Organic waste materials as fuels in boilers: challenges and solutions

Waste organic matter used for fuels includes a large variety of materials derived from vegetables and animals and from industrial and human activities: Wood, leftovers from agricultural and forestry processes, seaweed, pulp mill liquor, sawdust, food processing waste, municipal garbage, coffee grounds, cardboard, bark from plywood operations, bagasse from sugar cane, among many others.

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Jul 17, 2017
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Authors: Enrique Posada ; Mateo Jaramillo ; Gilmar Saenz

Abstract

There are attractive opportunities to use organic waste materials as a source of energy for several industries. This allows for the elimination of waste and residues, and at the same time saving money and minimising the use of fossil fuels. This entails important challenges due to the special nature of wastes and the need to focus on appropriate equipment design that minimise the risks involved, really obtain the economic advantages, keep environmental emissions within limits and ensure process safety and equipment stability. This study presents a review of some of the challenges that involve the use of organic waste materials as fuels in boilers, and how these can be resolved. Some real cases are presented.

Introduction

Waste organic matter used for fuels includes a large variety of materials derived from vegetables and animals and from industrial and human activities: Wood, leftovers from agricultural and forestry processes, seaweed, pulp mill liquor, sawdust, food processing waste, municipal garbage, coffee grounds, cardboard, bark from plywood operations, bagasse from sugar cane, among many others. Organic matter can be burned to generate heat for steam, drying process, power generation and other process. Aside from the possibility that these materials could offer less expensive ways to obtain energy than is the case for coal, fuel oil, and natural gas [1], there are additional incentives for burning them for energy applications. First, as a means for solid waste disposal; second, especially in the case of vegetable waste, as a way to diminish global warming gases impact from fossil fuel combustion, as these vegetable materials are part of the natural cycle [2]. A third consideration has to be with the fact that properly managed landfills are not always available to dispose organic solid waste. Open dumps and poorly designed and managed landfills lead to ground and water pollution, leachate and contamination of surface water sources [3].

In many cases, the equipment for burning the wastes is improvised or adapted intuitively, as there is not yet a fully mature manufacturing capacity for industrial waste heat generation equipment, especially in developing countries. In recent years, more demanding environmental regulations have been put into place, causing that existing equipment has to be retrofitted with particulate matter control systems.

This paper presents a review of the use of organic waste materials as fuel for small boilers or heat generators and shows some basic considerations that should be taken into account, as a contribution to companies and engineers, mostly in developing countries, in the designing and operation of these systems.

The complexity of biomass fuels use

Biomass is associated with many aspects of agricultural, crop and forestry production. Large quantities of the residues associated with these production are generated worldwide, most of them vastly underutilised from the point of view of energy usage, although for several important industrial crops this is not the case. Some current farming good practices put these residues back into the soil or let them be used for animal feeding. However, frequently they are burnt or left to decompose in the open or taken to landfills. A number of agricultural and biomass studies, have concluded that it may be appropriate to remove and utilise a portion of crop residue for energy production, providing large volumes of low cost material. These residues could be processed into liquid fuels or combusted/gasified to produce electricity and heat [ ].

It is important to mention that not all organic waste types have been standardised or studied enough as to have complete information of their combustion and environmental emissions or by-products. This adds complexity to the design of combustion equipment and to the understanding of the use of these wastes in existing equipment.

Application problems can be minimised from the very design stage if there is sufficient knowledge of the characteristics of the organic material that will be burned. Generating energy with waste tends to be expensive in capital terms and more inefficient than is the case for using fossil fuels. Experience indicates that because of the temperatures required to avoid corrosion and the air to fuel ratio limitations, total system efficiency of waste to energy (WTE) plants will be on the low side, between 12 and 24% for the case of generating electricity [ ]. To these considerations, it should also be taken into account the typical problems that occur in combustion equipment when using organic material as fuel.

Fouling, deposits, slags and corrosion issues

Burning biomass tends to include severe corrosion problems. Although the chlorine contents of wood, peat and coal are relatively similar, there is considerably more sodium and potassium and less sulphur in wood fuels. It is suggested that the formation of complex alkali chlorides causes corrosion problems [ ].

Fouling or deposits collected on the surface of heat transfer equipment is considered a major issue that can affect the design, lifetime and operation of combustion equipment, increase the operating cost, decrease boiler efficiency, deteriorate materials, increase nitrogen oxides and carbon monoxide, reduce heat transfer and generate corrosion and erosion [1].

Most biomass fuels have high content of alkali metals compared with fossil fuels. An important alkali is potassium, which is released in the gas phase during the combustion and is present as potassium chloride and potassium hydroxide [7]. Alkali metal based compounds have in general low melting temperatures, which cause then to form aggressive deposits on superheater tubes and generate fouling problems and other malfunctions in the combustion stages [8].

