Biomass Energy

Introduction

Biomass, referring to various plant and animal-based materials that can be processed into fuel, includes food crops, wood, grass, agricultural and forest residues, and animal waste products. Figure 1 shows biomass is used as a source of energy and fuels. Different type biomass has different energy potential. 

Figure 1 Features of biomass energy technology

Virgin biomass includes all naturally occurring terrestrial plants such as trees, bushes and grass (Wiki). The growth of Virgin biomass is the most widespread and practical process for capture of this energy as organic fuels.  Figure 2 presents the relationship among energy supply and amount of virgin biomass needed (area). 

Figure 2 Required area and percent of U.S. area

The other type is waste biomass. It consists of a wide range of materials and includes municipal solid waste (MSW), sewage, industrial waste, animal manure and so on. Table 1 shows the potential biomass energy available in U.S. 

Biomass energy becomes a substantial contributor or commercial primary energy demand. Table 2 compares the consumption of biomass energy in U.S. in 1990 and projected for 2000.

Table 2 Consumption of biomass energy in United States

Biomass and waste fuels generated 71.4 billion kilowatt-hours of electricity in 2016, or 2% of total generation in the United States.  Wood solids accounted for nearly one-third the electricity generated from biomass and waste. Municipal solid waste (MSW), which comes from landfills, provided 20% of biomass- and waste-generated electricity in 2016. Landfill gas is created by decomposing organic material in landfills. Its composition is about half methane (the primary component of natural gas) and half carbon dioxide (CO2). Landfill gas provided nearly 16% of 2016 biomass-generated electricity.

Availability

The geographic distribution and quantity of biomass depend on the relationship between ecological zones and the climate conditions. Generally, a higher standard of living results in more waste. Socio-economic drivers such as government policy and social acceptance play a very important role in minimizing the generation of wastes and reducing landfilling by recycling and combustion to generate electricity. 

Based on the feasibility and accessibility of biomass utilization, the biomass resource can be divided into four categories. 

Agricultural residues 


Wood residues 


Municipal Discards 


Dedicated Energy Crops Case Studies 


Agricultural residues

Animal wastes have been transformed from a definite asset to a liability. By 1965, the disposal of animal excreta had become a serious problem (American Chemical Society, 1969). Today, the problem becomes even much more severe. Application of animal wastes to land is one of the most economical choices for disposal as well as providing fertilizing benefits. However, the utilization of livestock and poultry manures as waste biomass resources for energy applications could help mitigate pollution and at the same time open new markets.

Animal residues

Domestic farm animals and those confined to feedlot provide the greatest opportunity to serve as source of waste biomass. The animals that produce large, localized quantities of excreta are cattle, hogs and pigs, sheep and lambs, and poultry. U.S. populations of these animals in the mid-l990s, the estimated total, annual manure production for each species, and the human population equivalents in terms of solid waste generation are shown in Table 3.

Table 3 Livestock and poultry manures generated in the United States and their human population equivalent

Commercial broilers had the highest population, about 7 billion, and they produced the second largest amount of manure; cattle, which had a population of about 100 million, produced the largest amount.

Energy potential

Based on the measured heating values of air-dried samples including ash to dry waste, the energy potential of livestock/poultry can be estimated.  Cattle and commercial broilers are the two largest energy producers, and the total energy potential of all animal excreta is about 4.6 EJ/year, an amount 28 times that of the energy content estimated here for primary biosolids production in the United States. The following table 4 shows the energy potential of livestock and poultry manures.

Table 4 Energy Potential of Livestock and Poultry Manures Generated in United States

Methane is produced by the anaerobic decomposition of organic matter. Manure deposited on fields and pastures, or otherwise handled in a dry form, produces insignificant amounts of methane. Figure 4 shows the methane emissions.

Figure 4 Methane generation potential from animal manure

Crop residues

Agricultural crop residues are those left in the field or accumulated during sorting and cleaning of produce. The quantities of crop residues that can be available in each county were estimated using total grain production, crop to residue ratio, moisture content, and taking into consideration the amount of residue left on the field for soil protection, grazing, and other agricultural activities. The amount of crop residues available by county is shown on Figure 5.

Figure 5 Crop residues

Forest residues

Forestry residues consist of slash left on the forest floor following logging operations; stems, stumps, tops, foliage, and damaged trees that are not merchantable; and wood and bark residues accumulated at primary wood manufacturing plants during production of lumber. Underground tree roots can also be included in the list of forestry residues. Figure 6 displays the forest residues by counties in U.S.

Figure 6 Forest residues

Primary Mill Residues


 Primary mill residues are composed of wood materials (coarse and fine) and bark generated at manufacturing plants (primary wood using mills) when round wood products are processed into primary wood products, like slabs, edgings, trimmings, sawdust, veneer clippings and cores, and pulp screenings. It includes mill residues recycled as byproducts as well as those left un-utilized and disposed of as waste. Figure shows the primary mill residues recycled as byproducts (fuel or fiber) as well as those left un-utilized and disposed of as waste.

Figure 7 Primary Mill Residues

Secondary mill residues

Secondary mill residues include wood scraps and sawdust from woodworking shops— furniture factories, wood container and pallet mills, and wholesale lumberyards. The geographic distribution is shown in Figure 8.

