**3. Gasification of composite biomass feedstocks**

In this part, gasification of composite biomass feedstock is investigated. The blending of biofuels is one of the solutions for the suppliers of conventional fuel to alleviate the greenhouse gas intensity originated from the fossil fuels. As an example, a composite biomass composed of paper fiber and plastic waste originating from a municipal waste stream is tested to investigate the role of adding dissimilar agents on the productivity of the gasification technology. The study also elaborates the failure scenario of "Clinkering" and investigates the thermo-chemical properties of the generated by-products through gasification of composite fiber and plastic.

heating value of plastic waste is notably higher (over double) than the one for paper waste which also adds value to the composite pellet. Therefore, the lower ash content and higher heating value of plastic waste show the promising potential of making their composite pellets for producing biofuels [26]. The lower heating values of plastic waste and paper waste were calculated at 36.91 and 15.38 MJ kg−1. It should be noted that gasifying the plastic is not practical as single biomass despite its high heating value. Hence, it is inevitable to blend the plastic

**Testing item (%) Plastic waste Paper waste Composite biomass\***

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Volatile matter 94.19 85.78 86.62 Fixed carbon content <0.01 3.45 3.11 Ash content 1.03 4.5 4.15

Hydrogen 13.97 5.92 6.73 Nitrogen 0.29 0.18 0.19 Sulfur 0.19 0.19 0.19 Oxygen 2.93 38.18 34.66 Higher heating value (MJ kg−1) 40.01 17.09 19.38

Proximate analysis Moisture content 4.78 6.27 6.12

Ultimate analysis Carbon 77 44.95 48.16

**Table 2.** Proximate and ultimate analysis of composite pelletized paper and plastic waste [25].

The excess of plastic content in the composition of composite matrix results in production of contamination in different forms. In the case of composite pellet, the plastic content was mixed in the ratio of 10% and resulted in producing metallic chunks of semi-burnt pellet mixed with some other elements called as clinker. The clinkering or so called "agglomeration" happens after ash is generated within the reactor when ash sintering begins at higher temperatures. The clinker forms initially in combustion zone where the highest temperature resides. By increasing clinker formation, some move downward with the help of biomass flow and deposit on top of the bed material. Therefore, the developed clinker is less porous compared to the original one formed in combustion zone. This can be due to longer distance in which they travel resulting in increased retention time in a high temperature zone which would lead to more viscous slag, along with lower heat transfer when entering the reduction zone. At the highest plastic levels tested (10%) one big ball of aluminum slag with an average of around 912 g was generated from a 15 kg start sample. The clinkers are shown in **Figure 7** [25]. One possible source of aluminum component can be from the coated plastic, however, other small contaminants during the experiment and from the fibrous feedstock might be another reason for observing contamination in the clinker. The detailed analysis of the clinker

waste with other biomass to reduce its potential emissions.

\*

Ratio 1:10 for plastic to paper.

is explained in the next two paragraphs.

To date, experimental indices based on feedstock composition have been used to predict ash deposition and slagging potential [22]. Ash deposits or agglomerates are a major problem in the continuous operation of a thermo-chemical conversion reactor such as a gasification and combustion system [23]. There are several major factors in the bed representative agglomeration phenomenon such as particle size, feeding mode, reaction environment (oxidation/ reduction), temperature, fluidization velocity, and contents of alkaline earth and alkali mineral [23]. Although downdraft gasifiers are known to have their limitations such as a feed size requirement, low ash content, decreased scale-up potential and increased risk for bridging and clinkering, this technology normally produces less tars and is less complex which could be applied for smaller scale systems [24].

The composition of the current experimental biomass includes a blend of the fibrous and plastic portions of post-consumer solid waste in Montreal, Quebec, Canada. The majority of the feedstock (90–100%) is comprised of fiber, which includes newspaper, cardboard, office paper, flyers, etc. Plastics are included as the remainder and consists of a blend of mixed polymers that include HDPE, LDPE, PET, trace PVC, etc. The plastics portion combined with fiber does not account for the level of plastic contamination that already exists in recycled municipal solid waste fiber. This portion may be as high as 2–5 wt%.

The proximate and ultimate analysis to understand the elemental matters in the individual and composite biomass feedstock is listed in **Table 2**.