Figs 1 a and b show two particular cases of corrosion in pipes from the boiler, the first shows the corrosion generated by the combustion of biomass with high chlorine content and the second one deposits formed in a boiler burning wheat straw.

Fig. 1

Fig 1 corrosion issues in boilers pipeline a Corrosion with high chlorine biomass cofiring [9] b Photograph of deposit forming from wheat straw on a simulated boiler tube [10]

Some studies suggest the adding materials such as bauxite, kaolinite, limestone and magnesium oxide and other additives to induce high melting point in the ash formed during combustion [11]. Other possibilities include the use of special alloys and resistant coatings [12], such as 50% Ni–50% Cr, alloy 625, NiCrBSiFe and alloy 718 [13] and the preparation of the surfaces with techniques that provide wear resistance and corrosion protection, such as high velocity oxygen fuel, in which a molten or semi-molten materials are sprayed onto the surface by means of high temperature and velocity gas stream.

Biomass fly ash and sparks

Complete burning of organic waste is more difficult, due to the nature of these materials, which tend to be irregular in shapes and sizes, rich in fibres and heterogeneous. This causes the presence of unburned waste in the flay ashes. A high level of unburned carbon in ash not only indicates inefficient fuel use, but also lower ash stabilisation and could dramatically increases the ash volume. This, in turn, raises the cost of handling, transportation and spreading the resulting solid waste. A complicated problem may occur when incandescent ash or sparks are transported from the combustion chambers to the baghouse filter, as the possibility of causing fires is high. This situation is normally associated with lack of sufficient residence time of the biomass fuel in the combustion chamber.

Incorrectly specified equipment has led to several baghouse fires. When burning higher boiling point organics, such was waste fuel, the organics that desorb at the hot end of the boiler condense at the cold end and enter the baghouse as liquid droplets or oils condensed on particulates. This fouls the bag filters and provides fuel for baghouse fires. Sources of ignition are for example: Baghouse fans of non-sparking designs that are incorrectly placed upstream of the baghouse and sparks (usually cellulosic material in the feedstock) carried with airstream, and compounds, such as sulphur, which have low autoignition temperatures [14].

The general recommendation would be made to the problem of sparks in the system it is to ensure that the conditions of combustion (time, temperature, turbulence) are adequate to make it a complete combustion, that is, treat the problem from the cause and not from the symptom. However, the literature also recommends extinguishing sparks, for this is necessary to avoid igniting temperatures. The layer of hot air surrounding the spark should be moving at a relative velocity to the spark. This allows for getting low spark temperatures as heat is exchanged to the air from the spark. This can be achieved by creating a change in air velocity and turbulences. This generate eddies in the air stream and removes hot air from the spark. Once the layer is disturbed, sparks can be cooled in a fraction of a second and so, it is avoided that they ignite the filter bags [ 15 ]. The eddies can be originated by sudden changes in duct sizes in the system, single or multiple plates with orifices, changes in direction of duct and specifically designed spark traps. Fig.  shows a type of spark trap.

Fig 2: Spark arrestor [16]

Explosion risks

To comply with environmental regulations, many process and manufacturing industries install thermal oxidisers (specially afterburners) for the abatement of volatile organic compounds (VOCs) and gases in their process waste streams before discharge to atmosphere. Thermal oxidisers, by their very design, operate on generating a flame or intense heat in their combustion chambers for the destruction of process waste or emissions containing VOCs. If process conditions upstream, where these emissions are generated, are not properly managed or controlled, thermal oxidisers can pose a significant fire and explosion hazard [17].

Occasionally, with incomplete combustion, explosive gas mixtures can build up within a biomass boiler combustion chamber and duct, which are subsequently ignited and an explosion of some form can occur. Large chimney systems can present an explosion risk due to the potential build-up of explosive gases [18].

Other operational condition issues

There can be severe consequences in not taking into account the actual variations in operating conditions that the equipment will experience and making the corresponding allowances in equipment design when using waste combustion as so-called Murphy's law will tend to prevail [19].

In the process design, safety factors allow to have a wide horizon of variables to any changes that may occur, whether physical or chemical, that is, allows the process not to be governed by a single operating point, having enough flexibility to fluctuations in quantity and quality of the materials entering the system. However, in addition, too much safety factor can also have an adverse or unwanted effect. Overdesign is not always a good thing.

One of the considerations that can contribute to build confidence and versatility in combustion processes is the use of fouling factors, especially in the boiler design. Reported fouling factors can be confusing, as they incorporate different variables: Tube arrangement, gas velocity, gas temperature, dust loading, softening point of the ash and estimated ash deposit thickness on tubes in between soot-blowing cycles. Understanding all this, lead to a better approach to the dynamics of the processes, generating implications for decision-making of global instrumentation and control philosophy [20].