Figure 8 Secondary mill residues

Urban Wood Residues
 Urban wood residues contain three major categories: 

Figure 9 Urban wood waste

Municipal Solid Waste (MSW)

As the populations of urban areas grow, the production of MSW increases. The generation of MSW in the United States increased from about 80 million tonnes in 1960 to 180 million tonnes in 1990. During this same period, the corresponding per-capita generation of MSW in 10-year increments was 1.23 kgperson-day in 1960, 1.49 in 1970, 1.65 in 1980, and 1.97 in 1990. landfilled MSW can provide energy as fuel gas for heat, steam, and electric power production over long time periods. Surface-processing of MSW can also provide energy for the same end uses when MSW is used as a fuel or a feedstock. 

At a higher heating value of 12.7 MJ/dry kg of MSW (Table 3.3), the energy potentially available from the MSW generated in the United States in the 1990s is in the range of 2.5 EJ/year. Presuming the total combustibles in the recovered fractions of MSW are utilizable and that the energy recovery systems in operation continue to be used, the data for the United States indicate that about 60 to 65% of the MSW generated could have supplied up to an additional 1.6 EJ/year in the mid-1990s. New energy recovery plants supplied with MSW feedstock could also provide an additional benefit by increasing the life of landfills. Only the unrecyclable inorganic materials in the ash would be landfilled if thermal processing of the MSW is employed.

Figure 10 shows the methane emissions from landfills. The amount of methane emission depends on three factors: (1) total waste in place; (2) landfill size; and (3) location in an arid or non-arid climate.

Figure 10 Methane emissions from landfills

Domestic Wastewater Treatment

Municipal wastewater treatment plants in industrialized countries receive wastewaters from residential sources, industry, groundwater infiltration, and stormwater runoff. The pollutants associated with these sources include a wide range of suspended and dissolved compounds and oxygen-demanding materials, many of which are toxic. Pathogenic components are present, including certain bacteria, viruses, organic compounds, inorganic nutrients, and heavy metals. The purpose of most wastewater treatment processes is to remove or reduce these components, other pollutants, and biological oxygen demand before discharge to receiving waters. About 40 to 45% of the treated biosolids are disposed of in municipal landfills, 30% is applied to land or distributed or marketed as fertilizer, 20% is incinerated, and the remainder is disposed of in dedicated landfills or by a few other methods. The energy potential of municipal biosolids is small. At an average higher heating value of 19 MJ/dry kg (Table 3.3), the energy content of all the primary and treated biosolids produced in 1995 in the United States can be estimated to be 0.163 and 0.113 EJ/year, both of which are much less than the energy potential of MSW. This relationship is a permanent one because of the nature of MSW and biosolids generation.

Figure 11 shows the methane emissions from domestic wastewater treatment.

            Figure 11 methane generation potential  from domestic wastewater treatment

Biomass Energy Application

Biomass currently provides 12 to 13% of global primary energy. Compared with other renewable sources of energy, biomass has the advantage that it can be used for heat provision, electricity production, and the provision of liquid and gaseous fuels for transportation purpose. Bioenergy is generated in a multistage supply chain that starts with the collection of residues, by-products, or waste, and/or the cultivation of energy crops. This biomass then undergoes a variety of mechanical processing, storage, and transportation steps – depending on the local conditions – and perhaps industrial conversion processes to produce secondary solid, liquid, and/or gaseous energy carriers or biofuels. These fuels can be used to meet the given demand for different forms of useful energy.

The conversion of biomass (i.e., organic material of very different origin) into solid, liquid, and/or gaseous secondary energy carriers can be realized via heat-induced processes (i.e., thermochemical conversion), via transformation by bacteria (i.e., biochemical conversion), and/or via physicochemical processes. Figure 12 presents the biomass energy main conversion routes.

Figure 12 Biomass energy conversion

Thermochemical conversion

Biomass can be converted into useful energy or into secondary energy carriers using conversion processes based on heat.
Application example: Wood pellet stove

Automatically feeding wood pellet stove is one example of direct combustion application in biomass. During ordinary operation, the combustion residues (i.e., ashes) contain less than 1% carbon andcan thus be used as a fertilizer or taken to a landfill depending on the legal regulation. The thermal efficiency of such systems can reach 95% or more.

Physicochemical conversion

This conversion pathway is used to produce liquid biofuels from organic matter containing oils and/or fats (e.g., rapeseed, oil palm fruits, sunflower seeds).

Application example: oil production

Depending on the vegetable oil content, the oil characteristic, and the properties of the oil seed, different technical approaches have been developed throughout recent centuries to produce the oil or fat in a most cost-effective way especially to provide high-quality oils and fats in an economic viable way for the food and fodder industry on the one hand side and for the chemical, pharmaceutical, and/or beauty industry on the other side. Basically, these approaches consist of one or a combination of the two technical processes, pressing and extraction.

Crude natural vegetable oil needs to be converted to a liquid fuel for diesel engines. There are two ways to realize that, which are: (1) Transesterification. It is the most widely realized option. (2) Hydrogenation. vegetable oil can be treated with hydrogen under presence of a catalyst using the hydrotreating technology usually employed in conventional crude oil refineries.

Biochemical conversion

Existing organic matter can be decomposed naturally through biological processes.

Application example: Anaerobic digestion

During anaerobic digestion, organic material with a high share of water is decomposed in an oxygen free atmosphere by bacteria releasing an energy rich gas, called biogas. 

Figure 13 Schematics of anaerobic digestion system

Assessment

The assessment of biomass will contain the following tasks: