**2. Fluidized bed gasification**

#### **2.1 Biomass wastes conversion via fluidized bed gasification**

There are several Biomass waste conversion routes to valuable products or heat/ power production as summarized in **Figure 2**. The choice of technology depends on the desired end-use, the nature of biomass, resource availability, and other considerations like the techno-economic and environmental aspects [11]. The main technologies employed so far are categorized under thermochemical conversion, biochemical conversion, and extraction leading to a wide range of applications such as heat and electricity generation, bio-oil, hydrogen, and various synthetic chemicals production [8]. Thermochemical conversion is among the widely exploited routes given its versatility in accommodating a wide range of feedstocks, design options, and a wide range of final products and application domains.

A prevailing thermochemical conversion method is biomass gasification as portrayed in red arrows in **Figure 2**, which typically uses a fixed or fluidized bed reactor to convert biomass into gaseous fuels at low to moderate temperatures [12]. Due to its several advantages over fixed bed gasifiers, fluidized bed gasification has garnered increasing attention over the others, in the biomass to heat/power conversion. High mixing and reaction rates, accommodation of various biomass feedstock, and its potential for scaling are the main winning points [13]. The gasification technologies are summarized in the **Table 1** with their typical features, advantages, and limitations.

Gasification usually produces a gas mainly composed of CO and H2 with energy values between 5 and 20*MJ=Nm*<sup>3</sup> depending on the biomass characteristics, operating

#### **Figure 2.**

*Biomass conversion routes emphasizing gasification pathways with red arrows (adapted from [6–8]).*




#### **Table 1.**

*Summary of features of gasification systems.*

conditions, and gasifying agent used [2]. Integrated with a gas turbine, boiler, or steam turbine it can be a viable biomass-to-energy conversion component in heat, power, and combined heat and power systems. Syngas can also be an alternative fuel

#### *Opportunities and Challenges of Harnessing Biomass Wastes for Decentralized Heat… DOI: http://dx.doi.org/10.5772/intechopen.112533*

in modified internal combustion engines for electricity generation. Small and medium-scale Combined Heat and Power (CHP) systems also known as 'cogeneration' are used to convert biomass into electricity while extracting waste heat, a promising application for commercial buildings such as hospitals, schools, or office building blocks but can also be used for decentralized power generation in remote and rural areas [31]. For existing coal plants, burning solid biomass in traditional power plants alongside coal, in a process known as 'co-firing', is a cost-effective, more efficient, and clean option with only minor technical adjustments. Caputo et al. [11] calculated and compared the overall system efficiencies for ï¬'uidized bed combustor with steam turbine cycle and fluidized bed gasifier with combined gas-steam cycle in a black-box model using literature data. The system with the fluidized bed gasifier has an efficiency between 36 and 45% while the former has an efficiency between 25 and 28% for a power scale between 5 and 50*MW*.

#### **2.2 Fluidized bed performance indexes and important process parameters**

The knowledge of performance indexes of any energy conversion system is crucial to determine plant size, investment cost, techno-economic feasibility, and environmental impact. The overall plant performance is the sum of the upstream, downstream, and gasification in-bed performances considering the gasification reactor as the powerhouse of the plant. The upstream process may require biomass feedstock treatment and in the downstream, there are auxiliary components such as gas cleaning units, boilers, engines, and turbines, depending on the plant's application. The commonly used performance indexes in fluidized bed gasification are gas yield, the high heating value of gas, cold gas efficiency, carbon conversion efficiency, and thermal efficiency [31, 32]. All performance indexes are attributed to the in-bed gasification process, except the thermal efficiency is calculated for the whole plant.

*Gas yield*

$$\gamma = \left(\frac{\mathbf{Q}\_{\text{syngas}}}{Wb(\mathbf{1} - \mathbf{X}\_a)}\right) \tag{1}$$

where *Qsyngas* is the flow rate of syngas (producer gas) (*Nm*<sup>3</sup> *=h*Þ, *Wb* is the mass flow rate of biomass (*kg=h*), *Xa* is the ash content in the feed (on a dry basis). *High heating value*

$$HHV\left(\text{MJ}/\text{N}m^3\right) = \sum HHV\_i \mathbf{x}\_i \tag{2}$$

where *xi* (vol%) and *HHVi* (*MJ=Nm*<sup>3</sup> ) represent the volumetric percentage and the higher heating value of each component in the dry product gas (mainly *CO*, *H*2, and *CH*<sup>4</sup> in gasification).