Several problems that may occur in equipment design of combustion of organic waste materials are shown in Fig .

Fig 3: Some failures in the combustion system to burn organic waste material [21]

Conclusions

Although there are great advantages in the use of WTE systems, they tend to be lower in efficiency and more intensive in capital cost as compared to fossil fuel systems. This has to be considered when planning for this WTE unit.

There are serious safety and stability issues associated to WTE systems. They are related to fire, explosion, material damage, deposits, corrosion and baghouse saturation problems. These have to be taken into account by proper design, enough design allowances, complete knowledge of waste properties and a very good understanding of the supplier and the customer about the possible difficulties and the need for good control and operation of equipment.

In general, experimental work and essays will be required, not only in the design stage, but also when operating the system and attending problems and failures.

Acknowledgment 

The authors thank Mr. F. Michael Lewis for assistance with their articles, their knowledge, and their comments that greatly improved the manuscript.

References

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  14. Santoreli J. Reynolds J. Theodore L.: ‘Introduction to hazardous waste incinerators’ (John Wiley & Sons, Inc. Publication, 2000, 2nd edn.).
  15. ‘Duct Collection Guide’, http://www.qamanage.com/blog/category/fires-and-explosions/spark-arrester/, accessed April 04th 2016.
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  22. Saénz G. Posada E.: ‘Thermal treatment and energy recovering equipment for waste water and gas emissions from a condensation polymerization plant’. 33rd Int. Conf. on Thermal Treatment Technologies & Hazardous Waste Combustors. Baltimore, United States of America, October 13, pp. 1–12.

Case Studies

Case 1: fires in the baghouse of a small hand operated vegetal fibre boiler

A Nomex filter baghouse was installed for a hand operated 100 BHP two pass boiler for a customer, which burned fique vegetal fibre wastes in it. The dust collector started to present conflagrations that burned the bags, which were operated at maximum temperature of 160°C (by using a controlled air injection system before the bag house). The fires were slow, neither explosive nor violent, but completely destroyed the bags. The bags were replaced several times, even after improving on the operation and on the control of the waste feeding and the combustion air supply. The fires kept happening at irregular intervals, of between 1 and 11 months.

Fig 4: Photographs of the fire in the baghouse filter

In the analysis, several considerations and alternatives were proposed:

  • To carry on a detailed study of the combustion gases to detect concentrations of CO, H  and CH  during a prolonged series of measurements and apply this knowledge to the solution of the problem. This was discarded by the customer because of high costs and the difficulties to find locally a supplier with experience and analytical equipment for this.
  • Increase the size of the baghouse to be able to ensure additional dilution in the incoming gases and in this way, avoiding any burning conditions. This was discarded by the customer, due to the involved investments.
  • Install a preventive system based on measurements of temperature before the cyclone collectors, with injection of water by nozzles. This system was installed, but did not work properly.
  • Change the baghouse to a wet scrubber. This option was discarded because of higher investments, corrosion issues, water management and possible visibility problems in the stack.
  • Change conditions in the boiler with the support of a specialist for this type of applications. This was discarded because of the high cost.

In all cases, the customer demanded that the supplier carry the full cost of any new investment and change. After complex negotiations, a rational examination of the situation was presented which allowed both, customer and supplier, to find common grounds for a final solution, now in place. It was found that the cost of replacement of the filters in the baghouse was much lower than the savings obtained by the customer by being able to burn the wastes. It was decided to operate the boiler during a test period, by the supplier, and share any savings among the two companies, after careful accounting of savings based in previously convened parameters.

Under this premises, the supplier considered to install a commercial spark arrester, but it was discarded because of the very variable gas flow conditions that this boiler experiments. Alternatively, it was decided to implement individual baffles (Fig.  ) in the boiler tubes to increase boiler efficiency and diminish exit temperatures. Of course, this required to clean the tubes and the baffles regularly.

Fig 5: Baffles in the boiler

After the test period, the customer found that it could operate the system with the baffles in place and assume the replacement cost in the filters based in the net savings obtained. The experience has shown, after some time, that with the baffles and the more controlled operating conditions, the frequency of bag replacements and damages have been diminished, but there it is still probable that the bags caught fire and be destroyed, as the fundamental root cause, associated to generation of sparks has been diminished, but not entirely avoided.

As discussed, to control the generation of sparks, the recommendation is to ensure complete combustion.

