Biomass combustion in fluidized bed boilers: Potential problems and remedies

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Abstract

Due to increasing environmental concerns especially related with the use of fossil fuels, new solutions to limit the greenhouse gas effect are continuously sought. Among the available alternative energy sources, including hydro, solar, wind etc. to mitigate greenhouse emissions, biomass is the only carbon-based sustainable option. On one hand, the versatile nature of biomass enables it to be utilized in all parts of the world, and on the other, this diversity makes biomass a complex and difficult fuel. Especially the high percentages of alkali (potassium) and chlorine, together with high ash content, in some brands of biomass prove to be a major source of concern. However, mechanisms leading to corrosion and high dust emissions problems have been identified and a range of possible solutions is already available. Among the technologies that can be used for biomass combustion, fluidized beds are emerging as the best due to their flexibility and high efficiency. Although agglomeration problems associated with fluidized bed combustors for certain herbaceous biofuels is still a major issue, however, but successful and applicable/implementable solutions have been reported. This review article presents the major issues concerned with biomass combustion with special reference to the small scale fluidized bed systems (small to pilot scale). Problems have been identified, mechanisms explained and solutions have been indicated. In conclusion, a range of concerns including environmental, economical and technical associated with biomass exist, but none of these issues represent an insurmountable obstacle for this sustainable energy source.

Introduction

Today biomass is seen as the most promising energy source to mitigate greenhouse gas emissions [1], [2], [3], [4], [5]. Large scale introduction of biomass energy could contribute to sustainable development on several fronts namely, environmental, social, and economical [4], [5], [6], [7]. World energy supplies have been dominated by fossil fuels for decades (approximately 80% of the total use of more than 400 EJ per year, see Fig. 1) [4], [8]. Today biomass contributes about 10 to 15% (or 45 ± 10 EJ) of this demand. On average, in the industrialized countries biomass contributes some 9 to 14% to the total energy supplies, but in developing countries this is as high as one-fifth to one-third [1], [6], [9]. In quite a number of these countries, biomass covers even over 50 to 90% of the total energy demand. It should be noted, though, that a large part of this biomass use is non-commercial and used for cooking and space heating, generally by the poorer part of the population [9]. Modern production of energy carriers from biomass (heat, electricity and fuels for transportation) or biofuels contributes a lower, but significant 7 EJ [4], [6]. The utilization of biomass within the European Union (EU) has strongly increased over the last decades, and the ambitions of the EU for the use of biomass are high. Up to 6000 PJ in 2010 are targeted (tripling the use compared to 1999 levels), and possibly even more beyond the targets [10].

Over the past decade, substantial growth figures of bio-energy in the EU, particularly for the modern energy carriers, like electricity, and increasingly biofuels for the transport sector, are observed. Heat production increased by some 2% per year between 1990 and 2000, bio-electricity increased by some 9% per year and biofuels production increased about eight-folds (over 20% growth per year) in the same period. The 1999 contribution of biomass to the EU energy supply was little less than 2000 PJ, some two-thirds of the total renewable energy production in the EU or 4% of the total energy supply [9], [11]. Fig. 1a shows the history and projection of world marketed energy consumption from 1980–2030 while Fig. 1b indicates a further renewable classified energy contribution for the year 1998 [12], [13].

Looking at the incentives that biomass offers, the governments in different countries are also committing more seriously for energy with least environmental impact. Every country in Europe has included bio-energy in its energy and climate policies. For the European Union (EU), targets have been set for bio-energy: in 2010 almost 10% of the energy supply of the EU is to come from biomass [11], [14], [15]. Few individual countries, especially in Scandinavia are setting up high goals for the future. For instance, Sweden formulated that 40% of its primary energy supply should be covered by biomass around 2020 [11], [16]. Another example is a recent EC-directive on biofuels for the transport sector, which has set targets for the use of biomass for transport fuels [11], [14]. Different global energy scenario studies indicate that in this century biomass may contribute much more: up to 30% of the 2100 energy supply from biomass [5], [17], an average 50–250 EJ/yr in 2060. A global (technical) potential of primary biomass in 2050 of 33–1135 EJ/yr [8] is foreseen, depending on population growth and food demand (diet), economic development, food production efficiency, energy crop productivity on various land types, competing biomaterial products, and land use choices. This range may be narrowed down to 300–675 EJ or 40–60% of the energy demand in 2050, which could be produced on 4–10% of the terrestrial surface [5], [8], [18]. Such targets are in line with various global scenario studies for bio-energy which indicate that biomass may contribute 100 to over 400 EJ (or 25% to almost 100% of the current world's energy use) to the world's energy supply during this century [11], [19], [20].

