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The Efficient Drying of Grain Residues & Sludges Using Biomass Fuels

IntroductionDrying of grain by-products from grain processing plants is currently done using convectivedryers directly fired with natural gas or contact dryers using steam. Typical products driedin this way are Corn Gluten Feed, Corn Gluten Meal, Wheat Feed and Distillers DriedGrains with Solubles (DDGS). Natural gas prices have soared in recent years thus drivingup costs for drying & process heating. Some waste burning technologies have beenadapted to burn the residues directly and recover some energy but in these systems muchof the fuel value of the residues is wasted in driving off water into the exhaust stack withoutregaining the energy. This paper describes a method of drying down to low moisturecontent such that the product can be sold dry as animal feed or burnt for its energy value.At the same time all the latent heat used to evaporate the water from the residue is madeavailable as process heat. This is a system which minimizes the generation of CO2,whether the fuel is biomass or a fossil fuel.

Barr Rosin & solid-fuel combustion specialists Doosan Babcock are working together todevelop biomass-fired dryer systems for integration with process plant to reduce energyconsumption and carbon emissions in the industrial sector. The concept is to use the fluegas from biomass combustion in a gas-gas heat exchanger to heat the drying medium inthe ring dryer. This will replace natural-gas fired systems and their fossil carbon dioxideemissions. To provide the correct process conditions for drying and to ensure aneconomic size heat exchanger, the flue gas is required at a temperature in excess of800C. This furnace exit condition is ideal for the efficient combustion of biomass,where the combustion temperature must be high enough to eliminate emissions of VOCs,dioxins, carbon monoxide and other products of incomplete or low temperature

combustion. The details of the combustor design, the heat exchanger and the operatingconditions are being finalised for commercial application.

Conventional drying and Exhaust gas recycle

Most large industrial drying is done on co-current convective dryers where the wet materialand heated drying gas pass together through the equipment. High inlet gas temperaturescan be used without damaging the material because the surface moisture on the wet feedrapidly evaporates at the dryer inlet lowering the gas temperature & protecting the surfaceof the particle. Air is the most commonly used drying medium but it has become commonpractice to recycle dryer exhaust gas to recover heat. In this case the drying gas contains

a lot of superheated water vapour and its oxygen content is reduced. Most commonlyRotary Dryers or Pneumatic Conveying (flash) dryers are used. The Diagram below showsa Ring Dryer system. This is a special type of flash dryer which has been widely used inthe grain processing industry to dry byproducts of Starch & Bio-Ethanol production andparticularly high quality DDGS.

The material to be dried often comprises dewatered suspended solids which have beenconcentrated in a centrifuge and then mixed with waste water which has beenconcentrated in an evaporator. These are mixed together & then conditioned with recycleddry product from the dryer to form a friable non sticky material which can be fed into thedryer. A high speed disperser throws the material in a finely divided stream into the hotdrying medium at the feed venture of the dryer where much of the moisture in the feed isflashed off. The drying medium conveys the product around to the manifold, which is acentrifugal classifier. The wetter & heavier particles are recycled back to the feed point forextra residence time in the dryer whilst the dryer & lighter material passes out to beseparated from the drying gas in a series of cyclonic separators.

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Partial Gas Recycle Ring DryerCourtesy of Barr Rosin Ltd

When directly firing a dryer where the combustion products become part of the dryingmedium the practical limit of recycling exhaust gas is around 70%. This limit is reachedbecause the combustion products form the rest of the drying medium. The exhaust of thedryer thus contains the combustion products & the evaporation. In dryers which processfermentation residues & other organic products some VOCs (volatile organic compounds)are evaporated into the drying gas stream together with the water vapour and these must

be removed from the bleed off gas stream before discharge to atmosphere. The usualmethod is to thermally oxidise the VOCs to CO2 and water vapour by heating the gasstream to over 800

0C.

Superheated Steam Drying

A further development of this principle is to recycle 100% of the drying medium by heatingthe dryer through a heat exchanger. In this case the combustion products are on the shellside of the exchanger and the drying medium is on the tube side. Because there is now nocombustion gas in the dryer process circuit the drying medium consists of superheatedsteam with a small quantity of incondensable gases & VOCs. The net evaporation of the

dryer is vented by a pressure control system and can be used as a heat source for otherprocesses. Typically multiple effect evaporator systems concentrating wastewater areheated by condensing the dryer exhaust so that all of the latent heat in the dryer exhaust isrecovered. Effectively the system is both a dryer and a boiler. The small amount ofincondensable gas left in the exhaust after condensation can be thermally oxidized in theair heater of the dryer thus eliminating the need for a separate oxidation system.