Case 2: problems in the baghouse of a 1000 BHP coffee grounds boiler

A customer in the business of processing soluble coffee and related products has a 1000 BHP boiler that uses a mixture of natural gas and coffee grounds as fuels. The outgoing gases are taken into direct rotary drier that dries the coffee grounds before being feed to the boiler. In this boiler, before installing the baghouse, there was a major increase in the heat generation due to the burning of larger amounts of biomass. After the drier, the gases are taken to the pollution control system. There is an auxiliary natural gas burner supporting the drier, to ensure enough energy to dry the waste, as shown in Fig .

Fig 6: Combustion system diagram for coffee grounds boiler

In this case, the observed problem was the premature saturation of the bags due to moisture problems and unburned materials. The designer tried to prevent this in the choosing of the burner for the drier and in the specification of a minimum temperature for the gas leaving the drier. However, the customer did not allow for this limit, considering that the specified minimum temperatures could cause fires and worked with lower temperatures. Probably because of this, the problems of saturation of the bags persisted, causing lack of satisfaction in the customer.

Indeed, there was an incident in which there was a fire, but it occurred in a by-pass duct used when operating the system with 100% gas natural. This was clearly due to the presence of combustible deposits in this duct, coming from non-controlled connections between this duct and the ones used to carry gases to the pollution control system that handles emissions when the boiler burns waste biomass. This situation was properly diagnosed and corrected.

There was also, despite working under the lower temperatures required by the customer, presence of fire in the bags, which happened once. When this problem occurred, it was traced to the generation of combustion because of the condensation of combustible oils and highly unburned particulate matter on the filter materials, all this associated with lack of proper combustion control in the boiler (Fig.  a). To confirm this, tests were performed with the material collected in the bag, which presented autoignition at temperatures of 220°C, generating incandescent materials capable of reaching and damaging the bags (Fig.  b).

Fig 7: Photo register of Case #2 a Presence of oily substances on the filter b Autoignition of the collected material

In this case, the following considerations have been made:

  • It is advisable to operate the auxiliary burner of the dryer to maintain a minimum incoming gas temperature to the bag house, above 94°C. With this, there should be a solution to the condensates and the saturation of the bags.
  • It is important to separate the circuit carrying the gases when operating with coffee grounds and operating with the filter, from the circuit of the outgoing gases when operating with natural gas.
  • Automatic control in this type of boiler is necessary to optimise burning conditions. Particularly, it was found that the control of some combustion parameters (notably oxygen) was manual. Besides, waste feeding and its water content are not yet correctly controlled or monitored and this could cause unburned fuels and excessive carbon black emissions.
  • Similarly as in the previous case, correct combustion is the way to solve the problem.

Case 3: problems with deposits in and afterburner for organic waste

A thermal equipment was designed for treatment of wastewater and gaseous emissions from a condensation polymerisation plant. The wastewater contains a considerable quantity of organic solvents and the gases have an important amount of VOC's. The equipment is a high temperature combustion chamber where the organic materials provide part of the energy, which is complemented with a natural gas burner to obtain the desired temperature, for the complete destruction of all the VOC's. Most of the energy is recovered from the hot flue gases with a heat exchanger, the exchanger was designed for preheat thermal oil before a boiler. The process is shown in Fig.  .

Fig 8: Process diagram for the thermal treatment system for wastewater and gases from condensation polymerisation plant

One of the operation difficulties is the presence of oxygen in gases because the mixture could be within flammability limits before the equipment, and the plant must be isolated from the equipment to avoid a risk safety situation that could be serious, so was necessary the installation of a flame arrestor. The VOC content in the gases was lower than expected and in principle, the gas consumption was going to be higher than expected. Fortunately, the gas burner was sized for the worst conditions and had enough capacity.

On the other hand, the organic contents on wastewater were significantly higher than expected, and instead of being an energy demander for evaporation, the wastewater was an energy source for the system compensating the low VOC content in the gases. This is sometimes a problem, in particular with a specific source of wastewater with a very high content of soluble organics (alcohols) that increase the heating value of the wastewater and causes temperature rise that system cannot control alone with the burner modulation and the system turns off for safety reasons under these conditions.

The main difficulty found in the operation of the equipment is due to the presence of inorganic salts in the wastewater. Sodium and calcium salts are found in the chamber and in the heat exchanger causing important problems in operation because the exchanger is obstructed making the system turn off because of the rise in temperature. Salts came from non-controlled streams of washing waste waters that can reach the reaction water system. The plant personnel knows the situation and currently is running a program to detect and correct these streams.

Except for the salt deposits in the chamber and exchanger, the other difficulties are now controlled. The equipment has been successful in the treatment of all the plant gases and wastewater, including waste water from a company's nearby plant [22]. The system is shown in Fig 9 .

Fig 9: Installed thermal treatment equipment for waste water and gases

Go to the profile of Enrique Posada

Enrique Posada

Director of projects, Indisa

engineering projects, energy, environmetal studies

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