A Comprehensive Literature Review of Thermochemical Conversion of Biomass for Syngas Production and Associated Challenges

The interest in the thermochemical conversion of biomass for producer gas production since last decade has increased because of the growing attention to the application of sustainable energy resources. Application of biomass resources is a valid alternative to fossil fuels as it is a renewable energy source. The valuable gaseous product obtained through thermochemical conversion of organic material is syngas, whereas the solid product obtained is char. This review deals with the state of the art of biomass gasification technologies and the quality of syngas gathered through the application of different gasifiers along with the effect of different operating parameters on the quality of producer gas. Main steps in gasification process including drying, oxidation, pyrolysis and reduction effects on syngas production and quality are presented in this review. An overview of various types of gasifiers used in lignocellulosic biomass gasification processes, fixed bed and fluidized bed and entrained flow gasifiers are discussed. The effects of various process parameters such as particle size, steam and biomass ratio, equivalence ratio, effects of temperature, pressure and gasifying agents are discussed. Depending on the priorities of several researchers, the optimum value of different anticipated productivities in the gasification process comprising better quality syngas production improved lower heating value, higher syngas production, improved cold gas efficiency, carbon conversion efficiency, production of char and tar have been reviewed.


INTRODUCTION
no environmental problems [1]. Various initiatives have been taken and multiple targets are set to meet stringent emission requirements like roadmap 2050 [2], the framework for climate change 2030 [3], particularly 20-20-20 targets as promoting up to 20% share of renewable energy in EU (European Union) countries, by reducing 20% GHG (Greenhouse Gas) emissions into environment and 20% rise in energy efficiency in 2020 [4]. Alternative energy resources include wind, solar, tidal geothermal and biomass etc. are renewable and environmental friendly as compared to the fossil fuels.
Among numerous alternative energy sources, the application of biomass as an alternative resource bring about many social, economic and environmental improvements. The management of biomass inappropriate manner reduces net carbon dioxide emissions almost zero and offers economic wellbeing of rural and semi-urban areas [1]. Biomass resources are extensive and copiously available in the world.
However, one-third of total energy is obtained from biomass in developing countries of Asia and Africa. As  [5]. The amount of char produced strongly rests on gasification method and operational parameters.
The lower heating value of char is in between 25-30MJ/ kg [6]. The heat required to carry on the reaction is generally provided at the oxidation stage through allothermal or auto-thermal method. In the auto-thermal method of gasification, takes place through the internal heating of gasifier with partial combustion and in allothermal gasification process energy necessary to carry on the reaction is supplied externally. Thus the syngas produced has wide application for the synthesis of various chemicals including methanol, MTBE (Methyl Turt Butyl Ether) and DME (Dimethyl Ether). Syngas can be converted into liquid transportation fuel through Fischer-Tropsch synthesis methods depending on different H 2 /CO ratios [7]. Further, in Integrated Gasification Combined Cycle both heat and power could be achieved. However, fossil fuels consumption produces a large amount of greenhouse gases [8], it is important to reduce the environmental impact caused by the non-renewable sources. As from numerous renewable energy means biomass is utmost significant environmental friendly resource widely available around the world for the generation of syngas and electricity [9]. Moreover, raw biomass when compared with coal has low utilization efficiency because raw biomass possesses ahigh quantity of moisture, high hydrogen to carbon ratio as well as high oxygen to carbon ratio.
The main stages in gasification are, drying (endothermic stage, heat is absorbed), oxidation (an exothermic stage, heat is evolved), pyrolysis, and reduction both are endothermic stages in gasification. The additional steps may be tar decomposition, in order to consider the creation of light hydrocarbons because of the disintegration of larger tar molecules [10]. Main steps in gasification processes are shown in Fig. 1 and are discussed in subsequent sections below. However, the overall energy structure consumes enormous quantities of natural resources and the most of the energy produced is derived from fossil fuels, which possesses serious environmental and health hazards. The more production of greenhouse gases results in severe global warming, which damages the ozone layer. Due to the high demand of energy, the number of fossil fuel power plants are installed that results in more production of carbon dioxide during fuel combustion, which is increased from 0-22x10 12 kg/ year in between 1890-2009.
Considering fossil fuels limitations, and the environmental glitches related to their application in the world is so far from attaining a sustainable energy future. Instead, we are strongly dependent on the nonrenewable energy sources [11].
The conversion of biomass into energy through different conversion routes is shown in Fig. 2. In this study,