The realization of all the above-mentioned target percentages will not be easy. Fuel availability over time, alternative applications, varying prices and sources of income are among the already foreseen problems. Dedicated production of biomass, the so-called ‘energy cropping’ or ‘energy farming’ is seen as the only measure to fulfill future renewable fuel needs with a variety of crops. In general, dedicated biomass production is more expensive per unit of energy produced than the use of available residues and wastes. Typical cost ranges for perennial woody crops under North Western European conditions are 3–6 euro/GJ (compared to some 1–2 euro/GJ for imported coal) [5], [11]. Biomass costs of dedicated production systems are especially dependent on the costs of land and labor and the (average) yield per hectare [7]. Using a crop only partially for energy and partially for material purposes may be a solution. This is because the material component of the crop may create (1) additional incomes and (2) additional GHG emissions reductions [21]. However, some studies have suggested that the large scale energy crop production including intercontinental trade of biofuels or bulk wood could be economically feasible and will not lead to dramatic energy losses [4], [22]. The eventual costs of electricity may prove competitive with present day fossil electricity. Biofuels remain slightly more expensive than fossil automotive fuels, but the gap can probably be bridged when system scales are increased and bio-processing technology improves.

Biomass offers a number of advantages compared to fossil fuels. Biomass is regarded as a renewable energy source with zero to low CO2 emissions if produced in a sustainable manner [23]. An evaluation of the CO2 balance shows that compared with the combustion of hard coal, the CO2 emissions can be reduced by 93% [24]. The thermal utilization of waste and residual material, as e.g., sewage sludge, along with reducing the CO2 emissions helps to solve the waste disposal problem.

Low SOx and possibly low NOx emissions are added benefits. The alkaline ash from biomass captures some of the SO2 produced during combustion and therefore co-firing can also reduce the net SO2 emissions. In addition, the fuel nitrogen content in biomass is in many cases much lower than in coals and may be converted to ammonia during the pyrolysis stage of combustion and thus can be utilized as reburning fuel. Hence, co-firing can also result in lower NOx levels. Blending can also result in the utilization of less expensive fuels with a reduction in fuel costs [25]. In addition, energy crops cultivated for sustainable biomass growth can be an employment generator, especially in rural areas. Furthermore, biomass can be an indigenous energy source for most countries, thus coal imports can be reduced [23].

However, apart from logistic and cost issues, there are certainly some big technical challenges associated with thermo-chemical biomass utilization. These include among others, NOx, N2O and dust emissions, deposit and corrosion problems, agglomeration and ash related issues. High impact factors for life cycle assessment (LCA) have been assigned to wood combustion in comparison to natural gas and light fuel oil [26]. The major contributors to wood combustion LCA come from NOx and PM10 emissions.

This paper focuses on these issues with special reference to fluidized bed combustion technology (small to pilot scale). The objective has been to provide more information related to the effect of the physical and chemical properties of the biomass on the combustion process and emission characteristics. Four major areas are considered. In Section 1, biomass properties, densification process, and a short description of fluidized bed technology and co-firing are given. Section 2 discusses issues related with organic and inorganic gaseous pollutants. In Section 3, operational problems, deposits, corrosion and agglomeration are briefly discussed and finally in Section 4, dust and trace metal emissions are presented.

Section snippets

Biomass classification

Biomass is a biological material derived from living, or recently living organisms. In the context of biomass for energy this is often used to mean plant based material, but biomass can equally apply to both animal and vegetable derived material [14], [27].

Biomass energy resources are diverse and therefore a great need exists for a comprehensive classification system. The motivation for such a system is to predict the behavior of biomass by identifying to which class it belongs. One approach

Emission limits

Table 4 provides an overview of the emission limits applied in the Netherlands. The decision which limit is enforced depends on the type of biomass and installation [44].

For all cases (besides combustion of own clean waste wood in an installation smaller than 5 MWth) where biomass is combusted for energy, NER or BEES A applies.