As shown in **Table 2**, the ash content of plastic is much less than paper fiber waste which causes a lower ash content in the matrix of the composite pellets. Furthermore, the higher


**Table 2.** Proximate and ultimate analysis of composite pelletized paper and plastic waste [25].

**a.** By increasing the elevation across the reactor (from top to bottom), the temperature initially rises particularly in combustion zone where the reduction bell and air nozzles are

**b.** Over the time, and by controlling the amount of air getting into the reactor, oxygen-starved environment forms in higher elevation where the air enters the reactor. The immediate im-

**c.** However, the value of carbon monoxide drops gradually as the formed gaseous products react with the generated steam and/or moisture from the biomass and produce higher

In this part, gasification of composite biomass feedstock is investigated. The blending of biofuels is one of the solutions for the suppliers of conventional fuel to alleviate the greenhouse gas intensity originated from the fossil fuels. As an example, a composite biomass composed of paper fiber and plastic waste originating from a municipal waste stream is tested to investigate the role of adding dissimilar agents on the productivity of the gasification technology. The study also elaborates the failure scenario of "Clinkering" and investigates the thermo-chemical properties of the generated by-products through gasification of composite fiber and plastic.

To date, experimental indices based on feedstock composition have been used to predict ash deposition and slagging potential [22]. Ash deposits or agglomerates are a major problem in the continuous operation of a thermo-chemical conversion reactor such as a gasification and combustion system [23]. There are several major factors in the bed representative agglomeration phenomenon such as particle size, feeding mode, reaction environment (oxidation/ reduction), temperature, fluidization velocity, and contents of alkaline earth and alkali mineral [23]. Although downdraft gasifiers are known to have their limitations such as a feed size requirement, low ash content, decreased scale-up potential and increased risk for bridging and clinkering, this technology normally produces less tars and is less complex which could

The composition of the current experimental biomass includes a blend of the fibrous and plastic portions of post-consumer solid waste in Montreal, Quebec, Canada. The majority of the feedstock (90–100%) is comprised of fiber, which includes newspaper, cardboard, office paper, flyers, etc. Plastics are included as the remainder and consists of a blend of mixed polymers that include HDPE, LDPE, PET, trace PVC, etc. The plastics portion combined with fiber does not account for the level of plastic contamination that already exists in recycled

The proximate and ultimate analysis to understand the elemental matters in the individual

As shown in **Table 2**, the ash content of plastic is much less than paper fiber waste which causes a lower ash content in the matrix of the composite pellets. Furthermore, the higher

municipal solid waste fiber. This portion may be as high as 2–5 wt%.

and composite biomass feedstock is listed in **Table 2**.

positioned. At this step, a rich combustion process happens due to excess of air.

pact of reduced environment is an increment in the level of carbon monoxide.

amount of hydrogen and carbon dioxide.

88 Gasification for Low-grade Feedstock

be applied for smaller scale systems [24].

**3. Gasification of composite biomass feedstocks**

heating value of plastic waste is notably higher (over double) than the one for paper waste which also adds value to the composite pellet. Therefore, the lower ash content and higher heating value of plastic waste show the promising potential of making their composite pellets for producing biofuels [26]. The lower heating values of plastic waste and paper waste were calculated at 36.91 and 15.38 MJ kg−1. It should be noted that gasifying the plastic is not practical as single biomass despite its high heating value. Hence, it is inevitable to blend the plastic waste with other biomass to reduce its potential emissions.

The excess of plastic content in the composition of composite matrix results in production of contamination in different forms. In the case of composite pellet, the plastic content was mixed in the ratio of 10% and resulted in producing metallic chunks of semi-burnt pellet mixed with some other elements called as clinker. The clinkering or so called "agglomeration" happens after ash is generated within the reactor when ash sintering begins at higher temperatures. The clinker forms initially in combustion zone where the highest temperature resides. By increasing clinker formation, some move downward with the help of biomass flow and deposit on top of the bed material. Therefore, the developed clinker is less porous compared to the original one formed in combustion zone. This can be due to longer distance in which they travel resulting in increased retention time in a high temperature zone which would lead to more viscous slag, along with lower heat transfer when entering the reduction zone. At the highest plastic levels tested (10%) one big ball of aluminum slag with an average of around 912 g was generated from a 15 kg start sample. The clinkers are shown in **Figure 7** [25]. One possible source of aluminum component can be from the coated plastic, however, other small contaminants during the experiment and from the fibrous feedstock might be another reason for observing contamination in the clinker. The detailed analysis of the clinker is explained in the next two paragraphs.

