Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
2002-04-15
2003-09-09
Lazarus, Ira S. (Department: 3749)
Furnaces
Process
Treating fuel constituent or combustion product
C110S342000, C110S346000, C044S605000
Reexamination Certificate
active
06615751
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for reducing agglomeration, sintering and deposit formation resulting from the gasification or combustion of a solid carbonaceous fuel material comprising a combustible portion and a non combustible inorganic portion, which non combustible inorganic portion comprises 4 to 50 parts by weight of K
w
+Na
w
, and 0 to 40 parts by weight of Si
w
, wherein K
w
is potassium, Na
w
is sodium, and Si
w
is silicon, all calculated in parts by weight of the elements per 100 parts by weight of the inorganic portion, and optionally also chloride, phosphorus, calcium and sulphur. This type of solid carbonaceous fuel material having a substantial content of potassium and in some cases also sodium is typically met in solid fuels having a high proportion of a biomass material, such as straw.
BACKGROUND ART
For the disposal of biomass side products and for the utilisation of the energy contained in biomass materials, a number of processes are being developed for the utilisation of biomass materials as fuels. In some of the processes the biomass may be used in combination with conventional fossil fuels including solid fuels, such as coal and coke.
The ash obtained from coal or coke often contains contaminating compounds, such as heavy metals, for which reason they cannot be utilised as fertilizers. Therefore other means for the disposal is used including the use as an ingredient in cement.
Generally ash from biomass materials contains much less amounts of such contaminating compounds. These compounds, on the other hand, are not usable in cement preparation due to a higher content of alkali metal compounds, especially potassium compounds. Furthermore the presence of chlorine, especially as alkali metal chlorides or HCl, gives rise to corrosion problems in connection with combustion and gasification.
The utilisation of biomass fuels in gasification plants is often carried out in two steps. The first step is a gasification carried out under limited oxygen supply. The subsequent step is a combustion of the gases with sufficient oxygen supply, optionally together with other fuel materials. Especially the gasification step often results in problems because the ash of the biomass, which in this case is the not gasified residue, either causes bed sintering and defluidization , corrosive deposits, or corrosive alkaline filter dust formation. This is due to the low melting point and high partial pressure of certain alkali metal compounds, which form part of or are derived from the non-combustible inorganic portion of the fuel. Silicon is another element typically abundant in the fuel. Alkali compounds, in combination with silicon and possibly small amounts of calcium, can produce low temperature eutectica. Local hot spots, where the temperature may increase far beyond the set point temperature, due to char or gas burning in zones where the oxygen to fuel ratio is relatively high, will exacerbate the problem causing further sintering and adhesion of neighbouring particles, which will raise the temperature even further.
CA 1 202 485 (Kekish et al.) discloses a method of raising the fusion point of slag by the addition of 0.5 to 10.0% by weight of the slag of a phosphate compound in fuels forming slag, with a high content of silicon oxide (up to 81%) and calcium oxide. The method is exemplified with a slag having a fusion point of about 1480° C. without the addition, and a fusion point above 1540° C. with addition of monoammonium phosphate, or phosphated alumina in an amount of 5% by weight of the slag.
The content and the composition of the non combustible inorganic portion of the fuel used by Kekish is not disclosed directly. The composition of the resulting slag is exemplified with 81% by weight of silicon oxide (45.4% Si), 9% by weight of calcium oxide (6.4% Ca), and only 3% by weight of potassium oxide (2.5% K). The high fusion temperatures relevant for this composition of slag is not a concern during biomass gasification or combustion according to the present invention. Kelish reports, that slag formation can be reduced and fusion temperatures raised to 1540° C. by adding as little as 0.025-0.1% H
3
PO
4
by weight of the solid fuel, irrespective of the fuel composition. Kekish, however, does not deal with fuel materials causing severe agglomeration, sintering, and deposit formation already in the temperature range of 700-1000° C., and especially fuels rich in potassium and with significant amounts of chloride, phosphorus, calcium and sulphur. An example of such fuel is straw, which typically contains more than 2.5% by weight of K, 0.1-3% by weight of Na, 0-15% by weight of Cl, 0-10% by weight of P, 2-10% by weight of Ca, and 0.5-3% by weight of S in the non combustible inorganic portion of the fuel material.
Kekish (CA 1 202 485) teaches, for coal and bark fired boilers, a broad range for the amount of phosphorous compound to be added relative to the amount of slag, especially slag rich in silicon oxide and calcium oxide. In case of a sufficient homogeneous source of fuel, the operator will be able to determine an appropriate amount of the phosphorous compound on the basis of a few test runs. However, for inhomogeneous fuel sources, this could lead to many troubles and utility shut downs due to variations in the amount and the composition of the non combustible inorganic portion of the fuel material. Addition of too small amounts of phosphorus will cause no beneficial increase in the ash fusion point, and in the worst cases it may even aggravate slag problems. Addition of too large amounts of phosphorus may be costly and in the presence of calcium lead to several low temperature eutectics below 800° C. Also superfluous phosphorus may be liberated as gaseous (P
2
O
5
)
2
or PO
2
under combustion conditions and gaseous (P
2
O
3
)
2
under gasification conditions, and contribute to the emission of harmful gasses. Thus the addition of an amount of phosphorous compound which in one situation is beneficial for the slag reduction may turn out to even aggravate the sintering problems in other situations or cause unwanted gas emission problems.
Accordingly it is still a problem to reduce sintering of ash from biomass materials, which by gasification or combustion develop ashes with a fusion point within the above mentioned gasification/combustion temperature range of 700-1000° C. Thus there is a need for an effective additive, as well as a method for the estimation of the necessary amount of such additive, for the gasification and combustion of biomass fuels with greatly varying amount and composition of inorganic compounds. In this way it will be possible to control the agglomeration and sintering problems in case of variations in the composition of the treated biomass material. Even when using the same type of biomass material, such as straw, substantial variations may occur due to different growth and weather conditions, or type of fertilizer used.
It has now been found that the above mentioned agglomeration and sintering problems connected with biomass materials with a high content of potassium and/or sodium can be reduced substantially by the addition of a reactive phosphorous compound, or a combination of reactive phosphorus and reactive calcium in certain forms and amounts, estimated on the basis of a simple analysis of the biomass containing material fed to the gasification or combustion unit.
DESCRIPTION OF THE INVENTION
Accordingly the present invention relates to a method for reducing agglomeration, sintering and deposit formation resulting from the gasification or combustion of a solid carbonaceous fuel material, comprising a combustible portion and a non combustible inorganic portion, which non combustible inorganic portion comprises 4 to 50 parts by weight of K
w
+Na
w
and 0 to 40 parts by weight of Si
w
, wherein K
w
is potassium, Na
w
is sodium, and Si
w
is silicon, all calculated in parts by weight of the elements per 100 parts by weight of the inorganic portion, and optionally also chloride, phosphorus, calcium and su
Fjellerup Jan
Henriksen Ulrik
Sørensen Lasse Holst
Brinks Hofer Gilson & Lione
Lazarus Ira S.
Raetech
Rinehart K. B.
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