A review on biomass as a fuel for boilers

Abstract

Currently, fossil fuels such as oil, coal and natural gas represent the prime energy sources in the world. However, it is anticipated that these sources of energy will deplete within the next 40–50 years. Moreover, the expected environmental damages such as the global warming, acid rain and urban smog due to the production of emissions from these sources have tempted the world to try to reduce carbon emissions by 80% and shift towards utilizing a variety of renewable energy resources (RES) which are less environmentally harmful such as solar, wind, biomass etc. in a sustainable way. Biomass is one of the earliest sources of energy with very specific properties. In this review, several aspects which are associated with burning biomass in boilers have been investigated such as composition of biomass, estimating the higher heating value of biomass, comparison between biomass and other fuels, combustion of biomass, co-firing of biomass and coal, impacts of biomass, economic and social analysis of biomass, transportation of biomass, densification of biomass, problems of biomass and future of biomass. It has been found that utilizing biomass in boilers offers many economical, social and environmental benefits such as financial net saving, conservation of fossil fuel resources, job opportunities creation and CO₂ and NO_x emissions reduction. However, care should be taken to other environmental impacts of biomass such as land and water resources, soil erosion, loss of biodiversity and deforestation. Fouling, marketing, low heating value, storage and collections and handling are all associated problems when burning biomass in boilers. The future of biomass in boilers depends upon the development of the markets for fossil fuels and on policy decisions regarding the biomass market.

Introduction

Currently, fossil fuels such as oil, coal and natural gas represent the prime energy sources in the world (approximately 80% of the total use of more than 400 EJ per year) as shown in Fig. 1. However, it is anticipated that these sources of energy will be depleted within the next 40–50 years. Moreover, the expected environmental damages such as the global warming, acid rain and urban smog due to the production of emissions from these sources have tempted the world to try to reduce carbon emissions by 80% and shift towards utilizing a variety of renewable energy resources (RES) which are less environmentally harmful such as solar, wind, biomass…etc. in a sustainable way [1], [2]. The Intergovernmental Panel on Climate Change (IPCC) reported that continued emissions from fossil fuels will lead to a temperature increase of between 1.4 and 5.8 °C over the period from 1990 to 2100 [3]. World energy supplies have been dominated by fossil fuels for decades. Today biomass contributes about 10–15% (or 45 ± 10 EJ) of this demand. On average, in the industrialized countries biomass contributes some 9–14% to the total energy supplies, but in developing countries this is as high as one-fifth to one-third [4]. According to the world energy council projections, if the adequate policy initiatives are provided in 2025, 30% of the direct fuel use and 60% of global electricity supplies would be met by renewable energy sources [1].

The major source of GHG emissions from a boiler system is carbon dioxide (CO₂) from the combustion of fossil fuels in the boiler. Other minor sources of GHGs can include methane (CH₄) from leaks in the natural gas distribution system and CH₄ and nitrous oxide (N₂O) as byproducts of combustion processes [5].

Steam systems are a part of almost every major industrial process today. Thirty-seven percent of the fossil fuel burned in US industry is burned to produce steam. This steam, in turn, is used to heat processes, to concentrate and distill liquids, or is used directly as a feedstock. All of the major industrial energy users devote significant proportions of their fossil fuel consumption to steam production: food processing (57%), pulp and paper (81%), chemicals (42%), petroleum refining (23%), and primary metals (10%). Since industrial systems are very diverse, but often have major steam systems in common, it makes a useful target for energy efficiency measures [6]. Saidur and Mekhilef [7] reported that process heat consumes about 20% of total energy in Malaysia rubber producing industries. Fig. 2 shows process heat energy used in Malaysian rubber industries along with other types of energy use.