The organic content and minerals of the biomass feedstock have different level of persistency in leaving the matrix of the composite pellets at different temperatures. Organic content of biomass has fairly lower resistance against high temperature gasification and is fully consumed in the combustion region of the reactor where highest temperature takes place. At higher temperature, the mineral content of the biomass transforms into ash and prepares the ash sintering process at which liquid and viscous slag is generated. The formation of slag is due to the interactive reaction between the melted ash and the mineral matter content of the ash matrix. This process continues until the generated molten ash accumulates and makes the chunk of clinkers which deposits in the bottom part of the reactor. It is also reported that liquid slag flows under the force of gravity and out of the bottom of the gasifier into a water quenching system which is not the case for the current system [25].

by direct scission, 1,5-radical transfer scission, and multiple step-radical transfer scissions as

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In this chapter, the potential of bioenergy production from each of which was investigated. The failure scenario of "bridging" was observed in this stage. Next, multiple feedstocks were examined for seeking the possible improvement in the quantity and quality of the produced gas. This chapter also aimed to find how the increase in plastic specifically in the recycled fiber stream would affect the performance of a downdraft gasifier. The failure scenario of "clinkering" was observed in this stage which could be considered as an individual project to work on. The chapter also showed that a mixture of silicon with aluminum, calcium and sodium under high temperatures would result in the generation of a solid clinker that ultimately moves through the reactor and is deposited at the bottom of the reactor. It may be concluded that due to the presence of plastic, the generated ash is superheated and melts into glass-like materials causing formation of metallic chunk. The chunk is cooled down through partially endothermic nature of the gasification and results in generation of clinker. This chapter presented informative tools for improving advanced biofuel production through gasification technology and using different types of biomass feedstock which can be continued

This study also focused on the development of gasification technology to enhance the efficiency of biomass conversion within the process. In a nutshell, the following recommenda-

**1.** Although this study worked on gasification of different types of biomass feedstock and identified the potential failure scenarios, it is still essential to elaborate the post-gasification process for syngas conditioning to produce enriched gas. This will have a significant con-

**2.** Developing/re-designing the down-draft gasification unit is recommended to examine the possibility of process optimization. The new design might apply a coupled reactor in which one produces the syngas and the other one works as a downstream unit to condition

**3.** Designing a burner is recommended to enhance the efficiency of the process where the syngas comes out. A good burning process helps to preserve the syngas produced in the

**4.** Developing a feeding system which is independent of the physical properties of feedstock is strongly recommended. This will help to reduce the cost related to supply chain.

**5.** A detailed investigation of tar and char modeling through different types of reactor configurations could help to understand the formation process and minimize the detrimental

reactor and boost the performance of the technology.

tribution to approaching integrated gasification combined cycle (IGCC) concept.

temperature climbs over 800°C [27].

**4. Conclusion**

in further researches.

the produced gas.

effects of by-products.

tions are offered for future research:

The inductively coupled plasma (ICP) mass spectrometry analysis of individual and composite paper and plastic waste indicates presence of six major elements in the matrix of the composite pellet including sodium (Na), magnesium (Mg), aluminum (Al), potassium (K), calcium (Ca) and silicon (Si). The majority of the contaminants (i.e. Na, Mg, Al, K and Ca) originates from paper fiber existed in the municipal waste stream, and silicon is derived from plastic wastes. It is reported that these elements could be detected in typical paper waste such as brown toilet paper, cardboard, industrial paper towel, magazine paper and printer papers. The use of these elements is for the purpose of grading and sizing of the papers [25, 27]. Characteristics of the clinker are also a function of degradation mechanism depending on the composition of volatiles and consequently the gaseous phased products are affected. The thermal degradation of polymers and plastics typically begins with random scission followed

**Figure 7.** Clinkering during gasification of composite pellet of fiber and plastic (unit: cm) [25].

by direct scission, 1,5-radical transfer scission, and multiple step-radical transfer scissions as temperature climbs over 800°C [27].
