The porosity characteristics of activated hydrochar were measured, including pore size, pore volume, and specific surface area, in view of potential industrial applications as an adsorbent. Additionally, the contents of 5-hydroxymethylfurfural (HMF), as high-value product, were determined in the liquid products obtained from HTC processing, as well as in the condensate obtained after WT processing. Specific surface areas of the activated hydrochars may still be too low for application as adsorbent material. Hence, its use as a biofertilizer and soil improver should be preferred. Interestingly, the liquid fraction obtained from the innovative WT process displayed a significantly higher 5-HMF content compared to the conventional HTC process.

Efficiency is one of the major objectives when designing energy systems. Irreversibilities of combustion processes can be characterized by analyzing entropy generation which is proportional to exergy destruction. In this paper, entropy generation is investigated in turbulent non-premixed counter-flow syngas flames at a high strain rate over a wide range of hydrogen percentage (H2/CO molar fraction from 0.4 to 2.0). The aim is to define the most efficient syngas composition to reduce irreversibilities. Irreversibilities involved in NO formation process are also examined. RANS (Reynolds Averaged Navier Stokes) technique including k-e turbulence model is used for the flow field estimation. Flame structure is calculated using SLFM (Steady Laminar Flamelet Model) and EPFM (Eulerian Particle Flamelet Model) is applied for NOx predictions. Total entropy generation rate accounts for chemical, heat conduction, mixing and viscous effects. Computational results show that the total volumetric entropy generation decreases with H2 enrichment as well as its different contributing effects. Chemical effect is dominant, followed by heat conduction and mixing effects. Viscous effect is negligible. The maximum of both thermal and prompt NO formation routes are influenced by the three main entropy generation modes, with the predominance of the chemical effect. At high strain rates, H2-rich syngas flames are efficient in regards to irreversibilities and NO emissions reduction.

The environmental and economic objectives for the reduction of fuel consumption and reduction in the polluting emissions are always of crucial news. Thus, the elucidation of new techniques of combustion using the diffusion flame (more secure) in a confined environment and the comprehension of the involved physical phenomena remains of capital challenge. The dilution of the combustion air by CO 2 has confirmed its technological interest; the objective of this work is to quantify numerically the polluting emissions in a turbulent diffusion flame. The air is diluted by CO 2 and attention has been paid to its thermal and chemical effects on the internal structure of the flame. It is found that CO 2 addition reduces flame temperature by both thermal and chemical effects. Most of the emissions are also reduced by CO 2 increasing in the air.

This paper presents a computational study to evaluate the influence of turbulence and combustion models as well as chemistry schemes on the combustion of a 8–11 kW small lab-scale biomass furnace. The analysis is conducted in the zone above the bed (freeboard) where the volatiles are burned. The turbulence models tested are standard k - e , RNG k - e and Realizable k - e ; and the combustion models are SFM (Steady Flamelet Model), UFM (Unsteady Flamelet Model) and EDC (Eddy Dissipation Concept). In addition, several chemical mechanisms with different complexity (reduced and detailed chemical kinetics) are considered. The predictions of the velocity, species, and temperature fields are compared with their counterparts’ experimental measurements. The present findings reveal that all tested combustion models (SFM, UFM and EDC) are capable of predicting temperature and major species profiles; whereas only EDC is able to reliably predict slow-chemistry species.

This study addresses the influence of several operating conditions (composition and ambient pressure) on biogas diffusion flame structure and NO emissions with particular attention on thermal and chemical effect of CO2. The biogas flame is modeled by a counter flow diffusion flame and analyzed in mixture fraction space using flamelet approach. The GRI Mech-3.0 mechanism that involves 53 species and 325 reactions is adopted for the oxidation chemistry. It has been observed that flame properties are very sensitive to biogas composition and pressure. CO2 addition decreases flame temperature by both thermal and chemical effects. Added CO2 may participate in chemical reaction due to thermal dissociation (chemical effect). Excessively supplied CO2 plays the role of pure diluent (thermal effect). The ambient pressure rise increases temperature and reduces flame thickness, radiation losses and dissociation amount. At high pressure, recombination reactions coupled with chain carrier radicals reduction, diminishes NO mass fraction.

This article reports on the effect of swirl intensity on the flow and combustion characteristics of pulverized olive waste (OW) burning. Numerical simulations are carried out using the computationa...

This paper presents the efforts toward a reliable baseline computational fluid dynamics (CFD) model for domestic biomass heating systems with a special emphasis on CO and CO 2 emissions. The model accounts for the different phases of biomass thermochemical conversion steps (drying, pyrolysis, gasification, gaseous combustion and char combustion). The conversion of biomass is described using a bed model approach which is considered to be an arrangement of three layers: drying layer, pyrolysis layer and char layer. The fuel release conditions are provided by these layers via a set of mixture fractions. Standard k-e model is employed for turbulence modelling. Turbulence – chemistry interactions are described by a pre-assumed PDF (Probability Density Function) – mixture fraction method. Radiation is accounted for by the DOM model (Discrete Ordinates Model). The model is developed in the framework of Code_Saturne (open source software). It is applied to analyze the design of a commercial wood log stove with a nominal thermal capacity of 10 kWt. Experiments have been also performed to provide data for model input and validation. The simulation results revealed a considerable potential for stove geometry optimization and emissions reduction. The CFD results can be taken as a guideline for an improved design with regards to emissions and efficiency.