The diagram below shows such a system comprising an indirectly heated Ring Dryeroperating in a superheated steam atmosphere and connected to a waste heat recoveryevaporator. The remaining small flow of uncondensed gas is then fed back into thecombustion chamber of the air heater to oxidise any remaining VOCs.

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Superheated Steam Ring DryerCourtesy of Barr Rosin Ltd

The drawing on the right shows a

Ring Superheated Steam Dryer. Thering dryer can be seen on the upperleft with its 2 product collectioncyclones and the product recycle &conditioning system below. In theforeground at the bottom is the mainre-circulating fan with the heatexchanger to the right

Evaporative capacity 37 T/hr

Heat input 29 mW

Steam Output 34 T/hr

Courtesy of Barr Rosin Ltd

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Fluidised bed combustorThe indirectly heated dryer can be heated with a number of different fuels withoutcontaminating the product because the combustion products are not in contact with thematerial to be dried. In particular solid biomass fuels can be used but it will be possible touse any type of solid fuel to heat the dryer. In order to provide the maximum flexibility it isnecessary to use a combustor which can handle a wide range of fuels and the mostflexible is a fluidised bed combustor. Other types of combustor such as chain gratestokers or pulverised fuel burners can be used but the fluidized bed is generally

recognised to be the most flexible.

All of the above mentioned types of burner have been used to burn biomass before butmost biomass combustion systems to date have been built as waste reduction systemswith energy production as a byproduct. The biomass in these systems is usually fed intothe combustor at a high moisture content, typically 30% to 60% moisture. This means thatthe water in the fuel is evaporated in the combustion system and the latent heat to do thisevaporation is lost in the combustor exhaust. Some sensible heat recovery is done at theexhaust of boilers but the water vapour must be condensed to make use of the latent heat.This is not practical when the vapour has been mixed with incondensable combustionproducts. The table below shows the effect of moisture on calorific value of a biomass.

Heat available in wet DDGS

-

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

0% 10% 20% 30% 40% 50% 60%

Moisture

mJ/kg

Heat available

To maximize the heat available in the biomass and hence reduce the amount of CO 2produced per net unit of useful heat produced the biomass must be as dry as possible.Burning dry fuel has the following advantages:

Higher net calorific value & hence less fuel usage Dry fuel is easier to store meter which gives better combustion control Fuel is stable & will not rot in storage Higher efficiency

Less CO2 emissions Smaller gas cleaning systems Lower exhaust volume Less visible plume

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Combination of dryer & Boiler

If we can combine the process steps discussed above with the energy centre of the plantthen an energy efficient and fuel flexible system can be installed. The block diagram belowshows a system for drying distillers spent grains (DDGS) from a bio-ethanol plant. The

DDGS consists of all the residual material left after fermenting the grain into alcohol andthe refining it to produce anhydrous ethanol. The DDGS is dried down to 10% moisturewhere it can be easily stored & metered. The dried material has a calorific value of 18 19mJ/kg and more importantly a net calorific value of 17 18 mJ/kg.

Condensate

Exhaust to

Atmosphere

Combustor

Ring Dryer

Wet Cake

DDGS

To storage

Syrup

Baghouse

HEX

Comb

air

SO2

Adsorption

NOx

Reduction

Waste

Heat

Evap

Incondensable gases

DDGS

from storage

Superheated Steam

Whole

Stillage

Thin Stillage

Boiler

Energy Integration, DDGS-Fired Ethanol Plant

Pr oc es sEngineering DivisionBarr-Ros in

About one third of the fuel is used to dry the DDGS & produce the vapour removed assteam which is used to pre-concentrate the wastewater from the ethanol production into a

syrup. The syrup is mixed with dewatered solid waste prior to drying. The solid fuelcombustor heats both the boiler and the dryer. At start-up when there is little or no materialto dry the boiler works at close to its maximum capacity supplying all the steamrequirements of the plant. However as soon as the dryer starts to operate it providessteam and the boiler can turn down. The combustor however does not turn down butsupplies the excess heat to the dryer as the boiler turns down. At normal operatingconditions the dryer will supply up to 40% of the steam demand of the bio-ethanol plant.The small amount of incondensable gases coming out of the evaporator shell carry VOCsand are passed into the combustor to be thermally oxidised.

This system also allows the process operator to minimize risk and maximize flexibility. If

the market for DDGS as an animal feed picks up then another biomass material can beused in the combustor, as can a large range of fossil fuels in the event of short supply ofbiomass.

GeorgeSvonja 6thMarch 2007