OFFER TO PURCHASE FOR CASH
                 ANY AND ALL OUTSTANDING SHARES OF COMMON STOCK
                                       OF
                           TRIGEN ENERGY CORPORATION
                                       AT
                              $23.50 NET PER SHARE
                                       BY
                              T ACQUISITION CORP.
                     AN INDIRECT WHOLLY OWNED SUBSIDIARY OF
                                      ELYO
                     AN INDIRECT WHOLLY OWNED SUBSIDIARY OF
                            SUEZ LYONNAISE DES EAUX

  THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
         TIME, ON FRIDAY, MARCH 24, 2000, UNLESS THE OFFER IS EXTENDED.

To Brokers, Dealers, Commercial Banks,

Trust Companies and Other Nominees:                            February 28, 2000

    We are writing to you in connection with the offer by T Acquisition Corp., a
Delaware corporation ("Purchaser") and an indirect wholly owned subsidiary of
Elyo, a SOCIETE ANONYME organized and existing under the laws of the Republic of
France ("Parent") and an indirect wholly owned subsidiary of Suez Lyonnaise des
Eaux, a SOCIETE ANONYME organized and existing under the laws of the Republic of
France ("Suez Lyonnaise"), to purchase any and all outstanding shares of Common
Stock, par value $0.01 per share (the "Shares"), of Trigen Energy Corporation, a
Delaware corporation (the "Company"), at a price of $23.50 per Share, net to the
seller in cash, without interest thereon, upon the terms and subject to the
conditions set forth in the Offer to Purchase, dated February 28, 2000 (the
"Offer to Purchase"), and in the related Letter of Transmittal (which, together
with any amendments or supplements thereto, collectively constitute the "Offer")
enclosed herewith. The Offer is being made in connection with the Agreement and
Plan of Merger, dated as of January 19, 2000 (the "Merger Agreement"), among
Parent, Purchaser and the Company. Holders of Shares whose certificates for such
Shares (the "Certificates") are not immediately available or who cannot deliver
their Certificates and all other required documents to Harris Trust Company of
New York (the "Depositary") or complete the procedures for book-entry transfer
prior to the Expiration Date (as defined under "THE TENDER OFFER--Terms of the
Offer" of the Offer to Purchase) must tender their Shares according to the
guaranteed delivery procedures set forth under "THE TENDER OFFER--Procedures for
Tendering Shares" of the Offer to Purchase.

    Please furnish copies of the enclosed materials to those of your clients for
whose accounts you hold Shares in your name or in the name of your nominee.

    Enclosed herewith for your information and for forwarding to your clients
are copies of the following documents:

    1. The Offer to Purchase, dated February 28, 2000.

    2. The Letter of Transmittal to tender Shares for your use and for the
information of your clients. Facsimile copies of the Letter of Transmittal may
be used to tender Shares.

    3. A letter to stockholders of the Company from Richard E. Kessel, President
and Chief Executive Officer of the Company, together with a
Solicitation/Recommendation Statement on Schedule 14D-9 filed by the Company
with the Securities and Exchange Commission and mailed to the stockholders of
the Company.

    4. The Notice of Guaranteed Delivery for Tender of Shares to be used to
accept the Offer if the guaranteed delivery procedures set forth under "THE
TENDER OFFER--Procedures for Tendering Shares" of the Offer to Purchase are to
be followed.

    5. A printed form of a letter which may be sent to your clients for whose
accounts you hold Shares registered in your name, with space provided for
obtaining such clients' instructions with regard to the Offer.

    6. Guidelines of the Internal Revenue Service for Certification of Taxpayer
Identification Number on Substitute Form W-9.

    7. A return envelope addressed to the Depositary.

    YOUR PROMPT ACTION IS REQUESTED. WE URGE YOU TO CONTACT YOUR CLIENTS AS
PROMPTLY AS POSSIBLE. PLEASE NOTE THAT THE OFFER AND WITHDRAWAL RIGHTS WILL
EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY TIME, ON FRIDAY, MARCH 24, 2000, UNLESS
THE OFFER IS EXTENDED.

    Please note the following:

    1. The tender price is $23.50 per Share, net to the seller in cash, without
interest.

    2. The Offer is being made for any and all of the outstanding Shares. There
is no minimum number of Shares required to be tendered.

