Heating value

The methods for determining the heating value are described in some detail in text section 04-00-01a. It should be reminded that the higher heating value assumes that the enthalpy of moisture in the combustion gases has been reclaimed by cooling them down to 25 °C, while the lower heating value has the moisture in the flue gas remaining in the gas phase, thus the fuel energy obtained is "lower”.

In a laboratory, it is the higher heating value of a biomass sample that is measured using a bomb calorimeter and the instructions of standard EN 14918. Accurate determination of the higher heating value usually requires knowledge of the fuel sulphur and hydrogen content. The hydrogen and the moisture content of the fuel must also be known to calculate the lower heating value. If no laboratory measured values for the sample in question are available, literature values can be used, although the accuracy of the results suffers to a certain extend.

Typical high heating values in a daf basis are provided in Chapter 03-00-02c. As previously discussed, the lower lignin content of herbaceous biomass also means lower HHVs compared to woody biomass fuels. In addition, the higher ash content also contributes to lowering the heating value obtained per kg of fuel. On a dry basis, herbaceous biomass fuels have a range of high heating values between 17 – 19 MJ/kg on a dry basis, while the lower heating value on as received basis is usually between 14 – 16 MJ/kg for straw type fuels.

The low moisture content means that the difference between the LHV and the HHV is not as high as in the case of woody biomass.

Since there may be differences between the moisture content measured in the laboratory and the actual moisture content in the field or at the point of delivery, the lower heating value of a lab analysis report may have to be converted to a different moisture basis before estimating basic engineering parameters of a bioenergy conversion system.

Emissions

As indicated in Chapter 01-00, biomass is considered a renewable fuel - CO2 is released in the atmosphere during its combustion but due to the carbon cycle of the plants it is considered as logistically "zero”. However, this does not mean that biomass combustion is without environmental consequences. Generally, pollutants from combustion can generally be divided in three groups:
•  Unburnt pollutants, which are caused by the incomplete combustion of the organic part of biomass. Typical examples are carbon monoxide and hydrocarbons. Methods for their reduction include all measures intending to increase the combustion efficiency, such as more efficient mixing of combustibles and air, temperature and residence time increases and staged combustion. Unburnt emissions tend to be more critical for small-scale applications, where the optimization of combustion conditions cannot be as thorough as in large-scale applications. This is further discussed in 04-00-08c.
•  Pollutants produced by the combustion process which do not directly relate to the completeness of the combustion. Typical examples would be NOx and halogenated hydrocarbons like dioxins. Their formation is a function of fuel properties and local combustion conditions and can be controlled either by fuel pre-treatment or by careful modifications of combustion parameters. Such pollutants will occur in both large-scale and small-scale applications. In large-scale applications there are options, namely to control these emissions through careful combustion control and through secondary measures like flue-gas cleaning, but this will typically be costly. The waste incineration directive is an example of how such demands are imposed on combustion plants and the large combustion plant directive (2001/80/EC) is another.
•  Pollutants that are directly related to the composition of the fuel. Typical examples will be SOx, HCl and heavy metals. The amounts of these can be reduced either by cleaning the fuel prior to the process or by cleaning the flue gases or, in case of a conversion process like gasification, liquefaction or pyrolysis, by cleaning the product fuel.

The following considerations regarding combustion emissions from biomass can be stated:
CO emissions are often used as an over-all benchmark of combustion efficiency, i.e. as an indicator substance or hydrocarbon emissions, but CO is also extremely toxic. Small-scale combustion applications suffer the most from CO emission issues. The correlation between hydrocarbon emissions and CO-emissions is weak unless the data are evaluated with a very high time resolution – at least one measurement per second – which in usually not the case. Hence, time average values do not give a good indication of combustion quality but a plant with a low CO-emission may still exhibit very high emissions of hydrocarbons and vice-versa. To abate CO-emissions, it is important that the combustion chamber is adapted to fuel and the use of herbaceous biomass in a boiler designed for woody biomass may lead to high CO-emissions (see also text section 04-00-01 and chapter 04-00 in general).

