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Joshua Turner
Joshua Turner

Handbook Biomass Gasification Knoef Pdf Download

Biomass gasification is an effective way to convert solid biomass into useful biofuels. Gasification is a theoretically complicated, often incompletely understood thermochemical process in which biomass materials experience incomplete combustion in a medium such as air, oxygen, or steam to produce a combustible gas called producer gas or synthesis gas. This gas is a mixture of hydrogen, carbon monoxide, carbon dioxide, water, nitrogen, and small amounts of methane and higher hydrocarbons (Lucas et al. 2004). The producer gas can be burned directly in furnaces, boilers, stoves, internal combustion engines, or micro-turbines for heat or power generation (Knoef 2005). It can also be further converted into a wide variety of useful, high-value petrochemicals or transportation fuels such as synthetic diesel (via the Fischer-Tropsch method), ethanol (via fermentation), and dimethyl ether and methanol (via catalytic reactions) (Hasan et al. 2010).

Handbook Biomass Gasification Knoef Pdf Download

The objective of this study was to better understand the effects of biomass type and air flow rate on the gasification performance of an updraft biomass gasifier. Sorghum stover, prairie hay, and woodchips were studied because of their local availability and potential as energy feedstocks. Three levels of airflow were tested (air-fuel equivalence ratios of 0.21, 0.25, and 0.29). The air-fuel equivalence ratio was calculated by dividing the mass of air used to gasify the biomass by the mass of air required to completely burn the biomass. Gasification performance was evaluated based on the producer gas composition, higher heating value (HHV), and tar content.

Using biomass residues from industrial processes and grasses can increase the overall efficiency of biomass gasification (Milbrandt 2005). In this study, three feedstocks were utilized to test the effect of the biomass type on gasification performance, and each experiment was performed three times. Prairie hay is a grass with a number of advantages, including its wild growth and the fact that it does not need to be fertilized or irrigated. In the same way, sorghum stover, a byproduct from agricultural crops, has potential for biofuel production. Prairie hay and sorghum biomass collected from a local farm were ground using a tub grinder (Model H-100, Haybuster Big Bite, Jamestown, ND). Furthermore, wood chips from a local transfer station were used. The wood chips selected were byproducts from construction and gardening applications. Hemicellulose and cellulose analyses of the biomass were performed in an ANKOM 2000 Fiber analyzer (Macedon, NY). The acid detergent lignin method was used to determine the lignin content (ANKOM, method 8). The ash content was determined as the residue remaining after combustion at 450 C overnight. An adiabatic bomb calorimeter (IKA-Calorimeter C 200, IKA-Werke GmbH and Co. KG, Staufen, Germany) was used to determine the higher heating values of the biomass feedstocks.

Three levels of air-fuel equivalence ratios (ER) were evaluated: 0.21, 0.25, and 0.29 with three replicates for each ER. At equivalence ratios close to 0.25, the producer gas from biomass gasification was found to have the highest energy potential (Knoef 2005). Several other studies have also found optimal gasification performance in this range (Lv et al. 2004; Sheth and Babu 2009; Ummadisingu et al. 2010). Equation 1 (Basu 2010) was used to calculate the mass of air needed for the complete combustion of the biomass. The mass of air required for gasification was calculated using the air flow and the reaction time of the gasification experiments. The air-fuel equivalence ratio was calculated using Eq. 2 (Basu 2010):

In each experiment, the gasifier was loaded with one type of biomass for a single batch reaction (e.g., 30 pounds (14 kg) of prairie hay or sorghum or 40 pounds (18 kg) of wood chips). All experiments were carried out for at least 60 min of stable gasification. The producer gas contents of hydrogen, carbon monoxide, and methane were used to calculate the heating value of the produced gas using Eq. 3 (Bejan 2006).

The results of prairie hay gasification at 0.21, 0.25, and 0.29 equivalence ratios are presented in Fig. 2. Prairie hay at an ER of 0.29 had the highest tar content, 3.1 g/m3. Increasing the ER increased the formation of tar species. Several researchers (Kinoshita et al. 1994; Chen et al. 2008) have reported that variation in the air available for gasification can affect tar yield during biomass gasification, increasing tar generation as the reaction air supplied increased. It is important to highlight the fact that prairie hay had a tar content of 1.67 g/m3 at an ER of 0.21, comparable to tar levels produced in downdraft gasifiers (Milne et al. 1998). The combustion zone temperature of prairie hay gasification decreased with increasing ER (Fig. 2B). The highest temperature (736 C) corresponded to the lowest tar content (ER of 0.21). Comparing Figs. 2(A) and 2(B), it can be seen that there was a negative correlation between tar content and combustion zone temperature. This study is in agreement with an earlier work (Chen et al. 2008), reporting that increases in the combustion zone temperature could increase the producer gas yield but decrease the formation of tar species.

