New methods for determination of KCl in straw

Transcription

1 SUMMARY for Eltra PSO-F&U project no July 2002 New methods for determination of KCl in straw The content of water-soluble salts as potassium chloride (KCl) in straw may cause a number of different operational problems during the energy conversion in the plants. The content of potassium and chloride in straw varies a lot and depends among other things on the conditions during cultivation and harvest. The colour of the straw has traditionally been used as an indication of the content of the problematic salts. The colour of the straw is however not a very reliable measure, for instance a grey colour is not necessarily synonymous to a low content of potassium and chloride in the straw. In the project a simple and practical method for determination of the content of chloride/potassium chloride to be used at the plants is developed. Furthermore, it is shown that a measure of the total content of potassium and chloride can be obtained from the content of chloride in the straw. The achieved results in the project are based on experiments carried out on 10 different samples of straw. Most of these samples had a rather low content of potassium and chloride. This is due to the wet harvest of 2001, making it difficult to get straw with a higher content of potassium and chloride. A preliminary determination of the content of potassium and chloride in the 10 straw samples, using the standard methods of the laboratory, showed that the content of potassium (K) in the 10 samples was more than twice the amount equivalent to chloride (Cl) in potassium chloride (KCl). That means that the equivalent amount of potassium is under the half of the content of potassium in the straw if the whole amount of chloride in the straw is considered to be in the form of KCl. By comparing with previous data of straw it was concluded, that this proportion reasonably could be assumed to apply in general. That means also for straw with a high content of potassium and chloride. In the project partly an existing thermal method is tested and partly methods based on dissolution in water of the potassium and chloride in the straw, followed by a quantification of the dissolved amounts. The thermal method is a method being developed to achieve a measure of the amount of KCl particles that might be emitted with the flue gas during the combustion of a present straw. In the project is investigated whether the method may be used more general, that is to achieve also a measure of the content of KCl in the straw. The investigation showed that this was not possible. dk-teknik ENERGY & ENVIRONMENT 15 Gladsaxe Møllevej DK-2860 Søborg Denmark Tel: Fax: dk-teknik@dk-teknik.dk

2 For a preliminary determined simple dissolution method, shaking up a portion of straw with a portion of water, the following principles of methods for a quantification of the dissolved contents of potassium and chloride were tested: Determination of the content of potassium and chloride using the standard methods Determination of the content of potassium and chloride using test sticks Determination of the content of potassium and chloride using ion-selective electrodes Determination of the conductivity Determination of the content of dissolved solids The testing showed that the content of chloride in the straw samples was brought into solution to a satisfactory extent for the chosen dissolution method. The content of potassium was however only brought into solution to a lesser and varying extent. For the determination of the content of chloride in the solution, a determination by a test stick from HACH was found to be both simple and good. A corresponding simple opportunity was not identified for the determination of potassium. As the combination of the chosen dissolution method with a determination of the chloride content by a test stick both fulfills the requirements of a simple and practical method and gives results comparable with the standard method of the laboratory, this combination was chosen to be the desired routine method. The method laid down by the project is thus a simple and practical method for the determination of the content of chloride in straw. The equivalent content of potassium chloride (KCl) can directly be calculated from the content of chloride. A measure for the total content of potassium and chloride can be achieved from the found proportion between the content of chloride and the total content of chloride and potassium in straw. Qualities of straw however appear, for which the content of potassium and chloride differ to some degree from the found proportion between the content of chloride and the total content of potassium and chloride. It is possible to identify such qualities by a supplementary determination of the conductivity of the solution prepared by the chosen dissolution method. By the testing it was found that there is a good relation between the conductivity of the solutions and the content of dissolved potassium and chloride in the solutions. As the content of potassium in the straw could not completely be brought into solution by the chosen dissolution method, a proportion between the dissolved content of potassium and chloride and the content of potassium and chloride in the straw determined by standard methods was determined. For the 10 straw samples of the project it was found, that the resulting proportion between the conductivity of the solutions and the total content of potassium and chloride in the straw gave a somewhat better measure of the total content than the calculation of the total content based on the content of chloride in the samples. The determined simple method for determination of chloride in straw was tested by repeated determinations on several days, using a number of different variations of the method. It was hereby found that it is necessary to carry out a coarse cutting up of the straw before the determination. Furthermore it was found that the uncertainty of the simple and practical method is of the same size as the determination by the laboratory method and that this uncertainty depends on the homogeneity of the present straw sample. The final procedure for the determined simple method for determination of the content of chloride in straw, and for the calculation of the equivalent amount of KCl, is given in the following appendix. In the procedure is also stated how a measure of the total content of potassium and chloride in the straw can be obtained from the determined content of chloride or from a possible supplementary determination of the conductivity of the solution. page 2 of 2

