Kraft Pulp mill
2.5 Emerging Techniques
A number of new promising technologies are presently being developed that result in reduction of emissions or energy savings. The improvements of environmental performance may not always have been the main driving force, but plays an important role in the development. Some of these techniques are described in the subsections below. This Chapter includes addressing environmental issues that have only recently gained interest and research programmes related to the development of environmentally improved solutions for the production of high-quality pulp and paper products.
2.5.1 Gasification of Black Liquor
Description: Gasification is a suitable promising technique for pulp mills for the generation of a surplus of electrical energy. Production of a combustible gas from various fuels (coal, wood residues, black liquor) is possible through different gasification techniques. The principle of the gasification of black liquor is to pyrolyse concentrated black liquor into an inorganic phase and a gas phase through reactions with oxygen (air) at high temperatures. A number of gasification processes for black liquor have been proposed. Conceptually, they fall into two categories. One is low temperature gasification, where the gasifier operates below the melting point of the inorganic salts (700-750 oC). Fluidised beds are suitable for a low temperature gasification process, and are used in all of the low temperature processes under development. The other category includes those processes which operate above the melting point and use a water quench to cool and dissolve the molten sodium salts. One example of gasification processes is described in this context:
Example: The Chemrec Process
At the Frövifors mill (Sweden), black liquor is taken from the evaporation plant in the pulp mill and heated to 130 to 135oC using indirect steam. The black liquor is at approximately 65 % dry solids content. The heated liquor then enters the first portion of the Chemrec process, which is a gasifier.
Figure 2.13: The Chemrec process with a gasifier and a quench dissolver for green liquor production and with a weak liquor gas scrubber for hydrogen sulphide absorption
The black liquor is introduced in the gasifier, where it is atomised by high-pressure air (12 bar) and sprayed into the upper part of the reactor. The main air flow needed for the process is pressurised to 0.5 bar and preheated from 80 to 500 oC. The atmosphere in the chamber is approximately 950 oC, and the atomised black liquor forms small droplets that are partially combusted. The inorganic compounds in the black liquor are converted to small smelt droplets of sodium sulphide and sodium carbonate, and this mist drops through the reactor and down to the quench cooler, which is an integrated portion of the reactor unit. The organic compounds are converted to a combustible gas containing carbon monoxide, methane and hydrogen.
The smelt droplets and combustible gas are separated when they are simultaneously brought into direct contact with a cooling liquid in the quench cooler that drops the temperature to 95oC.
The smelt droplets dissolve in the weak wash to form a green liquor solution that is pumped to the dissolving tank under the recovery boiler. Some weak green liquor is recirculated to the quench. A screen is used ahead of the pump to catch small particles and undissolved smelt droplets.
One of the most interesting opportunities with the black liquor gasification processes are to run a gas turbine in combination with a steam turbine in a combined cycle as shown in Figure 2.14 and Figure 2.15. The difficulty to be overcome is primarily the cleanness of the gas to avoid disturbances in the gas turbine.
Figure 2.14: Integrated gasification with combined cycle (IGCC)
Figure 2.15: Combined cycle for power production by means of gas turbine and steam turbine with
Status of development: The first demonstration plant of black liquor gasification using Chemrec concept has been operated at Frövifors Mill in Sweden since 1991. A pressurised demonstration gasifier of this type has also been in operation in Skoghall in Sweden. This type of process is also applied commercially in the U.S.A. since 1997.
The Integrated Gasification with Combined Cycle Technology (IGCC) can only gradually be introduced in the Paper Industry, mainly because of the life time of present recovery boiler.
Furthermore, the gasifiers will initially only be designed for smaller capacities than for a large pulp mill. Before the year 2010, the IGCC technology is only expected to play a marginal role in the overall kraft industry. It might be interesting for some mills where the recovery boiler is the bottleneck in the production and an increase of the chemical recovery capacity would solve these limitations.
Environmental implications: The possible advantages of black liquor gasifiers are:
• Increased electric power generation through the use of combined cycle (gas turbine plus steam turbine). Theoretical balance calculations show that a black liquor based IGCC concept may reach a power efficiency of about 30 % calculated on the heat value of the black liquor. This may be compared with 12-13 % for the conventional recovery boiler.
However, at the same time the overall efficiency (power + steam) would decrease by about 5 % to about 75 %. Thus, the production of process steam decreases. In a situation with a surplus of steam, this appears as an interesting option for an increased power production for export.
• Low emissions to the atmosphere
• Enabling pulp mills restricted in capacity, because of recovery boiler limitations, to increase production. The system is particularly beneficial in mills having a built-in but unused pulping capacity, and where fibre line modifications add more dry solids to the recovery system (e.g. low Kappa pulping, oxygen delignification, increased recycling of effluents from bleaching system)
If the kraft industry should introduce the IGCC technology the industry should have the potential of producing about 1700 kWh/ADt as compared with the present level of about 800 kWh/ADt. Thus the potential increase corresponds to about 900 kWh/ADt. At the same time the heat generation would be reduced by about 4 GJ/ADt, which is more than a typical surplus in a modern kraft mill.
