The PBC system

The combustion performance of a PBC system is very dependent on the actual conversion regime of the conversion system. Regime I is more likely to perform poorly, compared with the regime III, with respect to emissions, if the combustion system of the PBC system is not optimised. 

The experimental system consists of a PBC system and a measurement system. The PBC system consists of a conversion system and a combustion system, according to the three-step model, and primary and secondary air lines. A boiler system is not required to realise the measurement method. The measurement system consists of twelve measuring devices (sensors) and a data acquisition system. 

An experimental PBC system has been constructed according to the guidelines of the new measurement method modelled in Paper III. The PBC system comprises a conversion system, a combustion chamber, a flue gas duct, and a measurements system. The conversion system, which is the object of the measurement method, has the following conversion concept updraft, overfired, batch, and fixed horizontal grate. 

The analysis of the PBC system had to be analysed with a more fundamental approach, which could guarantee a result that could be achieved and applied. 

Lamb et al studied the conversion concept of a crosscurrent moving bed. The PBC system consisted of a trolley filled with a fuel bed of bagasse. The trolley rolled horizontally over a vertical air duct to simulate the moving bed (Figure 3). The speed of the trolley was controlled to keep the flame in a stationary position over the duct. An electrically heated platen, 30x15 cm, was placed horizontally 12 cm above the base of the bed, simulating radiation from furnace walls. 

As in classical simulations of biomolecules, there are two general frameworks for setting up QM/MM simulations for a biological system periodic boundary condition (PBC) and finite-size boundary condition (FBC). When the system of interest is small ( 200-300 amino acids), PBC is well suited because the entire system can be completely solvated and therefore structural fluctuations ranging from the residue level to domain scale can potentially be treated at equal footing, within the limit... 

The calculation of electrostatic interactions in FF-based tribological simulations also requires care. The typical model used in tribological simulations consists of two surfaces separated by a fluid, with the whole system subject to periodic boundary conditions (PBCs). If we define the system such that the surfaces extend in the x — y plane, it seems only natural to apply PBCs in these two dimensions. Flowever, care must be taken when treating the third dimension z, which lies normal to the surfaces. Specifically, one must ensure that the length of the simulation cell in the z direction is large enough to leave... 

The objectives of this project are consistent with the objectives (1) and (4) above. The general objective of this project has been to verify a new measurement method to analyse the thermochemical conversion of biofuels in the context of PBC, which is based on the three-step model mentioned above. The sought quantities of the method are the mass flow and stoichiometry of conversion gas, as well as air factors of conversion and combustion system. One of the specific aims of this project is to find a physical explanation why it is more difficult to obtain acceptable emissions from combustion of fuel wood than from for example wood pellets for the same conditions in a given PBC system. This project includes the following stages ... 

Firstly, it is this author s hope that the three-step model should be regarded as a fast and simple theory in the analysis of PBC systems. It can also be the natural starting point for more advanced theoretical approaches, such as partial differential theories. 

According to the three-step model (Figure 4), PBC system can ideally be defined as a complex process consisting of three consecutive subsystems (unit operations, functions) the conversion system (furthest upstream), the combustion (chamber) system, and the heat exchanger system (boiler system). The conversion system... 

Finally, the novel part of the three-step model is the identification of a separate unit operation (subsystem) in a PBC system, that is, the thermochemical conversion of the fuel bed, which by logical consequence requires the introduction of a third subsystem referred to as the conversion system. Commonly, PBC systems are modelled with two steps, that is, a two-step model [3,15], see Figure 7. In the two-step model the thermochemical conversion of solid fuels and the gas-phase combustion are lumped together. Several new concepts are deduced in the scope of the three-step model in general and the conversion system in particular, for example the conversion gas, conversion concept, conversion zone, conversion efficiency, which are all explained later in this summary. 

Paper I - Mass flow analysis of PBC systems in the context of the three-step model... 

Paper I presents a mathematical analysis of the three-step model with a focus on the mass flow of a PBC system, see Figure 11. The mathematical approach is based on a steady-state mass balance, which is also referred to as the simple three-step model. 

The mass flow and stoichiometry of conversion gas in a PBC system is analogue to the mass flow and stoichiometry of gas fuel into a gas fuel combustion system, see... 

Figure 12. In other words, the conversion gas of a PBC system is equivalent to the gas fuel of a gas-fired system. Consequently, the mass flow and stoichiometry of the conversion gas are key quantities in the calculation of the correct excess air number (see Paper I), the latent heat flow of combustion, and the conversion efficiency, and the combustion efficiency of a PBC system.

Figure 12 The difference and similarities between a gas-fired system and a PBC system...

What the three-step model really points out is that it is theoretically correct to carry out basic combustion calculations for a PBC system based on the mass flow and stoichiometry of the conversion gas from the conversion system and not based on the mass flow of solid fuel entering the conversion system. The two-step model approach applied on a PBC system, which is equivalent to assuming that the conversion efficiency is 100 %, is a functional engineering approach, because the conversion efficiency is in many cases very close to unity. However, there are cases where the two-step model approach results in a physical conflict, for example the mass flows in PBC sysfem of batch type cannot be theoretically analysed with a two-step model. 