The gasification of sorghum biomass presented comparable results to prairie hay gasification. The lowest tar content, 2.2 g/m3, was achieved at an ER of 0.21, and the highest tar content, 3.0 g/m3, was observed at an ER of 0.29, as shown in Fig. 3(A). In contrast with prairie hay gasification, the combustion zone temperature of sorghum stover was maximized at the moderate ER of 0.25 instead of at 0.21 ER as in prairie hay gasification, as shown in Fig. 3(B).

As presented in Fig. 4A, the producer gas from wood chip also exhibited increases in tar content when the ER increased, similar to the other two biomass types. Other researchers also found similar results. Increasing the equivalence ratio in biomass gasification had a negative effect on the tar content because of an increase in the formation of tar species (Kinoshita et al. 1994; Houben 2004). The combustion temperature decreased when the ER increased from 0.21 to 0.25, and then it started to increase as the ER further increased. A similar phenomenon was observed by Sheth and Babu (2009), who believed that the initial reduction in combustion temperature could be attributed to the increase of inert nitrogen as a heat carrier in the combustion zone. It is important to note that the combustion temperature of wood chip gasification was the highest at ER 0.29, at which the tar content was also the highest. This trend was totally different from those of prairie hay and sorghum stover, which may be related to the significant differences in the bulk density of the biomass. Wood chips had significantly higher bulk density (roughly 40 lb (18 kg) per load) than prairie hay and sorghum stover (approximately 30 lb (14 kg) per load). Such a difference could cause differences in the airflow through the gasifier chamber, altering gasification.

Carbon monoxide and hydrogen are the main sources of the heating power of the producer gas. These gases are the products of a large number of thermochemical reactions involving simple and complex molecules. Oxidation and reduction are some of the reactions taking place during the gasification process. Each is well-represented by several single reactions (Knoef 2005). As shown in Fig. 6, the hydrogen content did not exhibit significant differences when the averages of all biomass types were compared. However, the carbon monoxide content of the producer gas from wood chips was found to be the highest (21.3%) among the three biomass types, followed by prairie hay (16.7%) and sorghum biomass (14.4%). This could be related to the carbon content of the biomass, which appeared linearly related to the carbon monoxide composition. The carbon monoxide content in the producer gas increased on the same order as the carbon content of the biomass types (Table 1). The R2 value of the linear correlation between the CO content and the C content of the biomass was 0.92, indicating that the biomass carbon content significantly affected CO formation during the gasification process.

There are many bioenergy routes which can be used to convert raw biomass feedstock into a final energy product. Several conversion technologies have been developed that are adapted to the different physical nature and chemical composition of the feedstock and to the energy required (heat, power, and transport fuel) [28]. Recent years have therefore seen considerable effort devoted to the search for the best ways to use these potentially valuable sources of energy. Considering the methods for extracting the energy, they can be ordered by the complexity of the process involved as follows:(1)direct combustion of biomass;(2)thermochemical processing to upgrade the biofuel: processes in this category include pyrolysis, gasification, and liquefaction;(3)biological processing: natural processes such as anaerobic digestion and fermentation, encouraged by the provision of suitable conditions, again lead to a useful gaseous or liquid fuel.

During a gasification process, biomass is directly converted to synthesis gas (syngas) in a gasifier under a controlled amount of air. Syngas can be used in internal combustion (IC) engine to produce power or in a cogeneration system to produce heat and electricity. Previously, Kapur et al. calculated the unit cost of electricity of using rice husk gasifier based power generation system and evaluated its financial feasibility with utility supplier and diesel generated electricity [31]. Abe et al. [32] discussed the potential of rural electricity generation via biomass gasification system. The results suggest that even though agricultural residues such as rice husks may contain high energy potential, supplying a biomass gasification system in the long term may require tree farming in order to provide sufficient amount of resources [32]. These researches imply that the feasibility of these large-scale projects is greatly dependent on the plant location that affects the resource availability and the incurred logistic costs of the selected biomass feedstock. On an industrial scale, biomass gasification and power generation systems have been well-established.


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