Oxidation
In gasification process, the oxidation of biomass is performed to attain and sustain the thermal energy required to carry out endothermic processes at the desired level. In oxidation stage, restricted supply of oxygen is maintained in order to attain the stoichiometric ratio to oxidize the only portion of the material. Regardless of fractional oxidation involve entire carbonaceous types including tars; it is probably to make sure that only chars and hydrogen present in syngas take part in the partial oxidation process. During oxidation processes the main reactions take place are listed as follow: The thermal energy obtained in this step is required to maintain the whole process, whereas the combustion products are CO, CO 2 and water. When feedstock oxidation process is executed through the air the gas mixture may contain nitrogen, if only oxygen is used the nitrogen presence is practically absent in gas mixture.

Moisture Content and Drying of Biomass
When biomass feedstock used for gasification contains cellulosic material nearly 50% of the weight, in such type of reactions the feed material is specified using the formula of cellulose C

Reduction
In   Depending on the nature and type of reaction air blown gasifier comprises on four segments including drying, pyrolysis, combustion and reduction segments.

Updraft Gasifiers
Updraft gasifier is the oldest and simplest gasifier used In counter-currentgasification, syngas is collected from the low-temperature zone located at top of the reactor resulting in the substantial amount of tar production as presented in Table 1. Countercurrent gasifier can accept higher moisture containing biomass up to 60% [23]. The major problem associated with updraft gasifier is high tar content. If the gas produced is to be introduced to internal  It is important to maintain the temperature of oxidation zone greater than 1000 o C and the equal spreading of biomass and the gasifying agent is essential for smooth operation. The clean gas gathered from the gasifier is most appropriate for gas engines and gas turbines, since the gas taken off from the downdraft gasifier possess relatively high temperature, needs cooling before downstream application [29].  , CH 4 were 23, 13, 11, and 4% respectively with HHV of 5MJ N/m 3 , and during the gasification of hazelnut shells the carbon dioxide concentration was observed slightly low and the heat content of syngas enriched to 6.250MJ N/m 3 [30]. With the application of two-stage downdraft gasifier heating value was observed at 6.50 MJ N/m 3 having tar content of 0.0450gN/m 3 , whereas total combustible gases were more than 45% (

Entrained Flow Gasifiers
In flow gasifier is of fine quality powder in order to achieve maximum conversion efficiency. Furthermore, the high temperature and pressure cause low tar formation during conversion process [33]. EFG offer constant temperature, higher heating rate and short residence time, currently, these are employed mainly in coal and liquid fuels, thus have a little experience with biomass as a feedstock [34].
Several efforts have been carried out to maintain the required size of biomass to comply with gasifier requirements, but it adds to theeconomic burden of overall operational cost making it commercially unattractive option. Fuel particle size affects the entire successive steps such as fuel heating, reactantsand syngas quality.
Pre-treatment of feedstock is necessary to lessen the bulk density and moisture level available in biomass for the healthier operation of the process. Nevertheless, steam gasification upturns the hydrogen content of product gas, thereby increasing the lower heating value [22] ( Fig. 7).

FIG. 7. THE ARRANGEMENT OF ENTRAINED FLOW GASIFIER.
Biomass Type

Circulating Fluidized Bed Gasifiers
In CFBs excessive rate of the fluid is maintained within the reactor in order to create a turbulent stream regime which entrains the bed particles and char in the gas stream.

MAIN FACTORS IN FLUIDIZED BEDS
For proper working of a biomass gasifier and the effect of different operating parameters on gasification performance requires a complete understanding of the process parameters. Lignocellulosic biomass possesses different properties such as morphological chemical and physical properties that may upset the overall process parameters, if not taken into consideration before the design and operation of the gasifier [40]. The selection of biomass for gasification significantly depends on its heating value. Biomass materials possessing greater heat content improve the economy and performance of the plant. As fluidized beds provide effective heat transfer rates and can be operated at varying varieties of feedstocks [41]. The results of proximate and ultimate analysis of several feedstocks are shown in Table 3. For better syngas quality and minimizing the char and tar content in outlet streams, numerous researchers have been performed on different feedstocks in order to understand the kinetic characteristics before its gasification to happen.

