Patent Application
20080053348
The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which:
The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.
Without loss of generality, the incineration process is divided into four zones to be serially traversed by the waste: Drying zone 20, first combustion zone for pyrolysis and gasification/volatilization 21, residual zone for char oxidation or solid combustion 22, and ash treatment/sintering zone 23. These zones are actually not very well separated in the furnace and can overlap to a certain extent. A second combustion zone or flame zone 24, where the homogeneous gas phase combustion of the pyrolysis gases takes place, is identified above the waste bed. Primary air 30 is fed from below the grate in generally different amounts to the four abovementioned zones 20, 21, 22, 23. Secondary air 31 is fed above the grate to ensure complete combustion of the gasification and pyrolysis products in the second combustion zone 24.
In order to assess the steam flow in a somewhat systematic way, different kinds of energy balances are considered. First, assuming complete combustion of the fuel, no losses and unitary boiler efficiency, the total energy contained in the steam is equal to the sum of the energy in the waste and that of the combustion air. Accordingly, the energy balance can be written as
{dot over (m)}
steam
H
steam(Tsteam, Psteam)=[w0·ηLHV+u2(1)H(u2(2))+u3(1)H(u3(2))] (eq. 1)
wherein
Second, the steam production can likewise be expressed in terms of an energy balance over the boiler as
{dot over (m)}
steam
H
steam(Tsteam, Psteam)={tilde over (η)}{dot over (m)}gas(u2, u3, d1,gas(xLL, u2), Gcm(XGC, u2))Hgas(xGC) (eq. 2)
wherein, in addition to eq. 1,
Eq. 2 is strongly nonlinear and the influence of the incoming waste feed rate w0 is not clearly identifiable.
In order to avoid the respective drawbacks of the first and second approach above, an attempt is made to derive an expression for the steam flow rate {dot over (m)}steam in a semi-heuristic way. To this end, eq. 1 and eq. 2 are replaced by a polynomial having the general form
where θi are regression coefficients and φi are individual steam contributions. In other words, the steam flow rate is approximated as a linear combination of distinct steam contributions, each of which depends on operational parameters in distinct way inspired by the physical origin of the respective steam contribution or its corresponding heat source as detailed below.
According to the invention, any uncertainty based on the unknown quantities from eq. 1 and eq. 2 are incorporated in the regression coefficients θi. Eq. 3 is nonlinear in the operational parameters, i.e. the process inputs u2, u3, w0 and the process states xLL, xGC, but it is linear in the regression coefficients θi. This particular form of eq. 3 allows estimating the regression coefficients θi on-line without excessive computational power using e.g. a Recursive Least Square (RLS) method as detailed in the following.
A somewhat refined version according to an exemplary embodiment of the present invention has the form
{dot over (m)}
steam=(θ1φ1+θ2φ2+θ3)·cp·xGC+θ4 (eq. 3′)
where steam contributions take the form
The first steam contribution, denoted by φ1, represents the influence of the primary and secondary air, whereas the second steam contribution, denoted by φ2, represents the influence of the combustion gases originating from the solid and gaseous combustion. The latter contribution comprises a first exponential term giving the combustion rate of the solid phase as a function of the waste temperature xLL, and a second term giving the combustion rate of the gaseous phase as a function of the flame temperature xGC.
In this approach, the steam flow {dot over (m)}steam depends linearly on the waste feed rate w0, i.e. {dot over (m)}steam=M(θ)+w0N(θ). This relation can thus be solved analytically for the waste feed rate, which for a given steam set point {dot over (m)}steam determined by steam delivery or energy output contracts can be calculated in a straightforward way. On the other hand, it is to be noted that any approach involving less operational parameters (e.g. θ1+w0θ2 involving no process inputs or states at all other than w0), and in particular an approach that neglects the temperatures xGC and xLL, has proven to be less successful. Hence, the semi-heuristic model of eq. 3 must not be oversimplified.
As there is a delay between the feeding of the waste onto the grate and its effect on the steam production, a corresponding delay time A is introduced into the relations above. This time delay can be in the order of up to one hour, and physically relates to the waste residence time in the initial grate zone. The aforementioned linear relation then reads
{dot over (m)}
steam
=M(θ)+w0(t−Δ)N(θ) (eq. 4)
| Number | Date | Country | Kind |
|---|---|---|---|
| 06405102.2 | Sep 2006 | EP | regional |