Table 4 also shows the European draft directive for Large Combustion Plants (LCP) exceeding 300 MWth, from which the national emission legislation has been derived. In

Combustion

A detailed description of biomass combustion is beyond the scope of this article. Only the shortest description can be provided here. Biomass combustion is a complex process consisting of consecutive homogeneous and heterogeneous reactions. The essential process steps include drying, devolatilization, gasification, char combustion, and gas phase reactions. Fig. 3 shows a schematic description of these processes for wood. The biomass combustion process can also be briefly summarized as follows.

Co-firing

Co-combustion refers to simultaneous combustion of two fuels. It is the practice of introducing biofuels as a supplementary energy source in high efficiency utility boilers, but combustion of two different biomasses is also not unknown, at least in smaller scale units (1–5 MWth). In this section, the focus will be on biomass-coal co-combustion. Coal is the most abundant of the fossil fuels and is responsible for about 27% of the world's primary commercial energy use. It accounts for about 34%

Fluidized bed combustors

Bubbling fluidized bed boilers (BFBB) or circulating fluidized bed boilers (CFBB) are considered as mature technologies (for small scale). Fluidized bed combustion has been indicated as one of the most promising techniques, because of its flexibility, high combustion efficiency and low environmental impact [25]. As this article mainly deals with the small scale FB units, the following paragraphs give a short description of such a FB unit.

In FB furnaces, an initially stationary bed of solid

Pollutant emissions

Pollutants can be classified into two major classes:

  • 1.

    Unburnt pollutants and

  • 2.

    Pollutants that are produced by combustion.

The unburnt pollutant include CO, HC, tar, PAH, CxHy, and char particles. These pollutants are usually due to poor combustion which is a result of low combustion temperature, insufficient mixing of fuel with combustion air and too short residence time of the combustible gases in the combustion zone. They can be expected from all fuel categories depending on the furnace design and

Definition and sources

Deposits or fouling, in terms of combustion, are commonly known as the layers of materials (ash) collected on the surface of heat transfer equipment. Another important term, slagging, characterizes deposits on the furnace walls or other surfaces exposed to predominantly radiant heat. Corrosion is the deterioration of intrinsic properties of a material due to reaction with its environment. Corrosion can be caused either directly by gas phase species, by deposits or by a combination of both [156]

Definition and sources

Ash from any combustion system can essentially be divided into four classes:

  • 1.

    Bottom ash

  • 2.

    Cyclone ash

  • 3.

    Filter flyash

  • 4.

    Flue dust.

The fraction of fuel ash in each class essentially depends upon the technology and solid emissions control devices installed with the system. Bubbling fluidized bed systems contain nearly no bottom ash as all ash is entrained out of the bed by the flue gases. The cyclone ash, filter flyash and flue dust are discussed under dust emissions in the following section.

Although the

Definition and sources

Biomass combustion leads to relatively high emissions of particulates, i.e., well above 50 mg/m3 at 11 vol.% O2 [26]. The size distribution of the particles varies for different combustor setups. Particles in submicron (less than 1 µm) and supermicron (greater than 1 µm) range are usually emitted with size distribution differing with respect to technology used [43], [148].

Fine particles propagate far away from the source and have a high probability of penetrating into the alveolar regions of

Definition and sources

Trace elements (As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb,V, Zn) present in some biomass fuels play an important role in the various practices of this renewable feedstock. These elements are of great biochemical, nutritional, and clinical importance and also of great environmental concern for heavy metal emissions [202]. Cd in agricultural products is potentially a public health problem, owing to its accumulation in the kidneys and its effects on skeletal density [203].

The presence of the

Conclusion

With the growing concerns of greenhouse emissions, biomass is set to become an important contributor to the world energy need. Although present biomass economics are a bit more expensive than fossils fuels but trends show that with an efficient bio-cycle including cultivation, transportation and combustion, biomass can also compete economically.

Apart from small scale greenhouse or community boilers, the use of biomass as a sole energy source is unimaginable especially for electricity

Acknowledgement

European Union is acknowledged for funding the research in the framework of NNE5 (Project No.: E5-2001-00601) via the project ‘Safe co-combustion and extended use of biomass and biowaste in FB plants with accepted emissions’ (contract ENK5-CT2002-00638, FBCOBIOW).

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