Economic considerations: No data available
Literatur: [SEPA-Report 4713-2,1997]
2.5.2 Use of SNCR on the recovery boiler
Description: The NOxOUT-process is one of several existing processes which are utilising the principal of Selective Non Catalytic Reduction (”SNCR”) to cut down NOx emissions, which means the thermal reduction of nitrogen oxides by ammonia to nitrogen according to the following reaction equations:
2NO + 2NH3 + 12 O2 → 2N2 + 3H2O
3NO2 + 4NH3 → 72 N2 + 6H2O
If urea is used the following primary net reaction occurs in which ammonia is formed:
(NH2)2CO + H2O → 2NH3 + CO2
The reducing agent in a full-scale test in Sweden was a chemically enhanced water-based solution of urea. The process utilises the furnace as a 'chemical reactor' and does not require any additional equipment downstream of the boiler. The reaction normally occurs within a narrow temperature band around 1000 °C. When the temperature is too high more NOx is produced.
When the temperature is too low, ammonia is formed. In the NOxOUT process, the temperature band is widened and chemical enhancers suppress the by-production of ammonia. Ammonia (NH3-slip) produced by unwanted side-reactions and consumption of chemicals are the major parameters when optimising and running the NOxOUT process.
Status of development: A Swedish kraft pulp company commissioned a full-scale test of the patented NOxOUT-process in one of its existing recovery boilers. During the test period, the boiler operated between 95 and 105 % of maximum continuous rate. A number of injection ports for reduction chemicals was installed at several levels. The project has shown that thermal reduction of nitrogen oxides using the NOxOUT process can successfully be applied to recovery boilers.
Environmental implications: Compared with other combustion processes the recovery boiler shows low emissions of nitrogen oxides. Typical nitrogen oxide levels are found to be between 50 and 80 mg NOx/MJ. Despite of relative low flue gas NOx concentrations the recovery boiler is the largest source of NOx emissions in a kraft pulp mill (due to high gas flows). Thus, flue gas treatment measures applied to the recovery boiler would give the greatest effect on the total emissions. Furthermore, an increase of NOx emissions from modern, high efficiency recovery boilers may be expected mainly caused by the demand for increased dry content of the black liquor and higher furnace loads.
Analysis assuming a stochiometry of 1:1 indicates the following performance of the NOxOUT-process:
• An average NOx level without NOxOUT of 80 mg/m3n (standardised m3, dry gas at 3% O2)
• An average NOx level with NOxOUT of about 55 mg/m3n (about 30% reduction)
• A slight increase of ammonia (slip) in the order of 3-4 mg/m3n (stochiometry 1:1)
Depending on stochiometry applied up to 50% NOx-reduction (stochiometry 2:1) is achievable despite the low NOx level without treatment (but then linked to an increase of ammonia-slip).
No disturbances or other negative effects on the operation of the recovery boiler were observed during the full-scale test runs. No negative effect in the chemical recovery cycle was observed.
The total operating costs is relatively low. The alteration required on the recovery boiler may be done during a normal maintenance stop.
The use of urea in SNCR-processes can eventually cause corrosion problems due to the possible formation of corrosive by-products. For security reasons it has therefore in Sweden been recommended to avoid the use of urea-injection in recovery boilers. Because of that the Swedish kraft Pulp Company has performed several new trials with the use of NH3 (gaseous or liquid) instead of urea resulting in NOx-reductions between 20 - 50 % (stochiometry from 1:1 to 2:1) with varying NH3-slips. A NOx-reduction of about 30 % seems to be reachable with acceptable low NH3-slip. The company is planning a long-time trial before come to decision about a final installation or not. The investment costs for installation at recovery boilers are today much lower compared to the below figures and are estimated to be below 1 MEuros (with the use of liquid NH3).
Economic considerations: The investment costs for a complete installation of the NOxOUT process at a recovery boiler similar to that in the tested mill (black liquor load: 1600 t dry solids/day) amount to about 2.2 - 2.8 MEuros. Operating costs for the system include chemical
of the operating costs due to the variation in world-wide prices. However, during the test period in Sweden the urea costs about 154 Euros/t, the total operating cost calculated for the given recovery boiler was in the range of 1 to 1.4 Euros/kg reduced NOx.
Literatur: [Lövblad, 1991]
2.5.3 Removal of chelating agents by modest alkaline biological treatment or by use of kidneys
Description: The chelating agents ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) has been used for many years in the pulp and paper industry. They are applied because of good sequestering properties, i.e. their ability to suppress the activity of the dissolved transition metal ions without precipitation (in the following the shortage "Q" will be used for both of them). These metal ions are able to catalyse the decomposition of the bleaching agent hydrogen peroxide into radicals. Totally chlorine free (TCF) bleaching is currently only possible by treating the pulp with Q before the hydrogen peroxide stage. Increased concentrations of Q are therefore found in wastewaters generated from the production of TCF pulps. In wastewater analyses of a TCF mill producing market kraft pulp 25-40% of charged Q has been identified. This corresponds to Q contents of 10 and 15 mg Q/l in the effluent at a charge of 2 kg Q per tonne of pulp. Although EDTA is non-toxic to mammals at environmental concentrations, there is some concern about the potential of EDTA to remobilize toxic heavy metals out of sediments and the difficulties to biodegrade this substance. In the following only experiences from the use and fate of EDTA are discussed.
Option 1: Biological treatment with and without activated sludge systems commonly used in