Two new efficiencies are deduced in the context of the three-step model that is, conversion efficiency and combustion efficiency, which can be very useful in the optimization of existing PBC system and in the design of new advanced environmental-friendly PBC systems. However, to be able to quantify these new parameters, the mass flow and stoichiometry of the conversion gas need to be measured. 

The three-step model states that a PBC system which performs ideally with respect to the mass flow should operate in a mode of complete combustion and complete conversion, that is, conversion efficiency and combustion efficiency equal to one. In practice, a PBC system in general and a small-scale PBC system in particular, operate in different ranges of incomplete conversion and combustion that is, the ash flow contains fuel and the flue gas contains unbumt compounds, such as VOC, tar and CO. 

The three-step model was developed as a consequence of the extreme complexity of a PBC system. This author had a wish to describe the PBC-process as simple as possible and to define the main objectives of a PBC system. The main objectives of a PBC system are indicated by the efficiencies of each unit operation, that is, the conversion efficiency, the combustion efficiency, and the boiler efficiency. The advantage of the three-step model, as with any steady-state system theory, is that it presents a clear overview of the major objectives and relationships between main process flows of a PBC system. The disadvantage of a system theory is the low resolution, that is, the physical quantity of interest cannot be differentiated with respect to time and space. A partial differential theory of each subsystem is required to obtain higher resolution. However, a steady-state approach is often good enough. 

Appendix B includes a review and a classification of conversion concepts. It also investigates the potentials to develop an all-round bed model or CFSD code simulating the conversion system. This review also contains a great deal of information on the heat and mass transport phenomena taking place inside a packed bed in the context of PBC of biomass. The phenomena include conversion regimes, pyrolysis chemistry, char combustion chemistry, and wood fuel chemistry. The main conclusions from this review are ... 

The conversion system of a PBC system can be designed according to many conversion concepts. A classification of updraft conversion systems was carried out. It resulted in 18 different conversion concepts, which points to the complexity involved in trying to obtain an all-round CFSD code. 

The three-step model, which is a new system theory, identifies the least common functions (unit operations) of a packed-bed combustion system. The three steps are referred to as the conversion system, the combustion system, and the boiler system. Previously, PBC has been modelled in two steps [3,15]. The novel approach with the three-step model is the splitting of the combustion chamber (furnace) into a combustion system and a conversion system. The simple three-step model is a steady-state approach together with some other simplifications applied to the general three-step model. The simple three-step model implies a simplified approach to the mathematical analysis of the extremely differentiated and complex PBC process. 

The hardware of the method is a newly constructed experimental PBC system. It is designed according to the three-step model. The object (variable) of the measurement method is the conversion system, which can be varied that is, different conversion systems in small-scale range (<300 kW) can be studied by this method. The method works well on both continuous and batch conversion systems. However, the primary and secondary air flow need to be constant during the test. 

The conversion concept of Rogers PBC system was overfired, updraft, fixed horizontal grate, and cylindrical batch reactor (see Figure 1). 

The objective of AxelTs [11] experimental study is twofold (1) to develop methods to study the combustion process of a packed-bed of biomass (2) to study the effect of mass flow of air on the combustion process in different conditions with respect to fuel particle size, density, and shape. The results are planned to be applied to computer simulations of packed-bed combustion of wood fuels as well as design data for construction of PBC systems. 

The combustion system and the boiler system of a PBC system, according to the three-step model, can be modelled by means of CFD codes [4,5]. However, an allround bed model [6,7] to simulate the thermochemical conversion of the solid fuel inside the conversion system does not yet exist. Bed models of the conversion system will herein also be referred to as CFSD code computational fluid-solid dynamics), analogue to the CFD code. From the three-step model point of view, it is clear that without a CFSD code simulating the thermochemical conversion of the packed bed in the conversion system, simulation of the whole PBC system can never be completely successful. 

This appendix consists of two parts, (a) a conceptual classification and review of the conversion system and (b) a review of conceptual models applied to the heat and mass transport of the thermochemical conversion of biomass. Both these parts are analysed in the context of PBC and the three-step model. Mathematical modelling is outside the scope of this survey. 

Very often in the literature, the physical model used to describe a PBC system is a two-step model [18,19] that is, the conversion system and the combustion system are regarded as one unit referred to as the combustion system (combustion chamber, furnace), see Figure 14. The two-step model is based on the assumption that the conversion system is ideal that is, the conversion efficiency [3] is 100%, which is not the case in real solid-fuel fired systems. However, the two-step model is a functional engineering approach. 

The subsystem (function) located furthest upstream in a PBC system, according to the three-step model, is the conversion system, see Figure 16. This is where the thermochemical conversion takes place. The conversion system can be divided into the fuel-bed system (the packed bed), primary air supply system and the conversion technology, and can be designed according to several conversion concepts, see subsection 0. The main product of the conversion system is the conversion gas, which contains the heat of combustion. The by-product is the ash. 

The proceeding analysis and classification of conversion systems (fiiel-bed systems) in the next section will be confined to updraft fiiel-bed systems. The possible number of combinations, were all three air directions (up-, down- and crossdraft) considered, were too many to be included in the scope of this survey. Updraft fuel beds are chosen because they are the most common among PBC systems. 

---