Effect of Temperature
Bed temperature during the operational process of the gasifier effects on the heating value and composition of the syngas produced. Based on Le Chatelier's principle, the variations in temperature effects on syngas composition rest on thermodynamic characteristics of the reaction. In endothermic reactions high-temperature increase syngas production, while in exothermic reactions high temperature favors reactants. The purpose of gasification is to produce a syngas enriched in CH 4, CO and H 2 having medium to a high heating value that is appropriate for turbines and internal combustion engines [59]. Increase in temperature increases combustion rate producing additional amounts of CO 2 and H 2 O production.
It has been investigated that high temperature enhances carbon conversion efficiency, produce less tar and char quantities. Rice husk gasification was observed at 700-800 o C, it was monitored that rise in temperature from 700-

Effects of Gasifying Agent
Numerous  [63]. In gasification systems using steam-oxygen as a gasifying agent, the heat required is directed from partial oxidation reactions. The syngas formed in steamoxygen gasification has a high H 2 content and dilution with nitrogen is not favorable whereas, the cost incurred on pure O 2 is high making the overall process unfavorable on an industrial scale. For distinct feedstock flow rate, two ratios are necessary to be controlled for the evaluation of the gasification plant. One is Equivalence ratio when air or oxygen gasification is used while for steam to biomass ratio when steam is used as a gasifying agent.

Effect of Equivalence Ratio
In biomass gasification one of the most important parameters taken into consideration throughout the operation of the plant is ER (Equivalence Ratio). It is the amount of air to biomass weight ratio divided by stoichiometric air to biomass weight ratio required for complete burning of biomass [64]. It is noticed that maximum combustion happens at high ER when the higher quantity of air is supplied into the reactor, it increased char combustion to generate CO 2, reducing the amount of combustible gases production such as H 2, CH 4, CO. Besides that increase in ER results decrease in the LHV of syngas as more ER encumbers the production of CH 4 and light hydrocarbons possessing fairly greater heating values. At high ER ratio nitrogen available in air further, dilute the syngas reducing its energy content. Various studies carried out on ER have revealed that too small ER is also disparaging and adversely affect the gasification process as it causes the decrease in reactor temperature [14]. Hence, the appropriate value for equivalence ratio is from 0.20-0.40. ER value changes depending upon the required operating parameters and subsequent application of syngas [65]. Combustion of raw syngas in the downstream heating system, tar is not considered as a thoughtful concern, only product gas may possess high  The H 2 content of the syngas was 14-30 volume percent the decrease in H 2 was noticed as the steam to biomass ratio was improved or the steam to oxygen ratio was steadily lessened. However, when the quantity of O 2 was increased more quantity of H 2 was combusted in the gasifier, the alike tendency was found for CO, by changing the ratios and its concentration in the syngas was from 30-50 volume percent. As the gasifying agent to biomass ratio increased to a value above 1, it decreased the char yield to about 10%.Using steam in gasification process produce more H 2. Efforts are made to increase the production of syngas with a high concentration of H 2 and at the meantime capturing of CO 2 to increase the process performance. Limestone (CAO) is commonly used as a bed material to capture CO 2 in steam gasification process [50].

Effect of Biomass Size
Biomass size significantly affects the gasification efficiency, the smaller particle size of biomass raises the overall efficiency of gasification unit, however, thesmaller size of biomass increases the operational amount of the unit. A plant having 5-10 MW generation capacity, nearly 10% of the amount of energy produced is necessary to reduce the size of biomass [67]. While larger particle size decreases the initial treatment rate of biomass and increases devolatilization time. It is necessary to maintain balance by examining the outcome of biomass particle size on the overall efficiency of the process. Ly et. al. [14] experimented about the influence of particle size on the quality of Through the installation of the smaller size of power plants in biomass zones will reduce the transportation cost of the raw material [68]. The capital cost of energy produced using biomass and coal is lower when compared with wind energy and considerably lower than the electricity generated using diesel as a fuel. Therefore, the best option among various energy conversion technologies is thermochemical conversion of feedstock for energy production.

CONCLUSION
Several efforts are taken to reduce the fossil fuels consumption and finding alternative energy resources that could meet the energy demand at the global level.
In recent times, significant consideration has been Besides that, smaller size of feedstock produces more C 2 H 4 , CO, CH 4 and less quantity of CO 2 in contrast to larger particle size. Whereas using steam in gasification process as a gasifying agent produce more H 2 . Nevertheless, further exploration is necessary to increase the gas quality meant for its marketable uses with high energy content. For better syngas quality FBG with steam may increase the producer gas production and quality.