Biomass is the name given to any organic matter which is derived from plants. That is plant and animal materials such as wood from forests, crops, seaweed, material left over from agricultural and forestry processes, and organic industrial, human and animal wastes. Biomass is a general term which includes phytomass or plant biomass and zoomass or animal biomass. The sun's energy when intercepted by plants and converted by the process of photosynthesis into chemical energy, is ‘fixed’ or stored in the form of terrestrial and aquatic vegetation. The vegetation when grazed (used as food) by animals gets converted into zoomass (animal biomass) and excreta. The excreta from terrestrial animals, especially dairy animals, can be used as a source of energy, while the excreta from aquatic animals gets dispersed as it is not possible to collect it and process it for energy production [8] (Fig. 3).

Biomass is one of the earliest sources of energy especially in rural areas where it is often the only accessible and affordable source of energy [10]. Biomass is made up of carbohydrates. Biomass is a renewable energy source with very specific properties. Compared to other renewable technologies such as solar or wind, biomass has few problems with energy storage; in a sense, biomass is stored energy. Moreover, biomass is a versatile fuel that can produce biogas, liquid fuels and electricity [11]. Sometimes biomass is classified as combustible materials that can be used as an energy source. Biomass is a renewable energy source because its supplies are not limited. We can always grow tress and crops, and waste will always exist [12], [13].

The energy contained in biomass originally comes from the sun. Through photosynthesis carbon dioxide in the air is transformed into other carbon containing molecules (e.g. sugars…etc.) in plants. These sugars are called carbohydrates and stored in plants and animals or in their waste are called bio-energy (Fig. 4).

Biomass ranks as the fourth source of energy in the world, representing approximately 14% of world final energy consumption, a higher share than that of coal (12%) and comparable to those of gas (15%) and electricity (14%). Biomass is the main source of energy for many developing countries and most of it is noncommercial [8], [14].

Biomass sources provide about 3% of all energy consumed in the United States. Biomass supplied about 53% and 47% of all renewable energy consumed in the United States in 2000 and 2002 respectively. Biomass supplied almost six times the energy of geothermal, solar and wind energy combined (Fig. 5, Fig. 6) [8].

In 2003 biomass contributed 69 Mtoe (million tons of oil equivalent) to the energy system in European Union (EU25). That is 4% of the total primary energy input. Compared to other renewable heat sources such as solar thermal or geothermal, biomass is still the dominant source: it accounts for 96% of renewable heat [11].

Due to the nature of its natural resources, about 24% of the total energy consumption in Austria is covered by renewable sources of energy. Hydropower, biomass, solar energy and wind energy are promising renewable energy sources for Austria (Fig. 7) [15].

Virtually all countries in South and Southeast Asia are major woodfuel consumers and producers. At present, some 39% of the total energy consumption in the developing countries of the region consists of wood and other biomass fuels, and in absolute terms the consumption is still increasing. Most woodfuels do not originate from natural forests but from agricultural and other land [16]. Table 1 shows the importance of biomass in different world region

A comparison of industrial fuels must examine the following characteristics of each fuel: (1) cost per BTU as a raw material, (2) availability in any kind of weather and any international political climate; (3) complexity of the on-site equipment need to transport and burn the fuel; (4) problems associated with the storage of the fuel; (5) emissions caused by combustion; and (6) historical success of the technology for boilers using this fuel. Consider coal, fuel oil, and natural gas in the light of these characteristics [17].

Companies are continually searching for fuels less expensive than coal, fuel oil, and gas. Natural sources such as manufacturing and agricultural waste are inexpensive. Waste materials currently being used as fuels include pulp mill liquor, sawdust, food processing waste, municipal garbage, coal wash water coffee grounds, cardboard, hog fuel (wet bark from plywood operations), and bagasse (sugar cane after the liquid has been extracted). Using industrial wastes as fuels can simplify the disposal process as well as providing an inexpensive source of heat [17].

Still, there are some problems associated with burning any new fuel. The technology for dealing with coal, gas, or fuel oil is well-known. Using a new fuel, however, raises the following questions [17]:

• How high in the combustion chamber should the new fuel be injected into the boiler? (This is critical in burning municipal waste).

• What kind of problems will the ash or residue create?

• What modifications are needed to burners?

• How will the new fuel be transported to and within the facility?

• What storage problems can be expected?

• How regular will the supply be?