*Cold gas efficiency*

$$
\eta\_{CG} = \frac{\mathbf{M}\_{syn} \times \text{LHV}\_{syn}}{\mathbf{M}\_{biomass} \times \text{LHV}\_{biomass}} \tag{3}
$$

where *M* denotes mass.

$$LHV\left(\text{MJ}/\text{N}m^3\right) = \sum LHV\_i \mathfrak{x}\_i \tag{4}$$

where *xi* (vol%) and *LHVi* (*MJ=Nm*<sup>3</sup> ) represent the volumetric percentage and the higher heating value of each component in the dry product gas (mainly *CO*, *H*2, and *CH*<sup>4</sup> in gasification).

*Carbon conversion efficiency:*

$$
\eta\_C = \frac{C\_{\text{syngas}}}{C\_{\text{bio}}} \times 100\% \tag{5}
$$

where *Csyngas* is the total carbon amount in the syngas, (kg) and *Cbio* is the total carbon amount in solid fuel, (kg).

*Thermal efficiency*

$$\eta\_{th} = \frac{P\_{NET}}{M\_{biomass} \times LHV\_{biomass}} \tag{6}$$

where *PNET* is the net thermal power output of the plant, *Mbiomass* is the biomass feed rate and *LHVbiomass* is the lower heating value of the biomass.

The performance indexes of a gasification system are affected by a number of factors [33] such as biomass characteristics, temperature, pressure, residence time, catalytic effects (of catalytic bed material and ash), and type and ratios of gasifying agents used.

#### *2.2.1 Biomass characteristics*

Biomass residues are products of forestry, agricultural, municipal, and industrial waste and have significant variations in their physical, chemical, and morphological characteristics [34]. Biomass is characterized by its elemental composition (ultimate analysis), moisture content, fixed carbon content, ash content, heating value, density, porosity, and thermal conductivity. The heating value and composition of the product gas depend on the biomass type along with other process parameters. Also, the amount of tar, particulates (ash and elutriated char), and other impurities like heavy metals, are concerns of careful choice of gasification technology and in-bed process optimization and/or cost of downstream cleaning of gas. In the case of lignocellulosic biomass, the main representative building blocks are cellulose, hemicellulose, and lignin. Since these constituent species follow distinct kinetic pathways in the devolatilization step, their proportion in the biomass affects the product distribution. Therefore, the performance of a biomass-based conversion system is hugely dependent on biomass characteristics. Gonzalez et al. [32] studied the effect of biomass characteristics on different performance indexes using 10 different biomass residues. The results suggest a positive correlation between VM, C content, and *HHVbio* to the combustible gas concentration, calorific value, gas yield, and energy yield of the product gas, while the *H*<sup>2</sup> concentration is more favored with the H/O ratio of biomass. The reactivity of biomass is affected by the inorganic content present, which in turn affects the carbon conversion efficiency [34]. Also, alkaline is found in some lignocellulosic biomass, which reacts with bed material causing agglomeration [35]. In addition, the amount of ash in the biomass can impact the plant's operating cost due to stringent gas cleaning requirements in some applications [34].

*Opportunities and Challenges of Harnessing Biomass Wastes for Decentralized Heat… DOI: http://dx.doi.org/10.5772/intechopen.112533*

#### *2.2.2 Gasifying agent*

The choice of gasifying agent affects the performance and economic aspects of fluidized bed gasification. Syngas heating values typically range between 4 and 7, <sup>10</sup>–18, and 12 � <sup>28</sup>*MJ=Nm*<sup>3</sup> when air, steam, and oxygen are used as gasifying agents respectively [29]. In another study, a simple directly heated fluidized bed air gasification delivers syngas having low heating value (4 � <sup>6</sup>*MJ=Nm*<sup>3</sup> ) and high tar content (10 � <sup>40</sup>*g=Nm*<sup>3</sup> ) [13]. Alternatively, syngas with higher heating values (10 � <sup>40</sup>*g=Nm*<sup>3</sup> ) can be obtained when oxygen and steam are mixed as gasifying agents for a similar gasifier design [36]. It is also demonstrated that a medium heating value (10 � <sup>15</sup>*MJ=Nm*<sup>3</sup> ) gas can be produced in a dual bed gasifier using steam and air [37].

The high amount of nitrogen dilutes the gas resulting in a considerably lower heating value of yield gas when air is used as a gasifying agent. Steam gasification has the advantage of maximizing hydrogen production via water gas shift reaction (*H*2*O* þ *CO* ¼ >*CO*<sup>2</sup> þ *H*2). But the resulting yield gas has lower quality in other performance indexes. As per a report on steam gasification in a circulating fluidized bed there was a decrease in heating value, gas yield, carbon conversion and an increase in tar yield [29]. Oxygen can be an excellent gasification agent as it results in high gas yield, a high heating value of gas, and less tar yield due to high reaction rates. The limitation is the high cost of oxygen production and operation costs [12]. Oxygen is a by-product of green hydrogen production via electrolysis. As a result, the availability of such plants within a reasonable distance from gasification plants can create an affordable supply of oxygen given that there are no other competing interests such as medical use of oxygen.

#### *2.2.3 Equivalence ratio*

The equivalence ratio<sup>1</sup> is one of the most important operating parameters. Higher ER favors gas yield but decreases *CO*, *CH*4, and *H*<sup>2</sup> production and increases *CO*<sup>2</sup> production [29]. The higher the ER the higher the gas yield as it promotes oxidation and carbon conversion. The heating value of the gas is related to the amount of *CO*, *H*2, and *CH*<sup>4</sup> [31]. High ER results in lower heating value product gas because more *CO*<sup>2</sup> and *N*<sup>2</sup> dilute the combustible gas species. Previous studies demonstrate that lower ER is desirable but too low ER means reduced temperature and the optimum value of ER, usually between 0.2 and 0.4 [38] needs to be maintained. If tar is a concern in the application of the producer gas, a higher ER is desirable (0.3–0.4 [38]) so that the reaction operates at a higher temperature, which favors tar cracking [33]. The response surface method which is a statistical technique that employs regression analysis based on mathematical relations is used to optimize values of input factors like temperature and equivalence ratio for optimal system performance [39].

#### *2.2.4 Steam to biomass ratio*

Steam is used as a gasifying agent for improved gas yield, LHV, and carbon conversion efficiency [33]. Water gas shift (*CO* þ *H*2*O* ! *H*<sup>2</sup> þ *CO*2) reaction is

<sup>1</sup> Equivalence ratio is defined as the ratio of actual air to fuel ratio to stoichiometric air to fuel ratio in this chapter's context

favored for the SBR range of 1.35–4.04, which increases the *H*<sup>2</sup> and *CO*<sup>2</sup> fraction in the producer gas [33]. Due to the increased yield of *H*2, steam gasification is very suitable for *H*<sup>2</sup> production. When the SBR increases more than the optimum, the reaction temperature reduces due to too much low-temperature steam [40].

#### *2.2.5 Temperature*

Bed temperature is one of the predominant parameters affecting the reactions in the gasification process [35]. The chemical kinetics of multi-phase and multi-step reactions in the gasification process is governed by the Arrhenius law of rate constant, which defines the temperature dependence of reactions. As a result, temperature plays the main role in deciding the output gas distribution and the gasifier performance. In a fluidized bed, the temperature remains almost constant due to the high thermal inertia of the bed material. The wide range of operating temperature in fluidized bed gasifiers is between 700 and 1000°C [41]. Pooya et al. [35], studied the effect of operating temperature between 650 to 1050°C, on the gasification of two different biomass in a BFB and the increasing temperature is in favor of gas yield, *HHVgas*, carbon conversion efficiency, and cold gas efficiency. A higher reaction temperature is in favor of high carbon conversion and tar cracking, which means lower tar and char in the producer gas [33]. However, the gasification temperature is limited by agglomeration and sintering of bed material and ash [13]. Therefore, applications, such as gas engines, turbines, fuel cells, and conversion of gas for the synthesis of fuels or chemicals, need extensive and costly gas cleaning [42] as the temperature is usually kept below 850°C in typical fluidized bed gasifiers.

#### *2.2.6 Bed material*

In fluidized bed gasification, the bed material is used as a mixing and heat transfer medium and desirably has high thermal inertia to maintain a fairly uniform temperature in the bed [43]. It has high thermal inertia enough to maintain a nearly constant temperature throughout the bed. The secondary effect of bed material could be acting as a catalyst in the case of catalytic beds, which influences the reaction rates of typical reactions involved in turn affecting the species distribution of yield gas. Some literature reported insignificant tar produced when the catalyst Rh/CeO2/SiO2 is used in low-temperature gasifications [29, 44]. Gallucci et al. [4] conducted a lab-scale assessment of four different catalytic bed materials (olivine, k-feldspar, kaolinite, and calcite) on their potential for emission reduction of heavy metals and pollutants from PABR (plant-assisted bio-remediation) plant biomass. Several papers reported that Dolomite is an effective catalyst for tar-cracking and enhances gas yield [45, 46].

#### **2.3 Biomass potential via gasification**

Biomass resource potentials are large enough to deliver about a quarter (i.e. 200 300*EJ*) of the world's future energy supply [3] during the century. The share of biomass in the global energy mix has grown from 4% in 2004 to 7.7% in 2013 (which is about 65% share among the renewables) [2], and 10% in 2018 (two-thirds in developing countries) [47]. The sum effect of factors like biomass availability, logistics, technologically feasible options, and policies is what decides the extent of effectiveness of the use of biomass as a competitive energy resource [48]. The primary step *Opportunities and Challenges of Harnessing Biomass Wastes for Decentralized Heat… DOI: http://dx.doi.org/10.5772/intechopen.112533*

for the efficient development of a biomass conversion system is the quantification and energy-potential characterization of the available biomass with respect to the desired application [49]. Jaswinder et al. [48], studied the biomass potential, challenges, technological options, and government policies towards promoting biomass use in decentralized power generation in the Indian context.

#### *2.3.1 Technical potential of biomass*

Estimation of the biomass potential that can be utilized for heat power and biomass-derived fuels is a very important step. The potential of biomass is estimated from the annual main production (*Pi*) and the product-to-residue ratio (*Rj*) when agro-industrial wastes are considered for utilization [49]. The subscript *i* denotes the main biomass while the subscript *j* denotes the residue from its main biomass source more than a single residue can be produced from the main biomass source. The moisture content (*MCj*) and low heating values (*LHVj*) of the biomass define the theoretical energy potential. Availability factor *A* should be taken into account in order to estimate the technical energy potential of a biomass (*QTij*). In a black-box approach, the gasification system potential can be calculated considering the yearly operational hours *H*, gasification reactor efficiency (*ηgasific*), and the generator efficiency (*ηgen*). The heating values and other important characteristics of biomass residues used in thermochemical conversion are summarized in **Table 2**.

Biomass potential from annual main biomass production: *Bij* ¼ *Pi:Rj*. Theoretical energy potential of each biomass: *Qij* ¼ *Bij:* 1 � *MCj* � �*:LHVj*. The technical energy potential of each biomass: *QTij* ¼ *Qij:A*. Gasification potential of each biomass: *QGij* ¼ *QTij:H:ηgasific:ηgen*. Regional energy potential: *<sup>Q</sup>* <sup>¼</sup> <sup>P</sup>*Qij*, *QT* <sup>¼</sup> <sup>P</sup>*QTij*, *QG* <sup>¼</sup> <sup>P</sup>*QGij*.

Hiloidhari et al. [63] used this methodology to estimate India's biomass potential but not limited to gasification only. 686 MT [64] of biomass production with 34% surplus, and can roughly produce 23 GW [29] of power equivalent to the 17% of India's total primary energy demand. The case of India's biomass potential is reported in various papers.

Following the same methodology a 2017 paper [48] estimated India's biomass potential to be 30*GW* of electricity from all the surplus crops considering availability between 28% and 48% from the surplus residue of 223*MT*, rice husk, bagasse, and sawdust being the most abundant ones. The annual operation hours are considered as 6570*h=year* whereas the average lower calorific values, lowest thermal efficiency, and average energy requirements for selected feedstocks (Rice, Wheat, Coarse cereals, Total Cereal, Cotton, Sugarcane) are considered. The installed capacity of bio-energy in India as of 2017 was only 5*GW*, which is about 17% of its calculated potential.