3 Appendix to summary for Eltra PSO-F&U project no New methods for determination on KCl in straw Routine method for determination of content of chloride in straw (and estimation of the total content of cholride and potassium)

4 Routine method for determination of content of chloride in straw (and estimation of the total content of chloride and potassium) dk-teknik 2002 Page 1 of 3 Measure range: Reference: Content of chloride in straw above 0,12 %weight chloride. Project report New methods for determination of KCl in straw. July dk-teknik ENERGY & ENVIRONMENT. Eltra contract no Principle: The content of chloride in a straw sample is brought into solution by shaking up with water and a test stick determines the content of chloride in the solution. Hereby the content of chloride in the sample is determined. A measure for the total content of chloride and potassium in the sample may be calculated from a determined general proportion between the content of chloride in straw and the total content of chloride and potassium in straw. Apparatus: 1. Scissors or the like for a coarse cutting up of the straw sample. 2. Dissolution container glass or plastic, equipped with a tight fitting lid and a volume of 1½ - 2 liter. It is practical if the container has a wide neck as e.g. an ordinary jam glass. 3. Scale, with a reading of 1 gram or better (e.g. an electronic kitchen scale). 4. Measuring cup, for measuring out 1 litre of water. Reagens: 1. Water with an insignificant amount of chloride and a low conductivity, as e.g. demineralised (ion-exchanged) water, distilled water and membrane filtered (RO) water. Ordinary supplied drinking water cannot be used. 2. Chloride test sticks, HACH Cat. no (measure range ppm Cl - ) Pre-treatment: 1. Cut a portion of 25 gram of the straw into pieces of maximum 3 cm. Procedure: 1. Transfer the cut portion of straw of 25 gram into the dissolution container and pour 1 litre of water into the container. 2. Assemble the lid and control that the lid is completely tight. 3. Shake the container vigorously, up and down, 25 times. 4. Let the mixture be left for at least 30 minutes.

5 Routine method for determination of content of chloride in straw (and estimation of the total content of chloride and potassium) dk-teknik 2002 Page 2 of 3 Procedure: 5. Shake the container, up and down, 5 times and let the straw settle. NOTE: Small particles may block the intake hole of the test stick. 6. Remove the lid and dip a test stick into the water. Do not dip the stick below the yellow stop mark on the top of the stick. NOTE: Alternatively a portion of the water can be decanted into a little glass, to a height of water of 3 5 cm. 7. Hold the stick dipped until the yellow stop mark becomes dark (end of reaction). It takes about 5 minutes. 8. Read the result on the stick, being the number on the stick that the top of the white reaction peak has reached ( Quantab unit ), see the illustration on the packing of the test sticks. Results: 1. On the packing of the test sticks is a conversion table, converting the Quantab units to chloride contents, mg/l Cl -. The content of chloride in the solution, mg/l Cl -, is converted to the content of chloride in the straw sample in weight percent (%weight Cl - ) through a division by 250. Below an extract of the conversion table is shown (reservation for alterations of the producer s specifications is taken), supplied with the result of the content of chloride in the straw sample. Quantab mg/l Cl - %weight Cl Quantab mg/l Cl - %weight Cl Units in the straw Units in the straw NOTE: Notice that the stated conversion of the determined content of chloride in the solution to the content of chloride in the straw sample is based on that the described procedure is followed.

6 Routine method for determination of content of chloride in straw (and estimation of the total content of chloride and potassium) dk-teknik 2002 Page 3 of 3 Results: 2. The content of chloride in the straw sample can be converted into the equivalent amount of potassium chloride, KCl, in the straw sample: %weight KCl = 2,1 %weight Cl 3. The content of potassium (K) in straw is larger than the amount of potassium included in the equivalent amount of KCl calculated in point 2. A measure of the total content of potassium and chloride in the straw, K + Cl, can be achieved by the following expression, assuming a content of moisture in the straw between %weight: %weight (K + Cl) = 3,2 %weight Cl + 0,3 By moisture contents below 12 %weight, 0,1 is to be added to the result. Practical comments: 1. The more sub-samples taken and combined for a lot of straw (before the test portion is taken) the better measure for the mean content of chloride in the lot is obtained. 2. Some straw qualities may contain relatively more potassium and by that, more potassium and chloride than calculated by the expression in the point Results, no. 3. A somewhat better measure for the total content of potassium and chloride in the straw may in these cases be achieved by a supplementary determination of the conductivity of the prepared solution and the following expression, assuming a moisture content in the straw between %weight: %weight (K + Cl) = 0,0017 Conductivity + 0,15 as the value for the conductivity shall be in µs/cm at 25 o C. The moisture content in the straw is however without significant importance for the result. For completely dried straw 0,02 is to be added to the result and for a straw with moisture content of 25 %weight, 0,02 is to be subtracted from the result. NOTE: Notice that the stated expression is based on that the conductivity is measured for the water solution prepared exactly as described in the Procedure.