Emissions and environmental problems

Global warming as a result of greenhouse gases (GHG) is one of the major concerns among researchers, politicians, and the public in general. The main emission for global warming is carbon dioxide (CO2), while other emission gases like CH4 and N2O also contribute. The major source of CO2 is related to power production, i.e. combustion of fossil fuels in power plants.

Approximately 130% increase is expected for the CO2 level in the atmosphere by the end of 2050, which will increase the earth temperature with about 6 °C [IEA report 2008]. United Nations Intergovernmental Panel on Climate Change (IPCC) concluded that if energy demand/production continues with the current trend, global CO2 emissions in 2050 should be reduced at least 50% compared to the base year, 2000 [IEA report 2010], in order to keep the temperature rise between 2.0 °C and 2.4 °C.

Having fossil fuels as the dominant source of energy, we should try to reduce the CO2 level at the same time. This can be achieved with e.g.:

22 Introduction 2. Renewable energy such as hydropower, wind, and bioenergy

3. CO2 negative technologies; e.g. biomass combustion combined with CCS Biomass as a renewable source of energy has a lot of environmental advantages over other solid fuels. However, almost all biomass fuels contain some amount of sulfur, nitrogen, and chlorine. Growing plants for use as biomass fuels may also help keep carbon dioxide levels balanced. Plants remove carbon dioxide from the atmosphere when they grow, store carbon in their structure and release oxygen.

As mentioned earlier, combustion of biomass results in formation of CO2, which is necessary for plants growth. Hence, biomass is also known as almost CO2-free or carbon neutral fuel. If carbon capture and storage technology is implemented in a biomass combustion plant, the net CO2 level for the cycle will be negative. Biomass combustion is the only energy system capable of doing this, i.e. provide a carbon negative system. Although having a CO2 neutral cycle, burning biomass will result in different types of other pollutants, mainly classified into two groups:

1. unburnt emissions; and 2. combustion related pollutants.

Unburnt emissions are a result of incomplete combustion, due to temperature effects, mixing and residence time; forming CO, HC, tar, PAH, and unburnt char particles [Khan et al. 2009]. The second type of pollutants originates from the initial fuel composition such as nitrogen, sulfur and ash elements. It includes PM, NOx, N2O, SOx, HCl and heavy

metals [Khan et al. 2009]. Ash deposition and corrosion is also influenced directly by fuel composition.

Different reviews have been carried out on aspects of biomass environmental problems [Berndes et al. 2008]. Below different emissions based on the relevant elements in biomass are introduced.

2.4.1 CO

Emission of CO is due to incomplete combustion, affected by different parameters. Therefore, in general, an excess air ratio of less than one, or supply of too little oxygen to the combustion zone will be a condition for the formation of carbon monoxide. When air and fuel are not perfectly mixed, zones with low concentration of oxygen will be present, resulting in formation of CO instead of complete burnout of CO. In addition, a sufficient residence time is needed for complete combustion, otherwise CO formation rather than CO2 [Laryea-Goldsmith 2010] will result.

Since CO is an intermediate species for CO2 formation, and temperature is an important parameter in this reaction chemistry, zones with low temperatures favor formation of CO, while at higher temperatures the reactions proceed faster toward complete oxidation.

Emissions and environmental problems 23 On the other hand, reaction of CO with other gaseous species is an important factor. For example, HCl in the flue gas has a direct relation with the CO emission level, affected by the presence of radicals and catalytic reactions in presence of those species [Wei et al. 2004]. Higher NOx emissions also gives a higher CO level, while combined NOx and HCl

result in a higher CO due to a considerable decrease in CO oxidation in presence of these gases [Roesler et al. 1995]. Likewise, it has also been shown that SO2 in the presence of NO prevents CO oxidation by reducing the concentration of free radicals [Glarborg et al. 1996].

2.4.2 Nitrogen: N

2.4.2.1 NOx

Nitrogen oxides (NOx=NO+NO2) are a minor part of the combustion products in the combustion of all biomass fuels containing nitrogen. NOx has both a negative effect on

the climate (indirect effect on greenhouse gas through ozone formation, acid rain, vegetation damage, smog formation, etc.) and human health (to respiratory system, when reacting with ammonia and other compounds to form small particles which can penetrate deeply into sensitive parts of the lungs). The major part of NOx emissions is coming from

fuel-N content and the share of other mechanisms, mainly thermal NOx, which originate

from air nitrogen content, is very low, as the temperature in biomass combustion systems is not so high. Different classes of biomass have a range of nitrogen content of 0−2.5 wt% while special types such as sewage sludge may contain up to 7 wt%. Primary measures (fuel staging, air staging, fuel mixing) can be used to lower the NOx emissions from fuels

with low nitrogen content [Houshfar et al. 2010b; Houshfar et al. 2012d], while from fuels with high nitrogen content, secondary measures such as SNCR or SCR should also be applied to satisfy local or international regulations. This will be discussed in more detail in the next chapter.