## **3. Opportunities**

#### **3.1 GHG emission reduction and climate mitigation**

According to the International Energy Agency, by 2019 fossil fuels were responsible for 32.1 gigatonnes of *CO*<sup>2</sup> emissions [65]. Additionally, it predicted that by 2030,


#### **Table 2.**

*Some of the Biomass residues used in thermochemical conversion pathways.*

### *Opportunities and Challenges of Harnessing Biomass Wastes for Decentralized Heat… DOI: http://dx.doi.org/10.5772/intechopen.112533*

emissions would be around 42 gigatonnes with the current global trend unless climate agreements are implemented with stringent measures [65]. It is critical to abide by the 2015 Paris Agreement that states global temperature rise should not exceed above 2°C from the pre-industrial levels in order to combat the potentially catastrophic global climate change threat [66]. In accordance with this aim, the EU under the 2030 climate and energy framework plans GHG emissions reduction by 40% through increasing the share of renewable energy by 27% [67]. Resource and environmental sustainability measures primarily involve renewable energy production [6].

Burning of biomass is regarded as carbon neutral (also written in EU legislation [2]) because carbon is re-absorbed during the growth of biomass through photosynthesis where CO2 and H2O are converted to glucose and O2 as depicted in **Figure 3**. A briefing on EPRS(European Parliamentary Research Service) discussed the opportunities and challenges of biomass use for electricity and heat with regard to GHG emission, resource availability, environment, and human health [2]. It outlined that biomass thermochemical conversion for heat and power generation has as high as up to 70% savings in GHG emission in some cases, but is influenced by factors like feedstock type, transportation, and conversion efficiency. In order to compute CO2 savings and effective emission, the IPPC European Directive put CO2 emission factors of 0*:*43*kg=KWhCO*<sup>2</sup> for European energy mix for electricity generation (40% coal, 30% gas, 30% non-fossil) and 0*:*23*kg=KWhCO*<sup>2</sup> for European fuel mix for thermal applications (50% gas, 40% oil, 10% coal) [68]. **Table 3** shows the commitment of some exemplary countries, EU, and worldwide in terms of the total installed bio-fuel capacity.

Biomass has been utilized for millennia and simply utilizing it does not guarantee sound benefits over other alternatives. In fact, the traditional use of biomass is very

**Figure 3.** *Carbon cycle via photosynthesis.*


**Table 3.**

*Bio-fuel installed capacities of countries, EU and global.*

inefficient and can cause adverse effects on air quality and human health [2]. According to a 2017 World Energy Outlook special report [72], a third of the world's population (2.5 billion people) still traditionally serves with solid biomass, which is estimated to cause 2.8 million premature deaths annually due to indoor air pollution. Clean and efficient utilization of biomass with proven technologies like fluidized bed gasification is a promising technological solution for achieving GHG emission reduction and climate mitigation.

Biomass residues constitute municipal waste significantly, especially in rural and developing countries. The global municipal waste production in 2016 is 2.01 billion tonnes which is projected to grow to 2.2 billion in 2025 and 3.4 billion in 2050, according to World Bank statistics [73]. Out of which the trash-derived biomass accounts for 44% [26].

Direct and inefficient burning of biomass for heat and the production of charcoal is a common bio-energy utilization trend in developing countries. Wood, straws, cow dung, and other biomass residues are used for cooking, space heating, and lighting which accounts for about 30*:*7*EJ* and another 20 to 40% for informal sectors including charcoal production [74].

Potential deployment levels of bioenergy by 2050 could be in the range of 100 to 300*EJ* [3]. In sub-Saharan Africa alone, biomass has an outstanding potential of about 15,000*MW* from just 30% of residues from agricultural crops and forest logging residues [75]. However, there are large uncertainties in this potential such as market and policy conditions, and strong dependence on the rate of improvement in the agricultural sector too [3].

#### **3.2 Distributed heat and power generation from local resource**

Biomass conversion via fluidized bed gasification is a very good candidate for distributed power and heat generation. Biomass conversion is way more cost-effective when it is done locally than transported a long distance. In 2015, 72% of energy