PCDDs and PCDFs or dioxin is a general term employed for a group of halogenated organic compounds. Several types of dioxins are highly toxic and are suspected to have carcinogenic and mutagenic properties. Dioxins are lipophylic and tend to accumulate in living tissues; hence even small environmental concentrations may find their way up the food chain and quickly reach dangerous levels. Dioxins, as well as other halogenated hydrocarbons like furanes, chlorobenzenes and others are formed by hydrocarbons reacting with chlorine at elevated temperatures. Since, thus, the formation requires the presence of hydrocarbons, the most efficient means for abatement is to provide conditions for a complete burnout. This is the reason for the technical process demands posed in the waste incineration directive, that the gases in the combustion chamber must be retained at least two seconds at a temperature exceeding 850 oC and with an excess of oxygen before they are allowed to be cooled in the convection pass. The basic assumption is that such conditions will reduce the amount of unburned hydrocarbons to such low concentrations that the formation of halogenated compounds becomes negligible. With high-chlorine fuels – such as some herbaceous biomasses or waste fractions – it is thus crucial that the size and the over-all design of the combustion chamber are appropriate. In modern combustion applications, combustion temperatures tend to be sufficiently high to control and limit dioxin emissions to the levels required by legislation.

NOx emissions are one of the most important types of pollutants from combustion applications. The NOx formation mechanisms are quite complex. In biomass combustion applications, most of the NOx emission is attributed to what is known as the fuel NOx mechanism, which is related mostly to the fuel nitrogen content. Since herbaceous biomass fuels have increased nitrogen concentrations compared to woody biomass, NOx emissions from these fuels may be higher.

On the other hand, fuel NOx emissions also dependent on the relative distribution of nitrogen between the volatiles and the char and the nature of the fuel. Biomasses are "younger” fuels that coal; during combustion fuel-N is converted mostly to NH₃ instead of HCN, as is the case with coals. Since NH₃ has a lower conversion rate to nitrogen oxides compared to HCN, the NOx emissions from biomass tend to be lower than those of coal, under the same combustion conditions. Also, the nitrogen present in the form of HCN tends to be converted into nitrous oxide (N₂O). Hence, the ratio of N₂O/NOx tends to be higher in coal combustion applications than in biofuel applications.

Generally though, the estimation of the NOx emissions from any fuel would require complex computational methods. NOx emissions are typically controlled through proper design of the combustion chamber and distribution of air flows (what is called primary measures) – in large scale applications. Secondary measures, e.g. flue gas cleaning, may also be adopted.

SOx emissions depend heavily on the fuel sulphur content. For herbaceous biomass, this tends to be somewhat higher than for woody biomass, so emissions might increase. Compared to coal combustion though, SOx emissions are expected to be much lower. The presence of calcium in the ash also reduces the SOx emissions, as calcium oxide can react with SOx and form gypsum under favourable conditions. Further reduction of SOx emissions can be achieved by the bed material of fluidized beds, check section 04-02-03.

Aerosol emissions relate to ash particles with a diameter smaller than 1 μm (what is also known as PM1). While coarser ash particles may be easy to collect and to avoid their uncontrolled release in the environment, aerosols need highly efficient dust collectors in order to limit their emissions. Due to their small size, PM1 particles can be easily inhaled and cause several lung or respiratory problems. The formation of aerosols is closely linked to the nucleation and condensation of volatile ash elements – or, to put it more simply to fouling issues. It has already been discussed in section 04-02-02a how the alkalis and chlorine in herbaceous biomass are released during combustion as volatile alkali chlorides. Chlorine also tends to form volatile chlorides along with heavy metals such as zinc (Zn), lead (Pb) and chromium (Cr). Thus, aerosols from herbaceous biomass might be an issue, especially if the plant has absorbed such heavy metals during its growth.