2.4.2.2 N2O

Nitrous oxide is also a minor product from combustion of biomass, from nitrogen in the fuel. This emission is a greenhouse gas and was recently shown to be the most important ozone-depleting substance [Ravishankara et al. 2009]. N2O has an indirect effect on health through O3 depletion. However, the emission level of N2O is usually very low in biomass combustion systems.

A detailed discussion on N-emissions will be given in the following chapters.

2.4.3 Chlorine: Cl

Each biomass having Cl in the composition will produce hydrogen chloride (HCl) in the combustion process, which is toxic and has negative effects on the human respiratory system. HCl is a strong acid, therefore contributing to acid rain and vegetation damage,

24 Introduction but has not high corrosion risk in pipes and other materials in boilers and reactors. Fuel leaching [Dayton et al. 1999], automatic cleaning of parts, tubes coating and change of reactor materials are methods to reduce the effect of corrosion from Cl. To regulate the environmental effect of HCl, sorption (dry or in activated carbon) and scrubbers can also be applied. The major part of Cl in the fuel may be converted to salts such as KCl and NaCl for alkali-rich fuels, which are highly corrosive when condensing on superheater tubes or on fly ash particles depositing on superheater tubes. Some Cl can also remain in the bottom ash or will be released as fly ash, which is discussed later.

2.4.4 Sulfur: S

The sulfur content of biomass is relatively low compared to other fuels. However, sulfur oxides (SOx, mainly SO2) and alkali sulfates are formed from biomass fuel sulfur oxidation, and contribute to aerosol and smog formation, vegetation damage, acid rain and corrosion; in addition to the dangerous effect on humans by asthmatic effect and damage to the respiratory system. The sulfur retention in ash or release to the gas phase highly depends on the temperature and presence of other mineral compounds [Knudsen et al. 2004; Lang et al. 2006]. Sulfur containing components may enhance chlorine corrosion at high temperatures and some methods as described for Cl should be applied to avoid corrosion from sulfur. Injection of lime or limestone is a technique to reduce the SO2 emission, while co-combustion of biomass and coal is an alternative for coal fired boilers.

2.4.5 Calcium: Ca

Ca has a relatively high melting point which increases the melting temperature of ash, and therefore reduces ash sintering on the grate or in the reactor. However, the ratio between Ca and potassium is more important where a low Ca/K ratio can lead to a reduced melting point [Steenari et al. 2009]. In such cases, necessary control equipment should be used to reduce the temperature in the combustion system.

2.4.6 Potassium: K and Sodium: Na

K is the major alkali metal in biomass fuels and Na is an important element with respect to corrosion. Due to their low melting point, formation of components from K and Na cause problems in the combustion systems such as sintering in addition to aerosol formation, agglomeration, deposition, corrosion, slagging and fouling. To avoid such problems, it is important to use dust precipitation, fuel leaching, coatings and cleaning. As mentioned before, alkali chlorides (KCl and NaCl) are corrosive components formed by reaction of Cl and K or Na. Sulfation of gaseous KCl is one of the methods to reduce the corrosion problems from KCl [Kassman et al. 2010]. A recent study shows that sewage sludge can act as a controller for corrosion when co-combusted with high potassium content biomass fuels [Elled et al. 2010].

Emission reduction measures 25

2.4.7 Silicon: Si

This element is commonly the main ash forming element and deserves special attention in biomass combustion. Fuels such as straw contain more silicon, which is a problematic element causing ash deposition in the system at high or moderate temperatures. In case of high concentration of alkali, formation of alkali silicates will lead to ash melting at low temperatures, e.g. less than 700 °C, hence resulting in deposition problems [Khan et al. 2009]. Agglomeration in fluidized bed boilers is also a result of silica, alkali and chlorine.

2.4.8 Zinc: Zn and Cadmium: Cd

Zn and Cd are important heavy metals in the ash composition due to sustainability of ash (ash utilization and recycling) and particulate emissions. Heavy metals are toxic and accumulate in the food chain. Ash treatment and fractional heavy metal separation is used to regulate the amount of these elements in the ash composition while dust precipitation and treatment of condensates reduce the particulates.

2.4.9 Other problems

Emissions such as PM are affected by a series of the discussed elements in the fuel. PM contains normally alkali salts like potassium chloride and potassium sulfates; thus the initial fuel composition, especially the percentage of potassium, chloride and sulfur is very important for creation of PM emissions.