    3. Tendering stockholders will not be obligated to pay brokerage fees or
commissions or, except as set forth in Instruction 6 of the Letter of
Transmittal, stock transfer taxes on the transfer of Shares pursuant to the
Offer. However, federal income tax backup withholding at a rate of 31% may be
required unless an exemption is available or unless the required taxpayer
identification information is provided. See "Important Tax Information" of the
Letter of Transmittal.

    4. The special committee of the independent directors of the board of
directors of the Company (the "Company Board") has unanimously recommended that
the Company Board approve the Offer and the Merger (as defined in the Offer to
Purchase) and approve and adopt the Merger Agreement. The Company Board has
determined that the Merger is advisable and in the best interests of the
stockholders of the Company, and has approved the Merger. The Company Board has
also approved the Offer and recommended the Offer to the Company's stockholders.

    5. Notwithstanding any other provision of the Offer, payment for Shares
accepted for payment pursuant to the Offer will in all cases be made only after
timely receipt by the Depositary of (a) Certificates pursuant to the procedures
set forth under "THE TENDER OFFER--Procedures for Tendering Shares" of the Offer
to Purchase or a timely Book-Entry Confirmation (as defined in the Offer to
Purchase) with respect to such Shares, (b) a properly completed and duly
executed Letter of Transmittal (or a manually signed facsimile thereof) with any
required signature guarantees or an Agent's Message (as defined in the Offer to
Purchase) in connection with a book-entry delivery of Shares, and (c) any other
documents required by the Letter of Transmittal. Accordingly, tendering
stockholders may be paid at different times depending upon when Certificates for
Shares or Book-Entry Confirmations are actually received by the Depositary.

    In order to take advantage of the Offer, (i) a properly completed and duly
executed Letter of Transmittal (or a manually signed facsimile thereof) with any
required signature guarantees or an Agent's Message in connection with a
book-entry delivery of Shares, and any other documents required by the Letter of
Transmittal should be sent to the Depositary and (ii) Certificates representing
the tendered Shares or a timely Book-Entry Confirmation should be delivered to
the Depositary in accordance with the instructions set forth in the Letter of
Transmittal and the Offer to Purchase.

                                       2

    If holders of Shares wish to tender, but it is impracticable for them to
forward their Certificates or other required documents or complete the
procedures for book-entry transfer prior to the Expiration Date, a tender may be
effected by following the guaranteed delivery procedures specified under "THE
TENDER OFFER--Procedures for Tendering Shares" of the Offer to Purchase.

    None of Suez Lyonnaise, Parent or Purchaser or any officer, director, agent
or other representative of Purchaser, Parent or Suez Lyonnaise will pay any fees
or commissions to any broker, dealer or other person (other than the Depositary
and Morrow & Co., Inc. (the "Information Agent") as described in the Offer to
Purchase) for soliciting tenders of Shares pursuant to the Offer. Purchaser
will, however, upon request, reimburse you for customary mailing and handling
expenses incurred by you in forwarding any of the enclosed materials to your
clients. Purchaser will pay or cause to be paid any transfer taxes payable on
the transfer of Shares to it, except as otherwise provided in Instruction 6 of
the Letter of Transmittal.

    Any inquiries you may have with respect to the Offer should be addressed to
the Information Agent or the Dealer Manager at their respective addresses and
telephone number set forth on the back cover of the Offer to Purchase.

    Additional copies of the enclosed materials may be obtained from the
Information Agent or from brokers, dealers, commercial banks or trust companies.

                                          Very truly yours,
                                          T ACQUISITION CORP.

    NOTHING CONTAINED HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL CONSTITUTE YOU
OR ANY OTHER PERSON THE AGENT OF SUEZ LYONNAISE, PARENT, PURCHASER, THE
DEPOSITARY, THE INFORMATION AGENT, OR ANY AFFILIATE OF ANY OF THEM, OR AUTHORIZE
YOU OR ANY OTHER PERSON TO MAKE ANY STATEMENT OR USE ANY DOCUMENT ON BEHALF OF
ANY OF THEM IN CONNECTION WITH THE OFFER OTHER THAN THE ENCLOSED DOCUMENTS AND
THE STATEMENTS CONTAINED THEREIN.

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