COUPLED HEAT AND MASS TRANSFER ANALYSIS OF A NOVEL LAB-SCALE BATCH PYROLYSIS REACTOR FOR BIOCHAR PRODUCTION

 

Crop growth and yield have declined significantly due to soil fertility problems, there is evidence of inefficacious use of fertilizer most especially nitrogen loss from leaching. The continuous decline in soil fertility and the costs of fertilizer are major impediments to crop production in smallholder farms. Lack of ample amounts of nitrogen in most soils puts a setback on the farmers' goals of increasing crop yield per unit area. Naturally, biochar deposits this nitrogen to the soil. Biochar is a pyrogenic black carbon that has aroused increased interest in the academic arena (Verheijen et al., 2009; Yao et al., 2013). 

The International Biochar Initiative (IBI) (http://www.biochar-international.org/biochar; Mohan et al., 2014), states that, biochar is a black solid substance obtained from the combustion of biomass. Various heating rates produce different quality of biochar for use as fuel and adsorbents (Masto et al., 2013). Several studies have suggested that when biochar is applied to soils, it enhances soil fertility and crop productivity. It also, increases soil nutrients and water holding capacity, and reduced emissions of other greenhouse gases from soils (Cohen-Ofri et al., 2006; Verheijen et al., 2009; Yao et al., 2011), it also mitigates global warming. Biochar may be added to soil as a conditioner or carbon sink to reduce greenhouse CO2 emissions from decaying biomass (Mulabagal et al., 2020). However, its application to agricultural soil is considered a promising strategy to sustain fertility, while concurrently sequestering atmospheric CO2 (Woolf et al., 2010; Biederman and Harpole, 2013). It increases nutrient availability, microbial activity, soil organic matter, water retention, and crop yields, while mitigating greenhouse emissions, minimizing nutrient leaching, and soil erosion (Yeboah et al., 2009; Woolf et al., 2010; Mohan et al., 2006). All soils contain some amount of biochar deposited through natural occurrence, such as grassland and forest fires (Skjemstad et al., 2002). In fact, areas high in naturally occurring biochar, such as the west coast of the Mississippi river and east of the rocky-mountains in North American Prairie are some of the most fertile soils discovered in the world (O'Neill et al., 2009). In 1870, James Orton, an American geologist, observed that alongside the typically grey, acidic soils of the basin there are large fragments of 'black and very fertile' soil. A study confirmed that the half-life of biochar found in coastal temperate forest in Western Vancouver has been calculated to be 6623 years (Lehmann and Joseph, 2009) while biochar stocks found after Savannah in Zimbabwe had an average residence time of only a few decades. However, the global reason for biochar decline is due to climate change and urbanization which puts pressure on agricultural land resources. 

Although, alternative methods are adopted to produce biochar. Biochar can be produced as a high carbon content by-product, when biomass, such as plant wastes or animal waste undergoes combustion in a closed container with little or no air (Lehmann and Joseph, 2009). Traditionally biochar is known as charcoal when produced for fuel. However, the name biochar is used when the material is produced specifically for application to soil as a part of an agronomic or environmental management program (Brown, 2009; Deal et al., 2012). In more technical terms, biochar is produced by thermal decomposition of organic material under reduced amount of oxygen, and at relatively low temperatures (Lehmann and Joseph, 2009). This process is known as pyrolysis. Pyrolysis of biomass is carried out in reactors of different designs leading to the production of not only biochar but also bio-oil (tar) and syn-gas (gas) which is useful in electricity generation and other industrial applications. Although, significant investigations have been carried out on this area with various reactor configurations. The configuration of the reactors in most cases is used to determine the product yield and the number of by-product separated from the parent material. Additionally, different kind of biomass have been pyrolyzed most especially wood (detailed research has been carried out on wood), others include plastic, corn cob and stover, while few researchers have studied the pyrolysis of poultry litter etc. However, literature search has shown that most reactors are designed to produce only three products (Allyson, 2011). Again, the interest in most design is the production of mostly bio-oil and syn-gas. However, the technology for converting bio-oil and syn-gas to a useable form is not yet very much available in the country. In Nigeria, traditional methods of pyrolysis are inefficient in addressing the issues of environmental pollution, ease of production and exposure of workers to related risks (Hammed et al., 2014). The only product that can directly be utilized in its present form from this process for agricultural production is the biochar. Therefore, the emphasis in this research work is to initiate a novel biochar production process through a two-step parallel kinetics. The implication therefore, is to design a reactor configuration different from other existing ones to be able to synchronize this two-step process to produce more quantity of biochar.

Since, the technology of converting gas and tar to useable form is not available in the country presently, only biochar can be used directly for agricultural purposes. It is therefore imperative to design a reactor for biochar production following a multi-step pyrolysis kinetics which makes this study novel. So that biochar can be produced at two stages in a single reactor. This can be achieved by designing a batch reactor and performing a coupled heat and mass transfer analysis using numerical method since pyrolysis plants and experiment are very expensive as stated earlier in this section. 

The aim of this study is to enhance the understanding of the pyrolysis process involving a two-step parallel kinetic reaction scheme by giving physical insights into the pertinent variables affecting the process with an improvement on reactor geometrical characteristics. To achieve the aim of this research, five tasks which form the objectives of the study will be undertaken:

Despite the advantages of biochar for increasing soil fertility, there are food safety and environmental concerns about its application in agricultural sites in its unmodified form (Wilkinson et al., 2003; Chan et al., 2008). The misuse of biochar may result in environmental problems (Gay et al., 2003). However, biochar characteristics and properties are greatly affected by pyrolysis process and its parameters (mainly process temperature and residence time). These factors are particularly important in determining the nature of the final product and, consequently, its potential value in terms of carbon sequestration, agronomic performance, and/or environmental remediation. The use of biochar has other benefits as well, such as increased water holding capacity and higher availability of nutrients, resulting in healthier vegetation (Beck et al., 2011). Healthier vegetation will also make the urban space more livable and enjoyable for humans and wildlife (Bolund and Hunhammar, 1999). 

Most pyrolysis plant design configuration emphasizes on the production of bio-oil and gas as a result this poses a great degree of mass loss which would have resulted in biochar production. In this study, the main focus is more on biochar production as an eco-friendly nitrogenous fertilizer for agricultural production (soil amendment or conditioning). Therefore, there is need to re-conFig. the reactors to produce more biochar than bio-oil and gas.

This research work is focused on numerical modeling and simulation of coupled heat and mass transfer of a novel batch pyrolysis reactor under isothermal heating conditions via slow heating rates. Poultry litter has been selected as the parent material (biomass) for the prediction of the pyrolysis products of a two-step kinetic reaction (primary and secondary reactions) to determine the mass loss, concentration variation and heat transfer rates at different temperatures and residence time. Poultry litter was selected as the feedstock due its high nitrogen content. Also, mild steel was considered as the material for the reactor simulation of the reactor body due its availability in the country.