Applying gasification on the residue generated by the biogas process would increase flexibility and efficiency of waste treatment plants while leading to a simultaneous production of biogas and syngas. Existing electricity conversion systems at biogas and gasification plants are usually designed to operate with a single bio-fuel (biogas/syngas). It is, hence, necessary to identify the combustion characteristics of these blended fuels in order to implement them efficiently in the existing systems. The aim of this work is to characterize the structure and emission of mixtures of biogas and syngas in counter-flow flame configuration at laminar regime. Particular attention is paid to the chemical effects of CO2 contained in the biogas and radiative heat losses of the flame. Several compositions of biogas and syngas have been considered in the fuel mixture. The results show that the flame properties are very sensitive to fuel composition. An increase in the volume of methane in the biogas or hydrogen in the syngas improves the Low Heat Value (LHV) of the fuel. Moreover, the addition of a volume of syngas exceeding half of that of biogas allows a reduction of harmful emissions while maintaining a constant level of operating temperature. At the flame front, the NO species is found to be governed by thermal route whereas it is generally produced by prompt path before and after the flame front. In addition, the chemical effect of CO2 reduces the maxima of temperature and emission. This effect remains constant in the syngas mixtures while it is very sensitive to biogas composition where it decreases with the amount of CH4 in biogas.

This study addresses numerically the influence of several operating conditions on the structure and NO emissions of a biogas diffusion flame. The analysis is conducted at atmospheric pressure in counter-flow configuration and mixture fraction space. CO2 volume in biogas is varied from 25% to 60%, H2 enrichment from 0% to 20% and the scalar dissipation rate from near equilibrium to near extinction. Particular attention is paid to CO2 chemical effect. CO2 contained in biogas can have chemical effects when it participates in chemical reactions and thermal effects when it acts like a pure diluent. Chemical effects of CO2 are elucidated by using the inert species technique. Flame structure is characterized by solving flamelet equations with the consideration of radiation and detailed chemistry. It is observed that flame properties are very sensitive to biogas composition, hydrogen addition and scalar dissipation rate. CO2 increment decreases flame temperature, mass fraction of chain carrier radicals and NO emission index. Blending biogas with hydrogen increases the mixture heating value and makes the fuel more reactive. Hence, chain carrier radicals and NO index emission are all increased. The chemical effect of CO2 is found to be present overall scalar dissipation rate values where it reduces the maxima of temperature and OH mass fraction and increases the maxima of CO and NO mass fractions. H2 enrichment has a weak influence on CO2 chemical effect. Hydrogen-rich biogas flames produce less NO at high scalar dissipation rates.

This paper reports a numerical simulation of turbulent non-premixed counter-flow syngas flames structure and NO emissions at a high strain rate with a special focus on mixing. The analysis is conducted over a wide range of hydrogen percentage (H2/CO ratio between 0.4 and 2.0) and operating pressure (from 1 to 10 atm). Numerical model is based on RANS (Reynolds Averaged Navier–Stokes) technique including k-e turbulence model. SLFM (Steady laminar flamelet model) is used for flame structure calculations and EPFM (Eulerian Particle Flamelet Model) is applied for NOx predictions. Mixing is described by mixture fraction and its variance. Radiation effects are also considered. Computational results show an improvement of mixing with hydrogen enrichment and ambient pressure rise. Maximum flame temperature decreases with H2 addition and increases with pressure. NO levels decrease towards hydrogen-rich syngas flames and increase with pressure. Zeldovich route is found to be the main NO formation path in the operating conditions considered.

Biomass co-firing within the existing infrastructure of pulverized coal utility boilers is viewed as a practical near-term means of encouraging renewable energy while minimizing capital requirements and maintaining the high efficiency of pulverized coal boilers. Coal/biomass co-firing is a complex problem that involves gas and particle phases, along with the effect of turbulence on chemical reactions. Computation Fluid Dynamic (CFD) simulations can provide insight to design and operational issues, such as co-firing percentage, load swings, injection location, excess air and air/fuel staging, and predictions related to heat release, unburned carbon, and NOx emissions. The CFD based co-firing tools consist of models for turbulent flow, gas phase combustion, particles dispersion by turbulent flow, coal/biomass particles devolatilization, heterogeneous char reaction and radiation. Additional models related to slagging and fouling can also be found. This paper presents a review on CFD based modeling approaches used to predict the combustion characteristics of co-firing biomass with pulverized coal under air and oxy-fuel conditions.

The aim of the present study is to investigate the enhancement of syngas combustion using the promising oxygen-enrichment technology, with a particular attention on optimal operating conditions in regard to NOx emissions. For this purpose, a numerical study is conducted on a syngas counter-flow diffusion flame, using air enriched with oxygen as the oxidizer. The oxygen concentration ranges from 21% to 30% by volume. Two syngas compositions are considered with H2/CO rates equal to 0.25 and 4, respectively. Flame structure is characterized by solving flamelet equations with the consideration of radiation. The chemical reaction mechanism used is GRI 3.0. Computational results showed that oxygen addition increases the flame temperature and intensifies the radiative heat transfer. It also considerably extends flammability limits allowing stable flames at high values of the scalar dissipation rates and for lean syngas composition. NOx formation is substantially increased with oxygen increment, and Zeldovich mechanism is found to be the main route of NOx formation. H2-lean syngas flames produce less NOx at low scalar dissipation while H2-rich syngas flames NOx emissions are low at high scalar dissipation rates.

In the framework of a European project, a straw combustion boiler in conjunction with an organic Rankine cycle is developed. One objective of the project is the enhancement of the combustion chamber by numerical methods. A comprehensive simulation of the combustion chamber is prepared, which contains the necessary submodels for the thermo-chemical conversion of straw and for the homogeneous gas phase reactions. Part I introduces the modelling approach for the thermal decomposition of the biomass inside the fuel bed, whereas part II deals with the simulation of the gas phase reactions in the freeboard.